LECTURES ON
PLANT PHYSIOLOGY

BY

DR. LUDWIG JOST

PROFESSOR OF BOTANY IN THE UNIVERSITY OF STRASSBURG

AUTHORIZED ENGLISH TRANSLATION

BY

R. J. HARVEY GIBSON, M.A., F.L.S.

PROFESSOR OF BOTANY IN THE UNIVERSITY OF LIVERPOOL

WITH 172 ILLUSTRATIONS

OXFORD 3 3

AT THE CLARENDON PRESS 4 V] j
1907 \/ -i

HENRY FROWDE, M.A.

PUBLISHER TO THE UNIVERSITY OF OXFORD

LONDON, EDINBURGH
NEW YORK AND TORONTO

PREFACE TO THE GERMAN EDITION

OF late years the subject of Plant Physiology has been discussed in
a succession of admirable treatises. In addition to Pfeffer's Handbook,
which laid the foundation of modern Plant Physiology, there are the briefer
expositions of the subject in the textbooks of General Botany, such as
those of Noll and Wiesner, but a textbook of Plant Physiology of
moderate size has been for long a desideratum. The object of the present
volume is to introduce the subject to those who are already familiar with
the fundamentals of natural science, and inasmuch as it takes the form
of lectures which I have been in the habit of giving in this University for
a number of years, and is the direct outcome of them, I have styled the
book Lectures on Plant Physiology.

The characteristic feature of such a textbook, apart from the mere
mode of presentation, must lie in the selection and arrangement of its
subject-matter, and those who are familiar with the works of others will
be able to see at once wherein the present treatise differs from them. It
is unnecessary for me to refer here to the general line of argument of the
book, since that may be readily grasped from a study of the table of con-
tents. Still less need I attempt to justify it, for if it does not speak for
itself, it will not help matters to go into a detailed explanation of the
principles of arrangement and selection in a preface. I may, therefore,
confine myself to saying a few words on the treatment of the literature and
on the illustrations.

A bibliography will be found at the end of each lecture. This con-
tains, in addition to works of fundamental importance on the subject more
immediately concerned, references to numerous special papers recognized
as the authoritative statements for individual observations or views.
That the selection of the literature must be arbitrary goes without saying ;
for it would be possible in the long run to cite one or more authorities for
almost every word I have written. The place which Pfeffer's Handbook
has taken in modern botanical literature has rendered it necessary to
quote from it on almost every page ; rather than pursue this course, the
present general reference must suffice ; where, however, it has been specially
referred to, the citation is made in the abbreviated form, £ Pfeffer,
Phys.' (Pfeffer, W. Pflanzenphysiologie, ein Handbuch der Lehre vom
Stoffwechsel und Kraftwechsel in der Pflanze. Bd. I, 2. Aufl., Leipzig,
1897 ; Bd. II, i. Halfte, 2. Aufl., Leipzig, 1901 ; Bd. II, i. Aufl., Leipzig,
1881 .) Other works are indicated in the text, as a rule, by the author's name
and the year of publication only. The dates are intended to serve merely as

iv PREFACE TO THE GERMAN EDITION

reference numbers to the authorities cited at the end of the lecture dealing
with the subject, and must not be taken as in any sense indicating the
date of the discovery of a fact or of the establishment of a theory. I have
placed the more recent literature prominently in the foreground, simply
because, with its aid, the student can have no difficulty in tracing the older
works on the subject ; I do not desire, by this method of citation, to
imply that modern works only are worthy of consideration. It has not been
possible to enter into the history of the science, save only in a few places
where it seemed essential to the elucidation of the subject.

Only a few of the illustrations are original ; the great majority are
reproductions from memoirs or textbooks, and for their execution I am
greatly indebted to Fraulein A. Winnecke. A number of cliches have
been borrowed from the works of Detmer, Fischer, Goebel, Klebs, Kiister,
Strasburger, and Verworn, as well as from the ' Bonn Textbook ' (Lehrb. d.
Botanik. Strasburger, Noll, Schenck, u. Schimper). To the authors of
these works my best thanks are due for permission to use these illus-
trations.

LUDWIG JOST.

STRASSBURG, i. E., November, 1903.

AUTHOR'S PREFACE TO THE ENGLISH

EDITION

IT is both a pleasure and a duty to express to my friend, Professor
Harvey Gibson, of the University of Liverpool, my sincere thanks for
having made my Lectures on Plant Physiology more readily accessible
to English readers through the medium of the present translation.

I have taken the opportunity of making brief alterations, here and
there, in the original German text, with the view of correcting errors or re-
moving ambiguities. In order to bring the work up to date, and to indicate
the more important additions to the science that have been made since
the original manuscript was completed (spring of 1903), supplementary
notes, distinguished by being enclosed in square brackets, have been
appended to or introduced into each lecture. Obviously it is not possible
to do more than offer a selection of the extensive current literature in the
present translation any more than in the original text. Czapek's Bio-
chemie (Jena, 1905, vol. ii) is a work of special importance in relation to
Part I, and reference must be constantly made to it. Similarly with
regard to Part II, Pfeffer's Plant Physiology, vol. ii (Leipzig, 1904),
will be found to be equally invaluable as a work of reference. Since the
English translation of that treatise has been already published, it appeared
to me unnecessary to introduce special references to it. All those who
desire to study the subject of plant physiology more closely are strongly
recommended to consult that work.

LUDWIG JOST.

STRASSBURG, 1906.

TRANSLATOR'S PREFACE TO THE ENGLISH

EDITION

I HAVE been prompted to undertake a translation of Professor Jost's
Vorlesungen iiber Pflanzenphysiologie by the desire to see made available
for English-speaking students a critical modern exposition of so important
and progressive a subject, moderate in size, adequately full in detail, and
written in a lucid and interesting manner by a teacher of Dr. Jost's rank
and experience.

I have not attempted to ' edit ' Dr. Jost's pages in any way, feeling
that the author should be allowed to speak for himself without comments
and interpolations on the part of his translator. Such additions to and
alterations in the German edition as have been made are from Dr. Jost's
own pen, and are indicated, as he has said, by enclosure within square
brackets. At the Author's request, I have endeavoured to translate the
German text as literally as was possible without, I hope, unduly offending
against the rules and idioms of the English language. Under the circum-
stances, of course, it will be readily understood that absence of critical
comment does not signify concurrence with every view put forward by
the Author, or complete acquiescence in his mode of treatment of every
subject.

I have to acknowledge my indebtedness and to offer my grateful
thanks to several friends who have aided me in matters of interpretation
or expression, but more especially to Professor I. Bayley Balfour, F.R.S.,
who has been so kind as to look through the proof-sheets, and to furnish
me with many valuable suggestions and criticisms. I am also indebted
to my colleague, Dr. A. W. Titherley, for much help in the interpretation
of the German names for organic and inorganic chemical compounds.

R. J. H. G.

LIVERPOOL, 1907.

SUMMARY OF CONTENTS

Introduction

Problems of Physiology, i ; Methods of Physiology, 2.

PART I
METABOLISM

I. Material Composition of the Plant

Chemical analysis, 3. Microscopical analysis, 6. Structure of the cell, 7 ; chemistry
of the cell, more especially of the protoplasm, 8.

II. Absorption of Materials in General

Osmosis, ii. Osmotic characters of protoplasm, 12. Impermeability, 1 2 ; osmotic
pressure, 13 ; permeability, 13 ; storage, 20 ; selective power, 20.

III. Absorption of Materials in detail. Fate of the Substances absorbed

CHAPTER I

Water

1. Significance of water, 24.

2. Absorption of water. Through the root ; characters of the soil, 25 ; characters

of the root, 26 ; external influences, 31. Through aerial organs, 32.

3. Excretion of water. Transpiration. Proof, 35 ; cuticular and stomatal tran-

spiration, 38 ; external influences, 38 ; stomata, 39 ; retardation of tran-
spiration, 42 ; promotion of transpiration, 42 ; significance of transpira-
tion, 43. Excretion of liquid water, 50 (see under 4).

4. Conduction of water. Organs for the conduction of water. Cells, 45 ; vessels,

47 ; filling of vessels with water, 49. Bleeding, 50 ; conditions, 53 ; causes,
54. Excretion of water by hydathodes, 58 ; significance of the excretion
of liquid water, 59. Mode of conduction of water. Problems connected
with the quantity of water conducted, 61 ; root pressure as a cause of the
conduction of water, 63 ; leaf suction as a cause of the conduction of
water, 64 ; physical questions, 64 ; peculiarities of the plant, 66 ; Jamin's
chain, 71 ; influence of living cells, 74.

CHAPTER II
Mineral Constituents

1. Source of the mineral constituents, 77.

2. Absorption of the mineral constituents, 78.

3. Significance of the mineral constituents, 80 ; necessity for certain elements, 80 ;

evidence for this necessity, especially by means of water culture, 81 ; sig-
nificance of the essential elements, 83 ; unessential nature of other elements,
86 ; stimulants, 87.

viii SUMMARY OF CONTENTS

4. The mineral constituents of the soil. The soil. Origin, 90 ; mineral contents,

91 ; absorption, 92. The action of the plant on the soil. Union of root-
hairs and soil particles, 94 ; excretion of acids, 95 ; renewal of root-hairs,
96 ; branching of the root, 97.

5. Soil and plant distribution. Physico-chemical causes of plant distribution, 97 ;

association, 99 ; historical causes, 100.

6. Soil and Agriculture, 101.

CHAPTER III

Carbon and Nitrogen

i. Assimilation in Holophytic Plants and Fate of the Products of Assimilation

a. Assimilation of Carbon.

1. The fundamental facts. Proof of the decomposition of carbon-dioxide. Air-

bubble method, 104; gas-analysis method, 105 ; other methods, 105. Signifi-
cance of chlorophyll, 106 ; physical and chemical characters of chlorophyll,
107. Concomitant action of sunlight, no. Products of assimilation.
Nature, no; amount, 114.

2. Influence of external factors on carbon-dioxide assimilation. Influence of

carbon-dioxide. Carbon-dioxide in air, soil, water, 1 1 8 ; influence of con-
centration, 119; entry into the plant, 120. Influence of other factors;
(i) indirect, through modification of the plant, 121 ; (2) direct influence of
oxygen, 124; of temperature, 124 ; of light, 125 ; intensity of light, 125 ;
wave-length, 126; light as a source of energy, 129; light becomes
absorbed, 129.

3. Historical data as to the assimilation of carbon-dioxide, 131.

b. Assimilation of Nitrogen. Sources of nitrogen, 133 ; gain and loss of nitrogen in

nature, 135. Absorption of nitrogen, 137. Assimilation of nitrogen. Con-
struction of proteid, 138; chemistry of proteid, 138; assimilation of nitric
acid, 141 ; of ammonia, 142 ; organic compounds of nitrogen, 143. Assimilation
of sulphur and phosphorus, 145.

c. Fate of the products of assimilation.

1. Solution of reserves : — (a) in seeds, 148 ; starch, 149 ; diastase, 1 50 ; enzymes,

152 ; catalytic phenomena, 152 ; action of enzymes on hydrogen peroxide,
152 ; incompleteness of the enzyme reaction, 153 ; dissolution of starch
in seeds, 154; removal of the embryo, 1 56 ; inhibitory agents, 1 56 ; cellulose,
157 ; cell- wall, 157 ; cytase, 158 ; fats, 158 ; chemistry, 158 ; decomposition,
159; proteid, 159; pepsin and trypsin, 160; (b) in perennials, 162 ; (c) in
trees, 163 ; (d) in foliage leaves, 164.

2. Circulation of the dissolved reserves. Migration from the leaf, 166 ; from

other storehouses of reserves, 167. Causes of the translocation. Diffusion,
167 ; accelerating agents, 167. Organs concerned in the transport of material.
Parenchyma, 170; sieve-tubes, 170; vessels, 172. Objects of trans-
location, 172.

3. Fate of the translocated materials. Transformation into plasta, 173. Forma-

tion of reserves. Carbohydrates, 173 ; proteid, 173 ; fat, 175. Formation of
waste products, 176.

2. Acquisition of Carbon and Nitrogen by Heterotrophic Plants

a. Saprophytes.

1. Acquisition of carbon, 177 ; nutritive value of different sources of carbon,

178; omnivors and specialists, 180.

2. Acquisition of nitrogen, 181.

3. Saprophytes in nature, 182 ; dead organisms, humus, 183 ; excretion of

enzymes, 183.

b. Insectivorous plants, 184.

c. Parasites, 186.

d. Metabolism in heterotrophic plants, 188.

SUMMARY OF CONTENTS ix

3. Katabolism in Autotrophic and Heterotrophic Plants

a. Respiration. Decrease in dry weight, 191; economic coefficient, 191. Extent of

respiration. Proof, 191 ; respiration in chlorophyll-containing cells, 194. Res-
piratory materials. Carbohydrates: complete combustion, 196; formation of
acids, especially in Fungi and succulents, 197 ; fats, 199; proteid, 200. De-
pendence on external factors. Light and temperature, 201 ; influence of
materials, 201. Intra-mplecular respiration, 202 ; cause (204) and significance
(205) of respiration. Historical data, 205.

b. Fermentation, 207.

1. Alcoholic fermentation, especially of yeast, 207 ; nutritive materials, 208 :

fermentative materials of yeast, 208 ; products of fermentation, 210 ; cause
of fermentation, 211; zymase, 211; relation to enzymes, 211; zymase in
intra-molecular respiration, 212. Dependence on oxygen, 212; aerobes,
anaerobes, 213. Biological significance of fermentation, 213.

2. Butyric acid fermentation, 214 ; effect of oxygen, 215.

3. Further fermentation of alcohol into acetic and carbonic acids, 216.

4. Fermentation of carbohydrates, 217 ; formation of lactic acid, 217 ; formation

of butyric acid, 217; fermentation of cellulose, 217; fermentation of pectin,
218. Conversion of the products of fermentation by other organisms, 218.

5. Fermentation of proteids, 219.

4. Special Cases of Anabolism and Katabolism

a. Oxidation of inorganic materials (respiration).

1. Sulphur-bacteria. Beggiatoa. Origin of sulphuretted hydrogen, 221; oxida-

tion of sulphuretted hydrogen, 221 ; red sulphur bacteria, 223.

2. Iron-bacteria, 224.

3. Nitro-bacteria. Origin of ammonia. Fermentation of urea, 224 ; oxidation

of ammonia, 225 ; nitrate and nitrite bacteria, 226.

b. Assimilation in nitro- and sulphur-bacteria. Assimilation of carbonic acid

by nitre-bacteria, 227 ; oxidation of organic substances by nitro-bacteria, 228 ;
assimilation in sulphur-bacteria, 229 ; sensitivity of nitro-bacteria to organic
substances, 229.

c. Denitrification and fixation of nitrogen, 231.

1. Denitrification, 231.

2. Fixation of nitrogen, 232. Clostridium pasteurianum, 233 ; symbiotic union

with other bacteria, 233 ; cultures, 234 ; other micro-organisms alleged to
fix nitrogen, 234 ; Leguminosae, 235 ; symbiosis with B. radicicola, 237;
fixation of nitrogen, 237.

d. Symbiosis and metabiosis.

1. Instances related to the case of Leguminosae, 239.

2. Mycorhiza. Endotrophic forms, 240 ; ectotrophic forms, 241.

3. Lichens, 242.

4. Metabiosis. Circulation of carbon and nitrogen in organisms, 243.

PART II

METAMORPHOSIS
I. Problems of Developmental Physiology

1. First example : development of the Myxomycetes, 247.

2. Second example : development of Basidiobolus, 248.

3. Third example : flowering plants. Development, 250; differentiation and division

of labour, 250; correlations, 252. Influence of external conditions on develop-
ment, 252; formal conditions, 253 ; formative results, 254.

4. Fundamental problems of Biology. Mechanical explanation of life, 254 ; organi-

zation and mechanism, 254 ; matter and form, 256.

x SUMMARY OF CONTENTS

II. Growth and Formation under Constant External Conditions

a. Growth of the cell, 258.

1. Growth, 258 ; formation, 259.

2. Growth of the protoplasm, 259,

3. Growth and formation of the cell- wall, 260. Origin of the cell- wall, 260.

Surface growth, 260 ; mechanics, 261 ; apposition, 262 ; intussusception,
262; significance of turgor tension, 263. Growth in thickness, 265 ; cessation
of growth, 267 ; resume, 267.

4. Cell-division, 267 ; nuclear division (mitosis), 268 ; cell-wall formation, 269 ;

arrangement of walls, 269 ; amitosis, 271; extent of division, 271.

b. Growth of the entire plant, 272.

1. Somatophytes, asomatophytes, 273 ; embryonic growth, 273. Position and

activity of the growing point, 273; shoot (symmetry, 275; form, 276);
leaf, 277 ; root, 279. Structure of the growing point, 279 ; simple cases, 279 ;
arrangement of cells at the shoot apex, 280 ; at the point of origin of the
leaf, 281 ; at the point of origin of the root, 283. Adventitious growing
points, 284.

2. Growth in extent, 285. Longitudinal growth: — measurement of increase,

286 ; distribution of growth (root, 287 ; shoot, 200; leaf, 292) ; rate, 293 ;
duration, 294. Growth in thickness, 294.

3. Internal development. Evolution of cell- form, 296 (tissue tension, 297) ;

cell- wall, 297 ; cell-contents, 297.

III. Influence of External Conditions on Growth and Formation

A. The non-living environment.
General observations, 298.

1. Temperature, 209. Alteration in the rate of growth. Cardinal points of tem-

perature, 299 ; supra-maximum, 300 ; infra-minimum, 300. Alteration in
form, 301.

2. Light. Cardinal points, 302.

a> Influence of the intensity of light. Influence on the rate of growth,
303. Formative influences. Etiolation, 304; causes, 306; significance,
306 ; other results, 307 ; form and structure, 307 ; correlative influences,
307 ; colour, 308 ; juvenile forms, 308.

6. Influence of the direction of light on polarity and symmetry, 310.

c. Influence of the quality of light, 311.

3. Gravity, 312 ; mass acceleration, 313; intensity, 313; direction, 313; morpho-

genic results, 313 ; influence on growth, 314.

4. Mechanical influences, 314.

5. Influences of materials, 315; deficiency of nutrients, 315; oxygen, 316;

poisons, 316 ; chemical stimuli, 317 ; water, 317.

B. Other Organisms.

Association, 320; parasites, 321; fungus galls, 321; insect galls, 321; symbiosis, 324.

C. Parts of the same body
Correlations, 326.

a. Evidence from observation, 327.

b. Evidence from removal of parts, 327. Immediate results of wounding.

Healing of wounds, 327 ; regeneration, 328 ; different types of reparation,
329 ; polarity in regeneration, 330 ; qualitative results, 330.

c. Functional adaptation, 330.

d. Transplantation, 332 ; in normal orientation, 332 ; in inverted orientation,

332; on other species, 333; quantitative results, 334; qualitative results, 334.

e. Causes of correlations. Influences of materials, 335 ; in growth in thickness,

335 ; in regeneration, 336. Mechanical influences. Mechanical theory
of leaf arrangement, 337.
Conclusion : the facts determining plant form, 338.

SUMMARY OF CONTENTS xi

IV. The Development of the Plant under the Influence of Internal

and External Factors

CHAPTER I

Periodicity in Vegetative Life

a. Activity and rest, 341.

b. Daily periodicity, 342 ; after-effect, 343.

c. Annual periodicity, 343.

1. In longitudinal growth. Trees. Leaf -formation, 344 ; causes of the

periodicity, 345 ; root-formation, 346. Perennials, 346. Tropical plants, 347 .
Qualitative changes. Scale-leaves, 348 ; bracts, 349.

2. In growth in thickness, 350.

d. Periodicity in development as a whole, 351.

CHAPTER II
Periodicity in Reproduction

Definition of reproduction, 353 ; types of reproduction, 355.

a. Causes of reproduction in lower plants, 355 ; examples, 356 ; general results

of Klebs's researches, 357.

b. Reproduction in the higher plants, 358. Reproduction in ferns. Alternation of

generations, 358 ; apogamy and apospory, 359 ; accessory reproductive
organs, 359. Reproduction in flowering plants. Alternation of genera-
tions, 361 ; accessory reproductive organs, 362. Causes of reproduction.
Continuous vegetative growth, 362 ; causes of flower formation, 363 ; causes
of the formation of accessory reproductive organs, 365.

c. Significance of reproduction, 365 ; general significance, 365 ; significance of

fertilization, 366. Cessation of the check to development as a consequence
of fertilization, 367 ; deficiency of nuclein in the ovum, 367 ; deficiency of
chromosomes, 368 ; deficiency in kinoplasm, 369 ; parthenogenesis, 369 ;
stimuli to development, 370 ; merogeny, 371. Fertilization as amphimixis,
371. Hybrids, 372 ; (a) production, 372 ; appearance, 373 ; characters, 373 ;
(jS) second generation : segregation, 375 ; (y) its significance in estimating
the meaning of fertilization, 375.

CHAPTER III
Heredity and Variation

a. Heredity. Initials, 376.

1. Heredity in unicellular types, 376.

2. Heredity in multicellular types, 376 ; idioplasm, 377 ; idioplasm in the cell,

377 ; idioplasm in the entire plant, 379 ; somatic and germ-cells, 379 ;
disappearance and origin of initials, 380.

b. Origin of species. Theory of descent, 383 ; Darwinian theory, 384.

1. Selection. Mode of action in formation of species, 384; definition of a

species, 384 ; specific characters, 385 ; characters associated with adapta-
tion and organization, 386. Mode of action in adaptation, 386.

2. Variation. Fluctuating variations, 387. Adaptation, 389; capacity for

adaptation, 389 ; inheritance of acquired characters, 390. Mutation, 393.

xii SUMMARY OF CONTENTS

PART III
TRANSFORMATION OF ENERGY

Conservation of energy in the organism, 397.
The forms of energy in the plant :

1. Heat, 398 ; production of heat, 399 ; dependence on the state of development

and on external factors, 399 ; relation to respiration, 400 ; sources, 400 ;
significance, 401.

2. Light, 401.

3. Electricity, 402.

4. Mechanical energy, 403 ; sources, 403. Passive movements, 405 ; active

movements and their classification, 405.

I. Hygroscopic Movements

Types of the movement, 406.

a. Movements due to swelling.

1. Swelling, 406.

2. Movements due to swelling and contraction, 409 ; curvatures due to

differential swelling of parts, 410 ; due to arrangement of cells, 410 ;
due to stratification of the cell- wall, 411; due to striation in the cell-
wall, 411. Twinings, 412. Torsions, 414. Biological significance of
these movements, 414.

b. Movements due to the cohesion of imbibition water. Fern sporangia, 415 ;

anthers, 416.

II. Variation and Nutation Movements

Osmotic pressure : amount, 418 ; action on the cell- wall, 419 ; movements due to

osmotic pressure, 421.
Growth as a cause of movements, 421.
Performance of work during such movements, 421.

CHAPTER I
Ejaculatory Movements

Examples of ejaculatory movements, 422 ; tensions in single cells, 422 ; in

tissues, 424.
Significance of external shocks, 426 ; autonomous and paratonic movements, 426 ;

Catasetum, 426 ; general remarks on movements due to external stimuli, 427 ;

classification, 428.

CHAPTER II
Paratonic Movements

A. Tropisms (Directive Movements)
a. Geotropism, 429.

i. In orthotropic organs. Proof, 430; occurrence, 431. Geotropic curvature.
In the root, 431 ; in the stem, 433 (amputated stems, 435) ; after the
cessation of longitudinal growth, 435 . Mechanics of curvature, 435. Signifi-
cance of gravity. Action of the stimulus, 436 ; duration of the stimulus,
437 ; intensity (439) and direction (439) of gravity. Preliminary effects
of the stimulus. Perception and reaction, 44 1 ; hypotheses as to perception
(NOLL, 442 ; N£MEC, and HABERLANDT, 443). Protoplasmic movement,
444 ; precedent chemical phenomena, 444.

SUMMARY OF CONTENTS xiii

2. In plagiotropic organs. Radial organs, 446 ; rhizomes and lateral roots,
446 ; disposition due to internal influences, 447 ; autotropism, 448 ; branches
of trees, 449 ; disposition due to external influences, 450. Dorsi ventral
organs, 452; branches, 452; flowers, 452; leaves with nutation and
variation movements, 453. Twining plants, 455 ; revolving movements,
456; twining, 458.

b. Heliotropism. Likeness to geotropism, 460.

1. In orthotropic organs. Positive heliotropism, 460; disposition to negative

heliotropism, 462 ; significance of light intensity, 462; adaptability to light,
463. Behaviour in nature, 464.

2. In plagiotropic organs. Leaves. Growth movements, 464; variation

movements, 466 ; fixed light position, 466. Other organs, 467.

3. Precedent phenomena in stimulation. Separation of zones of perception

and reaction. Grasses, 468; Malvaceae, 470; roots, 470. Precedent
phenomena in perception, 471 ; direction or intensity of light, 471 ; liminal
intensity, 473 ; latent period, 473 ; quality of light, 474 ; immediate cause
of phototropic activity, 474. Other phenomena in the chain of stimula-
tion, 474.

c. Combined action of geotropism and heliotropism, 476.

d. Other tropisms, 478.

1. Thermotropism, 478.

2. Electrotropism and galvanotropism, 480.

3. Chemotropism, 481 ; aerotropism, 484 ; hydrotropism, 485 ; orientation with

reference to the substratum, 486.

4. Traumatotropism, 486.

5. Rheotropism, 486.

B. Nastic Curvatures (Bending Movements)

0. Haptotropism (transition from tropisms to nastic curvatures).

1. Tendrils, 487. Result of temporary contact. Tendrils which are sensitive on

one or on all sides, 490 ; more exact estimate of contact stimulus, 400 ;
other stimuli, 492; curvature, 492. Encircling of the support; produc-
tion of continuous twining, 494 ; further results, 494.

2. Leaf tendrils, climbing roots, Cuscuta, 495.

3. Drosera, 496. Character of the movement induced, 496. Stimuli and reac-

tions, 497 ; direct stimulation, 497 ; indirect stimuli, 498.

4. Heat stimuli in haptotropic organs, 499.

b. Nyctitropism.

1. Paratonic movements, 501. Growth movements; flowers which react to

changes in temperature (501) and in light (502); foliage leaves, 503.
Variation movements, 503 ; mechanics, 505 ; influence of gravity, 506.

2. Periodic movements, 508 ; movements due to after-effects, 508 ; genesis of

periodicity, 509 ; mechanics of periodic movements, 510.

c. Shock movements.

1. Mimosa, 512 ; character and significance of the movement, 5 1 3 ; preliminary

changes in the articulation, 514 ; recurrence of the capacity for reaction,
515; estimation of the shock stimulus, 516; other stimuli, 516; trans-
mission of the stimulus, 517.

2. Stamens of Cynareae, 519.

3. Other examples, 520.

C. Review of Paratonic Movements

Influence of external conditions on movements. Stimuli, 522. Releasing stimuli in
mechanisms and organisms, 522 ; perception and reaction, 522 ; formal con-
ditions, 526; general, 526; nature of their action, 527.

xiv SUMMARY OF CONTENTS

CHAPTER III
Autonomous Movements

Induced by internal stimuli, 528.

1. Autonomous variation movements, 528.

2. Autonomous growth movements, 529 ; circumnutation, 530 ; transitory and

periodic nutation, 530; hyponasty and epinasty, 530; torsions and
twinings, 531.

III. Locomotory Movements

CHAPTER I
Autonomous Locomotory Movements

Occurrence and classification, 532.

1. Natatory movements, 532.

2. Creeping movements, 534. Types of these : amoeboid movements, 534 ;

rotation and circulation, 536. Causes of the movement, 536.

3. Formal conditions, 539.

CHAPTER II
Locomotory Directive Movements (Taxis)

a. Taxis in free organisms, 541.

1. Chemotaxis, 542 ; significance, 542 ; antherozoids of ferns, 542 ; other

organisms, 544 ; strophic and apobatic reactions, 545 ; phenomena pre-
cedent to perception, 547.

2. Osmotaxis, 547 ; hydrotaxis, 548 ; rheotaxis, 548.

3. Thermotaxis, 548.

4. Phototaxis, 548 ; positive and negative reactions, 549 ; light direction or

light intensity, 549 ; apobatic phototaxis, 550.

5. Galvanotaxis, 551.

b. Taxis in protoplasm and its organs, 552.

1. Phototaxis of chloroplasts, 552.

2. Traumatotaxis of the nucleus of the cell, 553.

INDEX

555

PART L METABOLISM

LECTURE I

INTRODUCTION

Problems of Plant Physiology — Methods — Chemical composition
and structure of the plant.

THE saying of the Greek philosopher — irowra. pel — ' everything is in a state
of flux,' is in the highest degree applicable to the living organism, for continuous
changes, both physical and chemical, are the inseparable characteristics of
life, not only in the organism as a whole, but in the individual parts of which
it is composed. Among these changes may be noted first those occurring in
the non-living body ; for instance, expansion as a result of exposure to increased
temperature is not limited to the inorganic world, and the living body as well
as the dead may be temporarily or permanently altered in form by mechanical
means. Purely physical or purely chemical changes, such as these, are of
but little interest to the physiologist, while, on the other hand, those changes
which are peculiar to the living organism, the possession of which differentiates
it from non-living nature, and which it no longer shows when life has passed
away from it, are of paramount physiological importance. To inquire into
the nature of such changes and, as far as may be, to trace them to their ultimate
physical and chemical causes are the special tasks of physiology. The ulti-
mate aim of the science, in a word, is not merely to attempt the elucidation
of the several changes individually, but also to arrive at a clear comprehension
of their relation to each other and, if it be possible, to solve the problem of
the nature of life itself. Neither in its general nor in its special aspect has
this goal as yet been reached. Whether it ever will be reached is a question
to which varied and equally dogmatic answers have been given, some optimistic,
some pessimistic, yet none of them can be said to be founded on any secure
basis of evidence. The attraction which science exerts on the mind of man
lies, however, not so much in the rapid attainment of the goal aimed at as
in the actual scientific inquiry itself, and that is the reason why those who cry
' ignorabimus ' have not long ere this abandoned scientific research altogether.

On inquiry we find that the changes which are characteristic of the living
organism are as follows : —

1. Much the most easily observed are the changes in form which every
organism exhibits during its life. From minute and for the most part
simple beginnings, each organism gradually increases in size in obedience to
certain recognized laws ; it increases in complexity also, or, in other words,
undergoes development, finally giving rise once more to the rudiments of a new
organism, which in its turn passes through a similar developmental history.
To these changes we shall give the name of Metamorphosis.

2. Changes of position, or movement of the entire organism or of its
parts, are not readily observable in all forms. We shall find, however, that
all organisms do exhibit such movements, possible only in consequence of

JOST B

2 METABOLISM

the expenditure of a calculable amount of mechanical energy. Moreover, we
shall discover that organisms expend energies other than mechanical, such
for example as heat, light, and electricity. These energies must have entered
the organism in some form or another and must have undergone transforma-
tion within it. It follows that, in the organism, we meet with Transformation
of energy.

3. The third series of changes are those which are so familiar in the animal
world, viz. the absorption of materials from the external world, their trans-
formation within the organism and the excretion of certain of these transformed
bodies, that is to say, the changes summed up in the term Metabolism. In
plants, also, a similar metabolism is observable, although special expedients
are sometimes requisite to demonstrate its occurrence.

In the following pages we have to treat of transformation of form, of
energy, and of materials, and we may most conveniently commence our studies
by considering the last of these, transformation of materials or metabolism.
A discussion of the physiology of organisms in general is outside our present
task ; we will confine our attention to the physiology of plants only. At the
same time it must be borne in mind that the distinctions once believed to
exist between the physiology of plants and the physiology of animals have
become more and more obliterated, and that it has already become possible to
elaborate a general physiology ([BERNARD, Le9ons sur les phenomenes de la vie
communes aux animaux et vdgetaux. Paris, 1878-9] VERWORN, 1894).

Before we enter on the discussion of the first great division of our subject,
a few words on the methods of plant physiology will not be out of place. These
methods are identical with, or at least do not differ in any essential particular
from, those of physics and chemistry. In the first place, observation, and that
too of the most exact character, is necessary for the proper study of the changes
occurring in the organism ; but observation alone is insufficient for the accu-
rate determination of the causes of these changes. Plant life, as we shall
discover, is maintained only in the presence of a whole complex of conditions,
and it is only rarely possible to carry out a physiological observation under
conditions of such a nature that we can say with confidence that a certain
change takes place in the plant when, and only when, accompanied by a single
change in the environment ; then only can it be said that the special alteration
in the surroundings is the cause of the special phenomenon in the plant. We
have to employ the utmost care in contriving that only one of the many factors
which affect the plant is altered. Observations made under such conditions
are termed experiments. Owing to the nature of the case, physiological experi-
ments are generally restricted within narrower limits than those of physics and
chemistry. A purely physical experiment in plant physiology has for that
very reason not infrequently led to most serious error, as an example will show.
If a physicist were to fasten a wire by one end to some fixed point in a vertical
position, and attach a metallic knob to the other, he might reasonably
deduce that the wire bent over in consequence of the weight of the knob, and
his deduction would be confirmed if, on removal of the knob, the wire once
more became straight. A similar bending is seen in the peduncle of the flower-
bud of the poppy, and we might very well conclude that curvature there
also was due to gravity acting on the bud. If the bud be cut off, as in the
case of the knob in the physical experiment above described, the peduncle,
it is true, becomes straight, so we naturally conclude that the bud actually
pulls the passive peduncle downwards. VOCHTING (1882), however, has
shown that when the weight of the bud is upheld the peduncle bends all the
same, and that this curvature is maintained even when the pull from above
more than compensates the weight of the bud. A consideration of these facts
leads us to the conclusion that the weight of the bud has nothing to do with

INTRODUCTION 3

the bending of the peduncle and that if we remove the bud the lesion itself
induces the straightening of the peduncle. This experiment also teaches us
to be on our guard against introducing, during the course of a physiological
experiment, any new factor likely to produce special reactions on its own
account.

A discussion of plant metabolism presupposes a knowledge of the chemical
composition of the plant, and to this subject we must therefore first of all turn
our attention.

Qualitative analysis discloses the existence in the plant body of a relatively
small number of elements. Ignoring such as are found only in certain plants,
such as are present only when artificially supplied, such as occur only in
minute quantities, and which are obviously of no importance to the plant, there
are left thirteen elements for consideration, viz. hydrogen, oxygen, chlorine,
sulphur, nitrogen, phosphorus, silicon, carbon, potassium, sodium, calcium,
magnesium, and iron. The gain in scientific insight which such an analysis
affords us is, however, very limited.

Nor does quantitative analysis give us a much deeper insight into the
subject, although the following table, borrowed from EBERMAYER (1882, p. 47),
may prove useful as indicating the amount of carbon, hydrogen, oxygen,
nitrogen, and mineral matter present in 100 parts of plant substance, dried
at a temperature of 100° C. : —

C. H. O. N. Ash.

Wheat grains ... 46-1 5.8 43.4 2-3 2-4

Oats /o-7 6'4 36.7 2.2 4-0

Rye straw .... 49.9 5.6 40 6 o-j 3-6

Potatoes 44.0 5.8 44.7 1.5 4-0

Peas 46-5 6-2 40-0 4-2 3-1

Leaves of Red Beet . 38-1 5-1 30-8 4.5 21.;

On the other hand, information as to the chemical compounds occurring
in plants is of much greater value. The number of these is, however, so enor-
mous that it is out of the question to attempt an exhaustive enumeration of
them ; moreover, investigations as to their nature are as yet far from being
complete. We know, for example, of the existence of one or more compounds
in certain species only, while there are many which appear to be characteristic
of genera or families. The majority of these substances are by-products of
metabolism and have on that account scarcely received from physiologists the
attention they deserve. Putting on one side the inorganic compounds for the
most part absorbed from the environment, and also the organic substances above
mentioned which are of limited distribution, we have left for consideration a large
number of organic compounds common to all plants. These are compounds
of carbon with one or more of the elements, hydrogen, oxygen, nitrogen, sulphur,
and phosphorus. It will be convenient at this point to present a brief sum-
mary of the more important of these bodies, classifying them rather according
to their physiological value than their chemical constitution. It is impossible
at the present stage to enter into a discussion of their chemical characters,
although later on it may be necessary in special cases to give some details on
these points. For all general questions of chemical constitution and pecu-
liarities we must refer to the standard text-books on Chemistry and on Physio-
logical Chemistry, such as those of EBERMAYER (1882), HAMMERSTEN (1899),
and FURTH (1903). [Special reference must also be made to the recently
published treatise by CZAPEK (Biochemie der Pflanzen, Jena, 1905. 2 vols.).
It deals in a remarkably thorough manner with all the more important chemical
problems in Plant Physiology and constitutes a handbook quite indispensable
to the student of Biology.]

B 2

4 METABOLISM

We may distinguish : —

1. Organic acids. Many of these by their very names show that they were
primarily discovered in plants, e.g. oxalic, malic, tartaric, and citric acids,
although they are by no means confined to the species whence they derive
their names. The lower members of the fatty acid series are also very frequently
met with in plants, e. g. formic, acetic, propionic, and butyric acids.

2. The glycerides of the higher fatty acids are known as fats, especially
the glycerides of palmitic, stearic, and oleic acids. Suberin also is a glycerine
compound of a fatty acid (suberic acid) and may for that reason be included here.
Again the various vegetable waxes belong to the same category, for most of them
are true fats or glycerine esters, some, however, are esters of univalent alcohols
with fatty acids. Finally we may add lecithin and cholesterin, which have many
characters in common with fats, but which have a more complex composition.

3. Among the carbohydrates we may note first of all the monosaccharides,
which contain six carbon atoms (hexoses), such as glucose (dextrose), mannose,
galactose, and levulose, or only five (pentoses), such as xylose and arabinose.
The disaccharides have a larger molecular composition, and these bodies, by
taking up water, hydrolyse easily into two hexose molecules ; e. g. cane-sugar
decomposes into dextrose and levulose, milk-sugar into dextrose and galactose,
maltose into two molecules of dextrose. The largest molecule occurs in the
polysaccharides (starch, cellulose, &c.), which can be decomposed into several
hexose and even pentose molecules.

4. Amido-compounds, i.e. amido-acids and acid amides. The amido-acids
are derived from fatty acids by the substitution of NH2 for H ; e. g. aspartic
acid = amido-succinic acid ; leucin = amido-caproic acid ; alanin = amido-
propionic acid, occurring especially in conjunction with phenol to form tyrosin.
The acid-amides arise by substitution of NH2 for OH in carboxyl ; e. g. asparagin
= amido-succinic-acid-amide ; glutamin = glutaminic-acid-amide.

5. Etherial oils are familiarly recognized as the oily, volatile substances
which are the source of many vegetable perfumes. From the chemical point
of view we may distinguish (a) the terpins, simple hydrocarbons, e. g. of oil of
turpentine and the oils which occur in the Myrtaceae and Umbelliferae ; to
which group belong also caoutchouc and its relative guttapercha, the latter
differing from the former, however, in having oxygen in its composition ;
(b) oxygen-containing bodies, such as camphor and many of the oils of the
Labiatae ; (c) etherial oils containing sulphur, such as those of certain species
of Allium and of the Cruciferae.

6. The resins are related to the etherial oils, in which, as a matter of fact,
they are not infrequently dissolved ; these bodies are chemically difficult to
determine (TscniRCH, 1900).

7. The alkaloids are nitrogenous plant bases, familiar to us owing to the
fact that to them may be attributed the poisonous properties possessed by very
many plants. Their physiological significance is as yet but little known.

8. The gluco sides are readily distinguished by the ease with which they
can be decomposed into hexoses and various aromatic substances. Thus the
nitrogenous substance amygdalin, found in bitter almonds, decomposes into
glucose, oil of bitter almonds and hydrocyanic acid, the non-nitrogenous salicin
gives saligenin and glucose. Many tanning materials also are related to the
glucosides and yield by decomposition, in addition to gallic acid, a sugar or the
' aromatic sugar ' phloroglucin. These substances concern us, as physiologists,
but slightly.

9. The pigments in the plant are both chemically and physiologically
extremely varied. We need only note here chlorophyll as the most important
of them.

10. The proteids are at once the most important and also the most complex

INTRODUCTION

constituents of the plant ; they consist of carbon, hydrogen, nitrogen, oxygen,
sulphur, and possibly also phosphorus.

Only a few examples of quantitative analyses of entire plants or of their
larger parts are available. Such analyses have been made mainly in connexion
with nutritive media, and are not of special interest at the moment, since
they take into account substances belonging to too few groups. Still we may
quote in this relation a short table illustrating such an analysis taken from
KONIG (1882).

Percentage composition of fresh material.

I

II

III

Non-nitrogenous.

IV

V

VI

Water.

Nitro-
genous
material.

Fat.
Ether
extract.

Sugar. Dextrine. Starch.

Total.

Wood
fibre.

Ash.

Wheat (grain) . . 13-65

12-35

H-^2

J-75

I-7Q

(1-44) (2-38) (64-09)

67-91
67-81

2-53

2-OI

1-81
i-8r

Vidafaba . . . 14-76
Yellow Lupin
(seeds) . 12.88
Coconut * . 5-81
Potato tubers 75-48
Beetroot . . 87-71
Leek (leaves) 90-82
Lettuce (leaves) 94 33

24-27
36-52

8-88

i-95
1-09

2.10
I.4I

1-61

4-92
67-00
0-15
oai
0-44
0-31

6-53 2-73
0-81 — 3.74

49-01

27-60
12-44
20-69
9-26

4-55
2-19

7-09

14.04
4-06

o-75
0-98
1-27
o-73

3-26

4.04
1.81
0-98

°-95
o-8a
1-03

1 This estimate is taken from WIESNER, Rohstoffe des Pflanzenreiches, 2nd ed.

A few remarks may not be out of place with regard to the details of this
table (comp. KONIG, 1897). In the first column the proportion of water
present is indicated, from which it will be apparent that every part of the
plant contains water, and that even in air-dried seeds it amounts to from
12 per cent, to 15 per cent, of the original weight, while plants in the living
condition contain at least 75 per cent, of water, and usually, as a matter of
fact, considerably in excess of that amount. The maximum percentage of
water, viz. 98 per cent., occurs, as might be expected, in aquatic plants (Algae).
Similarly, the last column teaches us that from no plant are mineral matters
entirely wanting. Neither of these columns, so far as analysis is concerned,
presents any difficulty, and both are of great physiological value. The case
is quite otherwise with columns II, III, IV, and V. In order to arrive at an
estimate of the nitrogenous constituents of the plant, the nitrogen itself was
determined, and the number so obtained was multiplied by 6-25, because it
was assumed in the first place that nitrogen occurred only in proteid, and, in
the second place, that proteid contained 16 per cent, of nitrogen (N.B. 6-25 x 16 —
100 per cent.). Neither assumption, however, proves to be correct. Proteid
contains from 15 per cent, to 18-5 per cent, of nitrogen, and, further, nitrogen
occurs in amides and in other bodies as well, probably in relatively large quan-
tity. The information given in column II, therefore, is of limited value.
Column III shows how much material is soluble in ether, all of which, however,
is not fat ; it includes as well such bodies as wax, lecithin, cholesterin, hydro-
carbons, and chlorophyll. Column IV contains what is left over after sub-
tracting the sum of the contents of the other columns from 100. In it are
included by no means only carbohydrates, but everything soluble in dilute
solutions of sulphuric acid and caustic potash (1-25 per cent, solution). The
substances which resist such reagents are found in column V.

Quantitative analysis of a plant, though far more detailed and accurate
than those given above, cannot give us any satisfactory insight into its
chemical mechanism ; for it must be obvious that materials which, in the
process of analysis, become united in the distilling apparatus, occur in the

METABOLISM

m

living plant localized in definite places, and cannot, under such circumstances,
react in any way on each other. A glance through a microscope shows us
how extraordinarily complicated in structure a plant really is. Every single
organ is seen to be composed of innumerable units which we term cells, while,
on the other hand, the entire body of a microscopic alga may consist of but
one cell, similar to some one of those which occur in a higher plant. If we
collect a number of these unicellular Algae and submit them, in mass, to chemical
analysis, we shall obtain results in no respect differing from those which we
meet with in a chemical analysis of the most highly differentiated parts of plants
of much higher grade. It is manifestly of the highest importance that we
should make ourselves acquainted with the constituent parts of a cell as revealed
by the microscope, and ascertain, if possible, how the different substances
which we have classified above are distributed in the cell. For this purpose we
must employ not only the ordinary methods of chemical analysis, but also the
so-called ' microchemical reactions ', on whose further
development the extension of our knowledge in this
direction so largely depends. Even now, however, it is
possible to determine in situ under the microscope
a considerable number of chemical compounds. To
enumerate all the microchemical reagents and their
reactions would occupy far too much space ; we must,
therefore, content ourselves with a resume of the most
important results arrived at ; in doing so, however, it
will be impossible to avoid trenching on certain ques-
tions which are morphological rather than chemical.

We may select as a subject for study a cell of the
freshwater alga, Dmparnaldia glomerata, illustrated at
Fig. i. This cell is cylindrical in form, and in it may
be distinguished three primary constituents : (i) the
cell-wall (m), which forms a hollow cylinder enclosing
cell contents ; (2) a soft, viscid substance, the protoplasm
(pi), covering and closely applied to the inner surface
of the cell-wall, and forming, like it, a closed sac ; (3) the
cell-sap (the vacuole, v), occupying the remainder of the
space. Although no further structural differentiations
are obvious in the cell-wall or cell-sap, these are by no
means absent from the protoplasm. In the first place,
we notice an annular green band with ragged edges,
the chloroplast (ch), a spherical body known as the
nucleus (n), and finally the cytoplasm, i.e. the remainder
of the protoplasm, a colourless, translucent mass, whose exact nature it is
extremely difficult to determine, and in which the chloroplast and the nucleus
lie imbedded.

Structures of similar or nearly similar character are met with in the majority
of vegetable cells, the difference lying for the most part in the form of the
chloroplast. It is only rarely that that body has the characters seen in Dra-
parnaldia, it is generally much simpler in structure ; each cell may contain
many chloroplasts, or these may be wanting altogether. The parts of the cell
above enumerated are of very different value, for the functions fulfilled by the
protoplasm are of much greater moment than those carried out by the cell-wall
and cell-sap, the latter being really products of the former. Indeed, cells
are not unknown which consist either for a time or during their entire life
of protoplasm only. Protoplasm may be briefly defined as the living substance
of the plant (and of the animal also), for it is only in such parts as contain
protoplasm that we encounter those changes which we recognize as characteristic

Fig. i. Cell of Drapar-
naldia glomerata. m^ cell-
wall ; //, protoplasm ; ch,
chloroplast ; «, nucleus ; z>,
vacuole. (Magnified about
500.)

INTRODUCTION 7

of life. Obviously, then, a knowledge of the chemical nature of protoplasm
will be of primary importance to us, but first of all, we may take note of some
of the more important points in the chemistry of the cell-wall and cell-sap.

To start with, the cell-wall is obviously not a chemical entity, for in addition
to the carbohydrates, of which it is in the main composed, it contains mineral
matters and water as well. The carbohydrates which enter into its composition
are polysaccharides belonging to the cellulose group. It is rarely the case
that the cell-wall is composed of a single chemical compound ; far more com-
monly many closely related bodies enter into its composition. The water,
which is always present, is not, however, located in definite spaces in the wall,
but occurs as 'water of imbibition', that is to say, in a state of minute sub-
division between or within the ultimate molecules of the substance of the
cell-wall itself. This is not the appropriate place to enter into the study of
the subject of imbibition as a whole (Lecture XXXII), but we may note the
following points of interest. In the first place, no definite and stable chemical
union exists between the imbibed water and the substance of the cell-wall,
since it is possible to squeeze the water out, at least in part, by mere mechanical
pressure, or to permit it to evaporate into the atmosphere. What is still retained
in the wall after pressure or evaporation may be driven off by heat. If the
desiccated cell-wall be once more brought into contact with water, it absorbs
it again with considerable force, and in quantity dependent directly on tempera-
ture. Simultaneously with the absorption of the water the volume of the wall
is increased, and an important physical alteration takes place also in the swollen
mass. Just as a piece of glue, hard and brittle as it is in the dry condition,
becomes soft and pliable when wet, the cell-wall alters in character when
similarly treated. For example, a fresh stem of Cobaea scandens (a well-known
climber) can be twisted round one's finger like a piece of string, but if dried
it becomes as brittle as glass. The alteration effected in the character of the
cell-wall by the imbibition of water cannot be other than of the greatest
significance in plant economy.

The mineral matters occurring in the wall may be in part dissolved in the
water of imbibition, but most of them are distributed in the solid form in
a minute state of subdivision among the particles of carbohydrate.

The cell-sap consists for the most part of water, in which are always
dissolved a large number of organic and inorganic compounds, while there
are also present in addition solid particles in suspension, resulting from the
precipitation of substances normally soluble in the sap.

Microscopic investigation of the protoplasm reveals the presence of a hyaline
ground substance, in which are imbedded the special organs of the protoplasm
already referred to under the names of chloroplast and nucleus, as well as
a number of granules and vesicles (microsomata), some of known, some of
unknown composition. The varied streaming movements exhibited by this
ground substance, or hyaloplasm, shows that it contains much water ; indeed, it
is easy to demonstrate directly its presence in the protoplasm. We may further
assume that the relationship of the water and the protoplasm is similar in
character to the relation subsisting between the water and the cell-wall ; in
other words, the protoplasm may be regarded as a swollen body. So far then
as the hyaloplasm itself is concerned, we may consider it, when saturated
with water, as a complex of organic nitrogen- and sulphur-containing bodies,
or proteids, and looked at from this point of view it has often been the custom
to regard protoplasm as proteid in solution. Because the cell-sap may also
contain dissolved proteid, and because this proteid when removed from the
plant no longer exhibits that vitality which makes protoplasm so interesting,
a distinction was made between dead and living proteid, and the name proto-
plasm was confined to the latter. The chemical investigation of as pure

8 METABOLISM

a sample of protoplasm as could be obtained (such as that carried out by
REINKE and RODEWALD, 1881-3), was therefore of the greatest value. These
investigators selected for the purpose the plasmodium of one of the Myxomy-
cetes, viz. a mass of naked protoplasm, unenclosed by a cell-wall. Three quarters
of this plasmodium was found to consist of water. The substance, when air-
dried, gave on analysis about 5 per cent, of water and 28 per cent, of calcium
carbonate. Since neither of these bodies can be considered as the specific
medium of vital manifestations we may neglect them altogether. In addition,
the air-dried plasmodium contained a large number of chemical compounds
which could be estimated only approximately, and which did not lend them-
selves to accurate analytical determination. These estimates are summarized
in the following table (REINKE, 1901. 232) : —

1. Proteids containing phosphorus (plastin and nuclein) about 40-0

2. Proteids not containing phosphorus ....

3. Amide bodies .

4- Fats

5. Lecithin . . .

6. Cholesterin

7. Carbohydrates . .

8. Resin „

9. Salts of organic and inorganic acids „
10. Undetermined or not specified bodies ,,

100-0%

We have already drawn attention to the fact that a series of metabolic
processes takes place in the plant, consisting in the absorption of materials
from the environment, the alteration of these in the plant, and an excretion of
certain substances which have become of no further use ; to this we must
now add a further statement, viz. that these metabolic processes are primarily
associated with the protoplasm. Chemical analysis of the protoplasm of the
Myxomycetes does not give us any indication which of the compounds so
determined form part of the special living protoplasm and which are to be con-
sidered as products of metabolism. REINKE expressly states that the plas-
modia which he investigated were in the act of forming fructifications, and
therefore were not likely to contain any unaltered nutritive materials ab-
sorbed from the environment. Since, however, the proteid, fat, and carbo-
hydrate, known to occur in large quantities in the seeds of the higher plants,
cannot be considered as genuine protoplasm, but rather as dead material
deposited in these situations as plasta for the construction of the seedling,
we are bound to regard the greater part, it may be, of the materials determined
by REINKE as belonging to the category of so-called reserve materials. Again,
it would be quite an arbitrary proceeding to designate any of the substances
mentioned in the table quoted above (such as, e.g., the dominant proteids which
contain phosphorus) as the essential, still less as the only constituents of
protoplasm itself. It is possible that protoplasm, the true living substance,
consists essentially of a mixture of many materials ; there can, however, be
only one substance with vital characteristics, and there is considerable
evidence forthcoming, as we shall discover later on, tending to show that it
is not unlikely that this substance exists only in relatively small quantity.

A further problem now confronts us. Is life bound up with one definite
substance, which we may term ' the life-bearer ', or does life arise from some
special arrangement of lifeless bodies ? It has often been the habit to liken the
organism to a machine, and analogies between them are not wanting. But
the work of a machine depends, in the first instance, not on the chemical nature
of the substance of which its parts are composed, but on their mode of con-
struction and on their arrangement. Whether we make a machine of brass

INTRODUCTION 9

or of steel may well affect the durability and precision of the instrument, but
not the nature of its action. Similarly one cannot help assuming that the
mode of arrangement of the ultimate parts of the organism is of greater im-
portance than the chemical nature of these parts.

ERNST BRUCKE (1861) deserves the credit of having been the first to lay
stress on the more minute structure of protoplasm as the cause of its vital
manifestations. He wrote in 1861 (p. 386), ' we are well acquainted with the
fact that the structure of the molecules of the organic substances which enter
into the composition of the cell is undoubtedly of a very complicated nature.
But we cannot rest content with simply postulating a complicated molecular
structure for the cell. It is impossible to think of a living vegetative cell as
consisting of merely a homogeneous nucleus . . . and a proteid solution, for we
certainly cannot perceive the phenomena which we term vital in the proteid
as such. On the contrary we are forced to ascribe to the living cell an entirely
distinct complexity of structure, quite apart from the molecular structure
of the organic compounds of which it is composed, and to this complexity
we may apply the term * organization'.

This organization demands special study, and for its investigation the
newest and strongest objectives must be employed. As, however, A. FISCHER
(1899) has shown, investigations of this kind are certainly open to the possi-
bility of serious error, because those who aim at conducting research on proto-
plasm are compelled, from the nature of the case, to operate upon dead proto-
plasm ; since although great care be taken to preserve the natural structure in
killing and fixing the material, still very frequently artificially produced pre-
cipitates have been mistaken for genuine features of the protoplasm.

According to many observers, protoplasm is composed of minute granules,
according to others it is made up of threads or netlike aggregations of fibrillae,
but all these theories of the structure of protoplasm are subject to the criticism
that they are based on an examination of dead and stained protoplasm and
not of the living substance. BUTSCHLI' s (1892) alveolar theory of the structure
of protoplasm is free from this criticism, for undoubtedly there are many cases
where a structure may be seen in living protoplasm suggestive of that exhibited
by frothy liquids. [According to FISCHER (Roux's Archiv f. Entwicklungs-
mechanik, Vol. 13, 1902) and especially A. DEGEN (Bot. Ztg. Vol. 63, 163,
1905), a foam-like structure in protoplasm is to be considered as a pathological
phenomenon.] But whether only the walls of the honeycomb-like spaces, lying
as they do at the limits of visibility, or their contents as well are to be con-
sidered as protoplasm it is impossible to say. It is equally certain that in
other cases no such alveolar structure can be determined in living protoplasm,
and thus we are forced to accept the views of BERTHOLD (1886) and FISCHER
(1899) as most in accordance with our present knowledge, viz. that the con-
stitution of protoplasm is neither uniform nor constant. Even if an alveolar
structure, in the sense understood by BUTSCHLI, could be demonstrated as of
general occurrence in protoplasm, we should not gain much thereby, for
BUTSCHLI (1898) himself has shown that an alveolar structure may also be
demonstrated in dead bodies and so cannot be regarded as a definite character-
istic of living structure.

Up to the present, then, the organization of protoplasm has not been un-
ravelled, although the investigations which have been carried out in this relation
have done much to render clearer our conception of what protoplasm really is.
No one nowadays doubts that it has a very complex structure, and we know
that it cannot any longer be considered as a homogeneous solution. Further,
no one expects to meet with the same or even an analogous structure in proto-
plasm as in a machine, the comparison of protoplasm with which must mani-
festly not be pressed too far. Owing to the manifold nature of the work

io METABOLISM

accomplished, and to the predominance of the chemical over the mechanical
processes taking place in protoplasm, it is perhaps on the whole more appro-
priate to institute a comparison between protoplasm and a chemical laboratory
or manufactory. In such a manufactory many chemical operations are carried
out, possibly in the same room ; but many of these operations must be kept
entirely apart if the wished-for result is to be attained. So also within a single
cell both oxidation and reduction, anabolism and katabolism of protoplasm
may take place, and for this reason alone organization in the protoplasmic
body is essential, since conflicting operations must be kept isolated. F. HOF-
MEISTER (1901) has drawn attention to the fact that, looked at from this point
of view, an alveolar structure in the protoplasm may have a deep significance.
Every one of the countless cavities might be considered as a chamber cut off
temporarily or permanently from the exterior. Within the space of a cubic
/i the most heterogeneous reactions may take place. The aim of future
research must be, without over emphasizing either the material of which proto-
plasm is composed or its organization, to take both of these phenomena into
consideration, and to employ them as bases for wider research in this most
difficult subject. At the same time it may be expressly noted that purely
chemical research into the nature of protoplasm is by no means valueless,
and that it has not yet been shown that chemical peculiarities play no part in
protoplasmic phenomena. It is obvious that we have not as yet discovered
any living chemical compound or any mixture of chemical compounds which
may be considered as vitally active, since the physiological chemist must of
necessity kill the very living substance he desires to study in subjecting it to
analysis. Experiments in physiological chemistry have shown sufficiently that
in proteids we are dealing with substances not only exceedingly complex but
also very delicate and labile, substances which may become continually altered
owing to apparently quite trifling causes. One can scarcely doubt that the
transformations which are exhibited by protoplasm, e. g. through the influence
of gentle pressure, and which result in local or general death, depend not merely
on changes in organization but also on chemical changes which are irreparable.
Thus chemical alterations are set up if water enters a protoplasmic particle
previously surrounded by salt solution, and these alterations may be expected
to be even more pronounced when the contents of two alveoli or two vacuoles,
originally separate, become united. REINKE (1901) says that ' if one gently
bruises in a mortar some pure protoplasmic rudiments of fruits of Mycetozoa,
the substance remains unaltered quantitatively and, to all appearance, also
chemically, but its organization is irretrievably destroyed, so that subsequent
differentiation of fruits is completely inhibited ; in other words, the protoplasm
is killed by the pestle without chemical action of any sort'. This view we
cannot agree with, for although the rubbing in the mortar may not, it is true,
have affected the substances enumerated in the analysis given above, still the
albumins and phosphorus-containing proteids must have been greatly modified.

The data we have obtained as to the chemical composition and structure
of protoplasm are applicable not only to protoplasm as a whole, but individually
also to cytoplasm, nucleoplasm, and chloroplasts. Since the nucleus and chroma-
tophores are living parts of the plant they may be considered as organs of the
protoplasm. From the chemical standpoint there are many agreements as well
as many differences between the cytoplasm and the nucleoplasm, but we need
not follow out these in further detail at this stage, since they have not as yet
been shown to have any general physiological significance. For the same
reason we may omit here any reference to the results obtained from investi-
gations into the chemistry of the proteids. It is to be hoped, however, that
these results may soon prove to be of greater physiological value than heretofore.

To summarize, therefore, it may be said that our first attempt to obtain

INTRODUCTION n

an insight into the chemical architecture of the plant has brought us face to face
with questions of the most difficult character, questions which we must leave
without being able to give satisfactory answers to them. At the same time,
chemical investigation of a selected plant teaches us much that is of importance.
In the first place, we find the plant to be composed of elements all of which are
found in its surroundings, the soil, water, and air ; and, in the second place,
we see that these elements are grouped in the plant into more complex com-
pounds than occur in the inorganic environment. We have also seen that the
plant takes up its raw materials from this environment, and changes them
in its interior. We must now endeavour to make ourselves acquainted more
in detail with the absorption of these raw materials.

Bibliography to Lecture I.

BERTHOLD, G. 1886. Studien iiber Protoplasmamechanik. Leipzig.

BRUCKE, E. 1861. Die Elementarorganismen. Sitzber. Wien. Akad., Mat.-nat.

Kl. 44, Abt. ii, p. 381 (Ostwald's Klassiker, Nr. 95, Leipzig 1898).
BUTSCHLI. 1882. Unters. uber die mikrosk. Schaume u. d. Protoplasma. Leipzig.
BUTSCHLI. 1898. Unters. iiber Strukturen, etc. Leipzig.
EBERMAYER. 1882. Physiologische Chemie d. Pflanzen, I. Berlin.
FISCHER, A. 1899. Fixierung, Farbung und Bau des Protoplasmas. Jena.
FURTH, R. v. 1903. Vgl. chemische Physiologic d. niederen Tiere. Jena.
HAMMARSTEN. 1899. Lehrbuch der physiologischen Chemie, 4th ed. Wiesbaden.
HOFMEISTER, F. 190 1. Die chemische Organisation der Zelle. Braunschweig.
KONIG. 1882. Zusammensetzung der menschlichen Nahrungs- und Genussmittel,

2nd ed., Vol. i. Berlin 1882.

KONIG. 1887. Landwirtsch. Versuchsstationen, 48, 8 1.
REINKE and RODEWALD. 1881. Unters. aus d. bot. Lab. Gottingen, 2, i.
REINKE. 1881. Ibid. 2, 79.
REINKE. 1883. Ibid. 3, i.

REINKE. 1901. Einleitung in die theoretische Biologic. Berlin.
TSCHIRCH. 1900. Die Harze und die Harzbehalter. Berlin.
VERWORN, M. 1894. Allgemeine Physiologic, 3rd ed. Jena 1901.
VOCHTING, H. 1882. Die Bewegungen der Bluten und Fruchte. Bonn.

LECTURE II

THE OSMOTIC CHARACTERS OF THE CELL

THE mode of absorption of nutritive materials may be best studied by
a consideration of the process as it occurs in a single cell. If the cell be naked,
as it is, for example, in the Myxomycetes, the protoplasm can flow round and
absorb solid bodies ; but in the great majority of cases the existence of a rigid
cell-wall renders the absorption of solids impossible and compels the plant to
be dependent on fluid nutriment, and absorption of water and of substances
dissolved in it is almost the only method available in nature by which the
plant can obtain its food supply. That water can penetrate both cell-wall
and protoplasm is unquestionable, since, as we have already seen, both are
capable of swelling in water ; but whether the substances dissolved in the
water are also capable of entering is quite another question. As a matter of
fact, we shall find that by no means all substances soluble in water are able
to enter the cell. When we inquire whether this selective absorption depends
on the nature of the cell-wall or of the protoplasm we find ourselves compelled
to study the physical laws of diffusion and osmosis, for it is in this category
of phenomena that we must include the processes concerned in the absorption
of nutritive material by the cell.

It is well known that when two miscible liquids or solutions, e. g. alcohol
and water, or water and an aqueous solution of copper sulphate, are care-

12 METABOLISM

fully poured consecutively into a glass cylinder, they will remain at first
distinct from each other. Sooner or later, however, the sharp boundary be-
tween them vanishes, as the copper sulphate molecules begin to wander in
amongst the water molecules and vice versa. This phenomenon is known as
diffusion, and it continues until the intermixture of the two fluids is complete,
and until the whole becomes uniform in character and in concentration. Sup-
pose, however, that the two fluids have no free surfaces but are separated by
a porous wall, then the phenomenon takes place under altered conditions, and
is known as osmosis. Let us assume that the fluids are water and an aqueous
solution of copper sulphate, and, further, that these fluids are placed in the
legs of a U-tube, separated by an intervening wall of clay (s, Fig. 2), animal
bladder, or parchment ; it will be found that the two fluids do not pass through
the dividing wall with equal rapidity ; the water, as a matter of fact, passes
far more rapidly into the copper sulphate than the copper sulphate solution
into the water. The result is obviously an increase of fluid on the copper
sulphate side, the level of the fluid on that side rising in proportion as it falls
on the other. Similar results are obtained when copper sulphate is replaced
by selected salts or by alcohol. That the result is essentially dependent on
the nature of the separating wall is shown by the fact that when a thin
caoutchouc membrane is introduced between alcohol and water, more alcohol
passes through the membrane into the water than water into the alcohol.
In every case, however, so long as the separating membrane is permeable to
both bodies, a complete intermixture in the long run always takes place,
resulting in the same concentration of fluid on both sides of
the dividing wall.

The cell-wall comports itself precisely like the parchment
or animal bladder in this experiment, presenting less resistance
to the passage of water than of salt solutions. In course of
time the salts also manage to pass through, and it is only to
chemical compounds with large molecules, like gum, proteid,
&c., that the wall is quite, or almost quite, impermeable.

Protoplasm in its osmotic characters differs from the
Fi 2" cell-wall and agrees with the so-called semipermeable mem-

brane in one very important point, viz. that it is quite
impermeable to certain materials, such as many salts, sugar, &c., although
it is readily permeable to water. If one places such a semipermeable
membrane in the position of the dividing wall in the experiment just de-
scribed (Fig. 2) the water passes into the copper sulphate, but no trace of the
copper sulphate passes into the water. Under these conditions a uniform
mixture of the two fluids, resulting in similar concentration on either
side of the membrane, cannot take place ; one substance must always be in
excess on one side of the dividing wall. As an example of a semipermeable
membrane we may take -the so-called ' precipitation membranes ' produced
by the contact of aqueous solutions of ferrocyanide of potassium and copper
sulphate or of tannin and gelatine. The precipitation membranes thus ob-
tained, gelatine-tannate or copper-ferrocyanide, do not form suitable experi-
mental materials, nor can they be introduced conveniently into the U-tube
shown at Fig. 2. Even were their introduction possible, these membranes
are so fragile that they would be quite unsuitable for the purpose. For those
reasons PFEFFER (1877), in his fundamental researches on this subject, precipi-
tated the copper-ferrocyanide membrane in or on the wall of a porous pot,
such as is used in a Daniell's element. The precipitation membrane is thus
supported, and the apparatus takes the form represented at Fig. 3. If the
porous pot be filled with a 10 per cent, solution of cane sugar to which the
copper-ferrocyanide membrane is impermeable, and if the pot be then placed

THE OSMOTIC CHARACTERS OF THE CELL

in water, the latter at once begins to enter through the wall, and the diluted
sugar solution begins to rise in the tube (R). If the U-tube be now filled with
mercury (Qu) it will be found that the water enters with a force sufficient to
raise and support a column of mercury of considerable height. This pressure is
usually explained by postulating an attraction between the molecules of sugar
in the porous pot and the molecules of water outside. The attraction, however,
affects the water only, for the sugar does not pass out on account of the pre-
cipitation membrane being impermeable to that substance. The amount of the
attractive force may be estimated, starting from the position of rest, by
measuring the height of the mercury column which is supported by the inflow-
ing water.

PFEFFER'S osmotic cell may be legitimately compared with a vegetable
cell, especially if the precipitation membrane be laid down on the inside of the
porous pot, as shown in Fig. 3, as is possible for many experiments without
detriment to the rigidity of the membrane. The cell-sap (which itself
may contain cane sugar) would correspond to the contents of the pot, the
protoplasm to the copper-ferrocyanide membrane and the cell-wall to the
wall of the pot. If the cell of an alga (Fig. i) be placed in water, the water
streams through the cell-wall and protoplasm into the vacuole, and if it were
possible to attach a graduated manometer to this cell, the pressure exerted
on the inside of the cell-wall might be estimated. That such a pressure actually
exists may be easily demonstrated otherwise. The pressure of the inflowing
water induces tension in the elastic cell-wall, and if
this tension be relieved, e.g. by piercing the wall, a dis-
tinct contraction of the cell may, in many cases, be
observed. This internal pressure is termed osmotic or
turgor pressure, and by it the protoplasm is pressed
firmly against the membrane ; without such a resis-
tant layer the protoplasm would have been as little
able to resist this pressure as the copper-ferrocyanide
membrane would without the supporting clay cell.

Since protoplasm, however, differs widely in con-
sistence from a precipitation membrane made of
gelatine-tannate or copper-ferrocyanide, and may be
likened rather to a fluid than to a solid, it is im-
portant to note that genuine fluids also show the
phenomenon of semipermeability. Water, for example,
is permeable to ether, but not to benzol, and if a
membrane be saturated with water and abuts on one
side on pure ether, and on the other on benzol, all the
conditions are fulfilled for the establishment of an excessive osmotic pressure
on the benzol side (NERNST, 1890).

If we next ask ourselves what substances the protoplasm is permeable to
and what not, two methods are open to us, either to determine those capable
of diffusing outwards from the cell-sap to the exterior (exosmosis), or inwards
from the environment into the vacuole (endosmosis). Our knowledge of the
contents of the vacuole is, at least in some cases, sufficiently extensive to
enable us to determine that in the long run exosmosis will take place from it.
We know, for example, that the cells of sugar-beet are unusually rich in cane
sugar, a substance which may, by chemical methods, be detected even in
minute quantities. When thick slices of sugar-beet, thoroughly washed to
remove all traces of free sugar released from the cut cells, were laid in water,
DE VRIES (1877) found that even after fourteen days no sugar had passed from
the uninjured cells into the water. Again, if this experiment be repeated,
using red beet in place of white, we find that the protoplasm is as impermeable

Fig. 3. PFEFFER'S osmotic
cell. Tt porous pot; N, pre-
cipitation membrane; /?, mano-
meter containing mercury, Qu ;
Z, sugar solution.

METABOLISM

to the colouring matter as to the cane sugar. In certain cells, however (nec-
taries, Lecture V ; roothairs, Lecture VIII ; endosperm cells, Lectures XIII,
XIV), the protoplasm is found to be permeable to various substances, such as
sugar, asparagin, peptone and proteid. In other cases we must admit the fact
of exosmosis without being able to say definitely what the substances are which
undergo exosmosis. Take, for example, Bacteria, which require organic com-
pounds for their nourishment ; if these organisms are able to thrive at the
expense of green Algae it is manifest that exosmosis of organic materials must
take place from the cells of the Algae. Again, the seeds of Lathraea and
Orobanche germinate only in the presence of a host plant, and this fact cannot
be explained otherwise than by assuming that an exosmosis of organic com-
pounds takes place from the host. But in general it may be said, as shown by
DE VRIES in the case of beet, that no exosmosis of the contents of the vacuole
takes place.

A comprehensive knowledge of the phenomena connected with the per-
meability of protoplasm cannot, however, be obtained by a study of exosmosis
only, because we are limited to the accidental occurrence of substances in the
vacuole, our acquaintance with which, moreover, is only rarely exact. In
studying endosmosis, on the other hand, we are unlimited in our choice of the
materials which we may present to the cell ; whether we obtain results or not
will depend solely on our ability to establish a definite criterion for absorption or
non-absorption. Such a criterion of protoplasmic impermeability, indeed the

Fig. 4. Young cells from the cortical parenchyma of the peduncle of Cephalaria leucantha. m, cell-wall ;
pi, protoplasm ; v, vacuole. /, in water ; II, in a 4% solution of potassium nitrate ; HI, in a 6% solution of the
same ; IV, in a 10% solution of the same. (After DE VRIES, 1877.)

best one available, is plasmolysis, a phenomenon first enunciated by NAGELI
(1855), and afterwards so thoroughly worked out by DE VRIES (1877) and
PFEFFER (1877), that at the present day it is perhaps not only amongst the most
fully investigated phenomena in plant physiology, but, far beyond the special
branch of science concerned, has become a subject of the greatest interest to
students of general chemistry.

Suppose we revert to our alga cell and assume that we know the actual
contents of the fluid in the vacuole, that it contains a 10 per cent, solution of
cane sugar ; what happens when this cell is placed in pure water we have
already seen, but what will happen if we place such a cell in a solution of cane
sugar which has, let us say, a lower concentration than that presented by the
cell-sap ? From such a solution the vacuole will obviously withdraw less fluid
than from pure water, less and less in fact as the fluid without becomes more
and more concentrated. When the concentration of the fluid outside is equal
to that of the fluid in the vacuole, no water will be taken up at all from the
surroundings ; when, on the other hand, the fluid outside has a higher concen-

THE OSMOTIC CHARACTERS OF THE CELL 15

tration than the contents of the vacuole, water will be withdrawn from the vacuole,
which in consequence will become smaller. At this point the behaviour of
the cell membrane differs from that of the protoplasmic lining, for the pro-
toplasm is capable of contracting proportionally to the decrease in size of the
vacuole, while the rigid cell-wall, on the other hand, can contract only to
the extent of regaining the size it possessed before being elastically extended
by the osmotic pressure. When the tension of the cell-wall is relaxed
(Fig. 4, //) no further decrease in size is possible, and a state of plasmolysis
ensues, where the protoplasm becomes separated from the cell- wall. This
separation appears first at the angles of the cell (Fig. 4, ///) and proceeds
continuously therefrom, until the whole of the protoplasm has assumed the
form of an elliptical or spherical mass lying free in the interior of the cell
(Fig. 4, IV). If the solution employed for bringing about plasmolysis be
coloured by a suitable dye, e. g. indigo -carmine or aniline-blue, it will be seen
that the coloured fluid penetrates the cell-wall and fills the space between the
wall and protoplasm. Further, this experiment demonstrates the fact that
the membrane is permeable to the dye, but that the protoplasm is not. If we
once more immerse the plasmolysed cell in water, plasmolysis disappears and
the cell regains its original size and form, without apparently having suffered
any injury. On the death of the protoplasm, e. g. as a result of a sufficiently
high temperature, its diosmotic characters are completely altered ; dead proto-
plasm presents no impediment to the free passage of colouring matters,
salts, &c.

By employing the plasmolytic method of investigation we are able readily
to determine that protoplasm is impermeable to a large number of substances
even though these be soluble in water. If only the proper degree of concen-
tration be ascertained, plasmolysis can be affected by cane sugar as well as by
grape sugar, by common salt as well as by potassium nitrate, but the exact
concentration must be determined by experiment. With the view of ascer-
taining the relative plasmolytic or osmotic activities of different substances
it is necessary to determine the precise concentration of each which will bring
about the first trace of plasmatic retraction from the wall (Fig. 4, ///, lower
part of the cell). The degree of concentration which induces this change may
be considered as having a slightly higher osmotic value than that of the cell
contents, while a concentration exactly similar to that of the cell contents will
obviously produce no reaction at all. Thirty years ago, by the plasmolytic
method, DE VRIES studied empirically the degrees of concentration of a variety
of substances which were capable of giving equivalent osmotic effects (briefly
termed isosmotic concentrations}. The material he employed was red beet,
and the results he obtained were as follows : —

1. Cane sugar . * .. 27-28% 5. Sodium nitrate

2. Magnesium sulphate . . 26-28% 6. Potassium chloride

3. Sodium sulphate . . 17-18% 7. Sodium chloride .

4. Potassium nitrate . . 6-7%

At first sight these numbers appear to follow no law, but in 1884, DE VRIES,
after the examination of a very large number of variously constituted sub-
stances, successfully established a definite relation between the different isos-
motic solutions. He was able to show, as indeed was only to be expected, that
the osmotic effect depends not on the specific gravity of the substance but on the
number of the molecules dissolved. Now if substances be dissolved in quantities
proportional to their molecular weights, similar numbers of molecules will be
obtained in each solution. When as many grams are dissolved in a litre of water
as is indicated by the molecular weight of the substance we speak of this unit as
a gram-molecule to the litre (briefly ' G.M.'). Thus to obtain a G.M. of cane sugar,

16 METABOLISM

342 g. must be used ; a G.M. of KC1 requires 74 g., a G.M. of NaCl requires 58 g.,
per litre in each case. DE VRIES also discovered that a G.M. of invert sugar,
cane sugar, malic acid, tartaric acid, citric acid, &c. — in short, a G.M. of every
non-metallic organic substance soluble in water had exactly the same osmotic
value, whence we may deduce the law that equimolecular solutions are at the same
time isosmotic. It is worthy of note that the plasmolytic method may also be
employed in chemistry for the determination of molecular weights, as DE VRIES
(1888 b) has shown in the case of raffinose. Various formulae have been put
forward by different chemists for this sugar, each giving a different molecular
weight : —

1. CttH,, Olt+ 3H20 ; M.W. = 396

2. C18H32 016+ 5HaO; M.W. = 594

3. C36H64 O33+ ioH2O ; M.W. = 1188.

If now the same material be treated with equivalent plasmolytic solutions
of raffinose and cane sugar, we find that a 3-42 per cent, solution of cane sugar
is isosmotic and equimolecular with a 5-957 per cent, solution of raffinose.
But a 3-42 per cent, solution of cane sugar is the equivalent of o-oi G.M. to
the litre, therefore a 5-957 per cent, solution of raffinose must have a similar
G.M., and its molecular weight must be 595-7, a number which strikingly
resembles the second of the formulae quoted above. The correctness of this
conclusion has been confirmed lately by other methods.

What is true of organic non-metallic substances is, however, far from being
so of all compounds. We might expect a solution of 101 g. of potassium
nitrate in 1000 g. of water to give exactly the same osmotic effect as 342 g. of
cane sugar; in fact, it has almost the same effect as 1-5 G.M. of cane sugar, in
other words, one and a half times as much as one would expect. DE VRIES (1884)
has, moreover, shown that in the case of many substances one may obtain one
and a half, two, or two and a half times the value of an equimolecular sugar
solution. He assumed, to use round numbers, the value of a known sugar
solution to be 2, and found that other substances gave, more or less exactly, the
numbers 2, 3, 4, and 5. These numbers may be termed, following PFEFFER'S
nomenclature, isosmotic co-efficients, for they indicate how much greater the
osmotic value of the substance is as compared with sugar = 2, and a knowledge
of such co-efficients is of the greatest value in all plasmolytic research.

If we further bear in mind that these isosmotic co-efficients apply only to
dilute solutions, the fact that a solution of, e. g., potassium nitrate has a value
one and a half times as great as what we would expect from its molecular weight
becomes at once clearly explicable. From a consideration of countless instances,
modern chemistry has shown that in dilute solutions molecules in part dis-
sociate into their constituent ' ions ', each free ion having the same osmotic
value as the entire molecule, i. e. it attracts water with the same force as the
molecule does. The degree of dissociation depends on the one hand on the
degree of dilution of the solution, and on the other on the nature of the dis-
solved substance. In the end all the molecules may become dissociated, and
these isosmotic co-efficients give, not, it is true, an exact, but still an ap-
proximate, indication of the amount of dissociation which has taken place ;
their significance is purely practical and they are useful on the whole if we
desire to calculate the osmotic value of a definite solution.

We have spoken above of a force with which the dissolved substance
and the medium, water, attract each other. This conception was until re-
cently generally accepted ; but nowadays, owing to the advance in general
chemistry, it has come to be considered an old-fashioned. Osmotic pressures
developed in cells are now explained by reference to certain characteristics
exhibited by all bodies in solution. According to VAN'T HOFF'S theory, one
must ascribe to substances in solution the same characteristics as are ascribed

THE OSMOTIC CHARACTERS OF THE CELL 17

to gases, and since the kinetic theory of solutions and of osmotic pressure is
based experimentally on PFEFFER' s researches in vegetable physiology, it will
be appropriate to consider this theory briefly here.

In the course of his investigations on osmosis, PFEFFER, in 1877, demon-
strated that the high osmotic pressures whose existence in certain vegetable
cells had been proved by him a short time previously, were due to the presence
of crystallizable bodies, such as cane sugar, potassium nitrate, and other sub-
stances with relatively small molecules, although, until that date, physicists
had held that high osmotic pressures could be attained only by colloids, such
as proteids, gums, &c., with large molecules. Previously to Pfeffer's experiments
only such materials as parchment and animal bladder had been used as separa-
ting membranes in the osmometer, and with colloids these gave higher osmotic
pressures than crystalloids, the reason being that the crystalloid diffused with
great readiness. PFEFFER showed that by using semipermeable precipitation
membranes instead of such materials, high pressures could easily be obtained
with the aid of crystalloids. The artificial eel employed by PFEFFER was
a porous pot in the wall of which a copper-ferrocyanide membrane had been laid
down, as described above, and using sugar solutions of different concentrations he
obtained the following pressures as indicated by heights in centimetres of
mercury : —

Sugar in weight %. Height in cm. due to pressure.

1 53-8

2 IOI-6

4 208-2

6 307-5

i 53-5

We may note, in the first place, that the osmotic activity which brings
about the pressure is almost exactly proportional to the degfee of concentration.
An average of several experiments gives us 50-5 cm. of mercury for a I per cent,
solution, and thus a 34-2 per cent, solution ( = I G.M.) will give a pressure of
1,727 cm., or 22.7 atmospheres. This number forms the basis on which VAN'T
HOFF founds his comparison of osmotic pressure with the pressure of gases.
BOYLE'S law, governing the pressure of gases, states that the product of the
volume and of the pressure of a gas is a constant quantity ; if the volume of
the gas decreases, then the pressure must increase correspondingly. Under
ordinary atmospheric pressure i G.M. of oxygen (32 g.) or i G.M. of carbon-
dioxide (44g.) occupies 22-4 litres. To compress the gas until the G.M. occupies
only one litre will necessitate a pressure of 22-4 atmospheres. The pressure
of i G.M. of gas in the litre corresponds exactly, as one would expect, to the
pressure of i G.M. of cane sugar in PFEFFER'S osmotic cell, and on this corre-
spondence is based VAN'T HOFF'S theory, i. e. that osmotic pressure arises from
the impacts at the same time of the molecules dissolved in water and of the
ions on the wall of the cell. However, botanical investigations on the amount
of osmotic pressure are entirely independent of this physical theory.

As it is quite immaterial in the case of gases whether the separate mole-
cules in the vessel are all of one kind, or are chemically different, so in con-
sidering the amount of osmotic pressure in the plant cell it is immaterial whether
the cell-sap contains only cane sugar, as we have hitherto assumed, or whether
it contains a complex mixture. So long as these bodies do not react on each
other, so long as the number of molecules and ions remains unaltered, and so
long as the protoplasm remains impermeable, so long does the osmotic pressure
remain unaltered, and we can always estimate its amount by the plasmolytic
method whether we know the nature of the materials which produce it or not.
If we find that a 3*5 per cent, solution of cane sugar will bring about the first
trace of plasmolysis in certain plant cells, while a 3 per cent, solution
produces no effect, it follows that the cell-sap is approximately isosmotic
with a 3-5 per cent, solution of cane sugar, and that the cell exercises the same

JOST C

i8 METABOLISM

pressure on the cell-wall as the sugar solution would, viz. about two and a half
atmospheres. We may, however, employ other substances to bring about
plasmolysis, for so long as we know the molecular weights of these substances
and their isosmotic co-efficients, the estimation of osmotic pressure by the
plasmolytic method presents no difficulty. Potassium nitrate has often been
used in place of cane sugar because it has been found that protoplasm is
frequently quite impermeable to that salt. A I per cent, solution of potassium
nitrate is equivalent to a 5-13 per cent, solution of cane sugar.

All vegetable cells are not equally suitable for plasmolytic research. Young
growing cells have their membranes stretched owing to osmotic pressure, and
such cells contract, as we have seen, when that pressure is withdrawn. This
contraction must also take place in plasmolysis, and so complications in
endeavouring to estimate osmotic pressure in such cells are introduced, which
had best be avoided. But neither are all mature cells suitable for the purpose ;
often the first beginnings of plasmolysis are not readily observable, and yet
that is the point of importance. For plasmolytic research in general, rather
than the determination of osmotic pressure in specific cells, it is preferable
to employ mature cells containing a coloured cell-sap, in which the separation
of the protoplasm from the wall may be especially well seen. DE VRIES,
for example, recommends the epidermal cells from the underside of the leaf
of Tradescantia discolor, and this material is frequently used for this purpose.

This is not the place to quote detailed statistics as to the absolute amount
of osmotic pressure (compare Lecture XXXIII) ; it will be sufficient to note
that pressures of five to ten atmospheres are by no means infrequent in the
plant. Deviations from such average pressures are known to occur, both
above and below the mean. Osmotic pressure does not appear, however, to
sink under three atmospheres in starved cells — whilst it may reach fifteen to
twenty atmospheres in such plants as the beetroot and onion.

Young cells which contain no vacuoles also exhibit a turgor pressure.
In this case the osmotically active substance must be dissolved in the protoplasm;
later on, it preponderates in the vacuole, where it accumulates proportionally
as the volume of the vacuole increases pari passu with the volume of the cell in
the process of growth.

In the cells of the beet and the onion, which we have instanced as ex-
amples of plants showing especially high osmotic pressures, the turgor is mani-
festly due to the useless activity of the accumulated reserves. When these
substances, as happens frequently in other regions of storage, are condensed
into larger molecules, e. g. starch, and rendered insoluble, then the osmotic
pressure disappears. In many other cells also it is possible that the accumu-
lation of reserves may bring about a pressure capable of producing perhaps
a quite undesirable secondary effect ; but that is not of general occurrence.
Osmotic pressure has frequently one very definite function to fulfil. By its
means, as we have already seen, the cell membrane is kept tense, and so long
as the tension is maintained the cell is more rigid than in the plasmolysed
condition. Just as inflation of a caoutchouc bladder renders it rigid, owing
to the stretching of the membrane, so osmotic pressure induces a corresponding
rigidity in a plant cell. In thin-walled growing cells this rigidity is maintained
entirely by osmotic pressure, and a small loss of water at once relaxes the
tension of the cell-wall and annuls the rigidity of the cell. The condition of
tension is termed turgescence and we speak of it as caused by turgor pressure ;
turgor pressure is identical with osmotic pressure. (As to the part played by
turgor pressure in growth, see Lecture XXI.)

The net result of our discussion, looked at from the point of view of the
absorption of nutritive material, is that we have found protoplasm to be easily
permeable to water, but quite impermeable to many of the substances soluble
in it. This result appears all the more surprising when we investigate how

THE OSMOTIC CHARACTERS OF THE CELL 19

the materials met with in cells enter into it. From the fact that water is not
the only body found in the cell it follows of necessity that all materials do
not behave in the way that cane sugar, potassium nitrate, and the colouring
matter of red beet do ; there must be many materials which are capable of
passing through the protoplasm. The researches of later years have proved
to us the existence of many such bodies, whose capacity for penetrating the
protoplasm may be demonstrated in a great variety of ways.

The plasmolytic method is equally serviceable lor the demonstration both
of permeability and of impermeability. DE VRIES (1888 a) has shown that
glycerine produces plasmolysis at first, but that after several hours this plasmo-
lysis disappears, the reason for the disappearance being that glycerine begins
slowly to enter into the interior of the cell. When the degree of concentration
of the glycerine inside and outside the cell is the same, the turgor of the cell is
re-established, and the existence of glycerine on both sides of the cell has as
little significance in relation to turgor as if it were entirely absent. The osmotic
pressure has, however, increased, and if the cell be once more plasmolysed
it will be found necessary to use a more concentrated solution than before.

Similarly, plasmolysis produced by urea, erythrite, glycol, &c., ceases to
make itself apparent, sometimes more rapidly, sometimes more slowly (OVERTON,
1895). Only a few minutes are necessary for the cessation of plasmolysis in
Spirogyra when treated with glycol, acetamide, and succinamide, while plasmo-
lysis, produced by glycerine, continues for a couple of hours, by urea five hours,
and by erythrite twenty hours. OVERTON has, however, discovered that certain
subblances, e. g. alcohol, pass through the protoplasm more rapidly than
glycol, and produce practically no plasmolysis, behaving in this respect like
water. Since the molecular weight of alcohol is 46, a I per cent, solution of
that substance has the same osmotic value as a 7*5 per cent, solution of cane
sugar, so that, if an 8 per cent, solution of cane sugar produces plasmolysis
in a cell of Spirogyra, the same effect should be obtained by using a i-i per cent,
solution of alcohol. No plasmolysis is, however, observable on using either a I per
cent., 2 per cent., or even 3 per cent, of alcohol, for it passes too rapidly through
the protoplasm to give time for plasmolysis to take place. That this result
is due really to the rapid penetration of the plasma, and not at all to injury
induced by the alcohol, is shown by the fact that if the 3 per cent, solution
of alcohol be added to the 8 per cent, solution of sugar, plasmolysis takes place
at once just as when the sugar is dissolved in pure water. In a similar way
OVERTON has established the fact that protoplasm is easily permeable to a large
number of organic substances, such as ether, chloralhydrate, sulphonal, caffein,
antipyrine, &c.

The fact that many substances enter into the cell in this manner is indis-
putable, but their entry is not perfectly self -apparent. We cannot actually
see the entrance of he substance, we can only conclude that it enters. At least
we must assume this from what has been said, although it does not quite corre-
spond with the facts. Not a few of the substances referred to betray their
entrance by the changes which are set up by them in the cell, such, for example,
as the deposition of insoluble bodies in the cell-sap after the entry of caffein,
antipyrine, acetamide, ammonium carbonate, &c. Generally speaking, we are
quite ignorant of the nature of these precipitates, save that in certain cases,
e .g. caffein and antipyrine, it is known that they are associated with the for-
mation of insoluble tannin compounds. Tannin itself is soluble, but, since
these tannin compounds are insoluble, the tannin is itself precipitated. In the
case of ammonium carbonate, probably the alteration of the acid reaction of
the cell-sap only plays a part. This can be proved with greater certainty when
the cell-sap is naturally coloured (e.g. as in red beet and in red and blue flowers),
the colouring matter serving as an indication of the presence of acids or bases,
much in the same way as a solution of litmus would. Thus the entry of very

c 2

20 METABOLISM

dilute solutions of free acids and of alkalis may be observed to take place
without any injury ensuing to the protoplasm.

The penetration of protoplasm by aniline dyes, a knowledge of which we
owe to PFEFFER (1886), is a phenomenon of great interest and importance.
Since colouring matters naturally dissolved in the cell-sap are unable to undergo
exosmosis, so long as the protoplasm is in a normal condition, the
protoplasm has been generally regarded as impermeable to dyes ; at the
same time, we have long been aware that dead protoplasm can absorb and
accumulate many pigments. The majority of the aniline dyes are poisonous
to the cell, and unless very dilute solutions be employed, they kill the proto-
plasm, before they penetrate it. Among the relatively non-poisonous aniline
dyes, methylene blue ranks first, for the plant can endure a solution of I in
100,000, or even I in 10,000, without suffering any injury. A solution of
methylene blue of a strength of I in 100,000 exhibits a beautiful blue colour
when in a layer several centimetres thick, but the colour is scarcely noticeable
in a glass tube I mm. in diameter, and cannot be distinguished at all, even with
the aid of a microscope, in a capillary tube o-imm. broad. If the cell-sap of
Spirogyra consisted of such a solution it would show no coloration, so that
a colouring body may very easily penetrate the protoplasm without making
itself apparent in the vacuole. PFEFFER, however, found that the roothairs
of Trianea after a short time exhibited an obvious blue coloration when treated
with a solution of methylene blue of i in 100,000, whilst in Spirogyra blue
granular masses made their appearance in the cell-sap. Diffusion of the dye
through the protoplasm must therefore not only have occurred, but a subsequent
accumulation of the dye in the vacuole must also have taken place. This accu-
mulation results from the transformation of the substance, after entry, into a
form which cannot again pass out, thereby making room for the further entry of
the pigment. This phenomenon is of the greatest significance in relation to the
absorption of materials by the plant ; for diffusion may result in the accumu-
lation in the cell-sap of materials in request, until the degree of concentration
inside is equal to that outside. Since, in most cases, the solutions available
for the plant in nature are extremely dilute, only very small amounts of
material can enter it by diffusion only, but if these substances so entering can
be stored away in an insoluble, or, at all events, in another and non-diffusible
form, continued entry of the material in question is possible.

The cell shows, therefore, certain characteristics which are of fundamental
import in the well-being and life of the plant ; one of these is its capacity for
absorbing materials, not indiscriminately just as they are presented to it, but
selectively, both from a qualitative and quantitative point of view. Hence
a substance widely diffused in nature may be altogether wanting in the cell,
simply because it is not capable of diosmosis, whilst a comparatively rare sub-
stance may be accumulated in considerable quantity. It is only in a few cases
that we are exactly acquainted with the cause of the storage, e.g. that of methy-
lene blue by the root-cells of Lemna. In this case the dye is united with tannin,
and tannate of methylene blue is incapable of penetrating the protoplasm in
any direction. No absorption or storage of this body occurs if the cell be placed in
tannate of methylene blue instead of methylene blue itself, nor does any exos-
mosis of the tannate formed in the cell occur if the cell be again placed in water.
If, however, a drop of citric acid be added to the water, after a short time the
blue colour gradually disappears inasmuch as a process the converse of storage
takes place First of all a very little citric acid enters the cell and unites with
the methylene blue ; as a result of this union, room is created for the entry
of more citric acid, and as citrate of methylene blue is capable of penetrating
the protoplasm, all the blue colour in the cell in the long run undergoes
exosmosis. Storage of such compounds does not always take place in such a
simple manner as that just described ; sometimes the changes are less, some-

THE OSMOTIC CHARACTERS OF THE CELL 21

times more extensive. As an example of an elaborate alteration we may take
the formation of the insoluble, and therefore osmotically inactive, starch from
sugar absorbed by the cell, while the precipitates by ammonium carbonate
already alluded to, removable by mere washing in water, may be quoted as a
simple illustration of the same phenomenon. In most cases we are unacquainted
with the mode in which storage in the cell-sap is effected. For instance, when a
nitrate or other inorganic salt accumulates in the cell-sap and reaches a higher
concentration there than in the surrounding fluid, it is not impossible that a loose
union occurs between that body and some other, but such unions are not very
probable. When unilateral accumulation occurs without alteration of the sub-
stance, purely physical diffusion conditions, which we have hitherto accepted
as essential, cannot be operative, or, at least, not entirely so. On this question
we must await the results of further inquiry. NATHANSOHN (1902) has made
an attempt in this direction, but his results cannot be said to be above criticism.
He experimented with Codium tomentosum, the cell-sap of which he studied
by quantitative chemical methods, but he overlooked the very large inter-
cellular space system in that plant, which, in addition, communicates freely
with the exterior, an omission which vitiates his researches. [NATHANSOHN
has recently extended his osmotic studies (1904, Jahrb. f. wiss. Bot. 39, 607
and 40, 403). He finds that the permeability of protoplasm for any substance
is not constant in degree but varies with external conditions. Protoplasm
becomes, for example, impermeable to a substance if it be present in the
vacuole in an amount bearing a certain quantitative ratio to the concentration
of the same substance outside. This relation between the internal and external
concentration may be again restored if the solution outside the vacuole be
diluted, when a diffusion of the more dilute into the more concentrated solution
takes place. In opposition to H. FISCHER (1904, Ber. d. bot. Gesell. 22, 485)
who explains this phenomenon by the Law of Distribution, NATHANSOHN (1904,
Ber. d. bot. Gesell. 22, 556) holds that it is due to variations in protoplasmic
permeability, which cannot be accounted for in a purely physical manner.]

As a result of ingenious arguments, into which we cannot enter at
present, PFEFFER has shown that the permeability of protoplasm does not
depend on the protoplasm as a whole, but only on a very thin, microscopically
indistinguishable layer, which may be termed the plasmatic membrane. An
outer plasmatic membrane determines what substances shall enter the proto-
plasm, while an inner plasmatic membrane determines what shall enter the
vacuole. These two membranes would appear to have different properties,
since substances may enter the protoplasm in considerable quantities and yet
bring about plasmolysis, owing to the fact that they are unable to penetrate
the inner plasmatic membrane. Sugar, as we shall see later, behaves in this
way in many cases.

Further, a single cell not infrequently contains several vacuoles whose con-
tents differ from each other, and the plasmatic membranes of these vacuoles
have in all probability different permeabilities. The cell's organization, as
HOFMEISTER (1901) has shown, must operate so as to keep the varied chemical
products apart, and here the plasmatic membranes must be of service ;
should these membranes alter in character, the previously separated products
may come in contact with and react upon each other, and hence the variations
in the plasmatic membranes must be of fundamental importance in the life of
the cell.

Finally, we must inquire into the causes of varying permeability of proto-
plasm to different substances. So far as purely physical conditions determine
permeability, a survey of diosmotic substances first of all may possibly give
us a clue. OVERTON'S (1899) comprehensive observations, made with the aid
of various methods, enable us to present the following summary : —

22 METABOLISM

A. Members of the aliphatic series which pass through the protoplasm in
so far as they are soluble in water : —

I. Easily diosmotic : —

1. Univalent alcohols (methyl alcohol, ethyl alcohol, allyl alcohol,

ethyl ether).

2. Aldehydes (formaldehyde, chloralhydrate).

3. Ketones (acetone, sulphonal).

4. Halogen-hydrocarbons (chloroform).

5. Neutral esters of inorganic and organic acids, provided with one

O-H group.

II. Not readily diosmotic : — bivalent alcohols (glycol) and the amides of
univalent acids.

III. Diosmotic with difficulty : — trivalent (glycerine) and quadrivalent alco-
hols (erythrite), urea.

IV. Scarcely diosmotic : — hexivalent alcohols, hexoses, amido-acids, neutral
salts of organic acids.

B. Among substances which do not belong to the aliphatic series,
and which readily enter the protoplasm, the following are known :—

Benzol, xylol — anilin, formanilide, acetanilide — phenol, resorcin, orcin,
phloroglucin — antipyrine — free alkaloids, but not their salts — basic aniline dyes,
but not their sulphur containing salts.

The ready solubility of all these substances in ether, fatty oils, and similar
media is, according to OVERTON, characteristic, and this authority has assumed
that the limiting layer of the protoplasm is impregnated with a substance
with a similar power of solution, and thus that only those bodies which are
capable of solution in the limiting layer can enter the cell, that is to say, the
osmotic peculiarities of the plasma depend on phenomena of selective solubility
(compare TAMANN, 1892). OVERTON gives many examples of the similarity
existing between solutions in oils and in the plasmatic layer ; he shows
especially how by certain substitutions many bodies can be made to enter the
plasma by dissolving them in oils.

Certain poisons also, such as corrosive sublimate, iodine, picric acid, and
osmic acid which, owing to the rapidity with which they penetrate the proto-
plasm, have been considered as good fixing materials (compare p. 9), are shown
by OVERTON to be readily soluble in oil. Most salts, however, are insoluble
in oil, and appear to be incapable of penetrating the protoplasm. Further
research on this subject is urgently needed, for it will appear later that many
of these inorganic salts are indispensable and must be absorbed from without.
Further, accurate research is still required on the mode of absorption of the
ordinary gases of the atmosphere, viz. oxygen, nitrogen, and carbon-dioxide.
That these gases do penetrate the protoplasm cannot be doubted, as our ex-
position of gaseous exchange in the plant will show.

It is, however, by no means probable that the plasmatic layer consists of a
fatty oil, since Algae, for example, are capable of living for whole days without
suffering injury in a 2 per cent, solution of sodium carbonate, a substance
which would emulsify, and therefore destroy the oily layer. OVERTON there-
fore came to the conclusion that the plasmatic membrane derived its osmotic
characters from the presence of a large percentage of cholesterin and lecithin,
and this hypothesis he has elaborated in his more recent researches (1900), where
he investigates the solubility of various substances in cholesterin. He finds
that the solubility of various bodies, more especially aniline dyes, in cholesterin
corresponds much more closely with the absorption of such substances by
the protoplasm than with their solubility in oil. The presence of cholesterin
in the plasmatic layer would thus explain the absorption of oils (Lecture XIII)

THE OSMOTIC CHARACTERS OF THE CELL 23

and xylol (OVERTON, 1899), and also of substances insoluble in water, while it
would present no obstacle to the passage of water. It is worth noting in this
connexion that lanolin, a derivative of cholesterin, is able to absorb more
than double its weight of water.

Although OVERTON' s hypothesis appears in many respects inviting, still
it has not been proved either in principle or in detail. In the first place, there
are specific differences in permeability ; Penicillium, for instance, will not
permit the entry of copper salts, although these are readily absorbed by the
majority of plants. Similarly Beggiatoa is able to take up sulphuretted
hydrogen, although Algae closely allied do not do so. Such differences
may be explained by assuming specific chemical constitutions for the plasmatic
membranes of the species concerned. Moreover, the plasmatic membrane of
one and the same individual varies according to external conditions. These
variations may depend on changes taking place from time to time in the
cholesterin-lecithin mixture, although it is possible that, under certain con-
ditions, other substances may play a part in the constitution of the plasmatic
membrane. Further, we cannot escape from the criticism that the plasmatic
membrane may often be in our experiments not quite in a natural condition.
For instance, many plasmolysing substances are instrumental in forming pre-
cipitation membranes on the surface of the plasma, and thus it is possible that
we might be studying the characters, not of the plasmatic membrane in its
natural state, but those of a precipitation membrane artificially produced
(BERTHOLD, 1896, p. 152).

[WACHTER'S researches (1905, Jahrb. f. wiss. Bot. 41, 165) have shown
that exosmosis of sugar from the cells of the onion is prevented by salt-solutions,
but that quantities of sugar diffuse from cells when these are surrounded by
water. In the case of beet also he has observed exosmosis of sugar to take
place, in opposition to the results which DE VRIES obtained (p. 13). WACHTER'S
work does not, however, provide us with an explanation of the complicated
osmotic phenomena seen in beet, although his studies demonstrate very
clearly that the osmotic peculiarities of the plasmatic layer are exceedingly
variable (compare Lecture XIV).] v.,.*:^

As a matter of fact, a renovation of the plasmatic membrane has been
experimentally established. It occurs, for example, on the surface of the
protoplasm which exudes from a wound in Vaucheria, and may be observed
also after injuries inflicted on plasmodia of Myxomycetes. In the latter
case it may be easily shown that new plasmatic membranes are formed on
isolated parts of the general cell plasma (PFEFFER, 1890'

Bibliography to Lecture II.

BERTHOLD, E. 1886. Studien iiber Protoplasmamechanik. Leipzig.

HOFMEISTER, Fr. 1 90 1. Die chemische Organisation der Zelle. Braunschweig.

NAGELI. 1855. Pflanzenphys. Unters. i, 21.

NATHANSOHN. 1902. Jahrb. f. wiss. Bot. 38, 241.

NERNST. 1890. Zeitschr. f. physik. Chemie, 6, 37.

OVERTON. 1895. Vierteljahrsschr. d. Naturf.-Gesell. Zurich

OVERTON. 1899. Ibid.

OVERTON. 1900. Jahrb. f. wiss. Bot. 34, 669-701.

PFEFFER, W. 1877. Osmotische Untersuchungen. Leipzig.

PFEFFER, W. 1886. Unters. aus d. bot. Instit. Tubingen, 2, 179.

PFEFFER, W. 1890. Plasmahaut und Vakuolen (Abh. matn.-phys. Kl. Sachs.

Gesell. 1 6, 187).

TAMANN. 1892. Zeitschr. f. physik. Chemie, 10, 255.
DE VRIES, H. 1877. Die mechan. Ursachen d. Zellstreckung. Leipzig.
DE VRIES, H. 1884. Methode zur Analyse d. Turgorkraft. Jahrb. f. wiss, Bot.

14, 427.

24 METABOLISM

DE VRIES, H. 1884 A. Bot. Ztg. 46, 229.
DE VRIES, H. 1888 B. Ibid. 46, 393.
DE VRIES, H. 1889. Ibid. 47, 309.

[A complete exposition of osmosis and related phenomena may be found in
B. E. LIVINGSTON'S treatise, The Role of Diffusion and Osmotic Pressure in Plants.
Chicago. 1903. See also HOBER, 1902, Physikalische Chemie der Zelle und der
Gewebe. Leipzig.]

LECTURE III

THE ABSORPTION OF WATER

LEAVING now the consideration of the simple relationships of the single cell
which, when surrounded by water on all sides, is able to absorb water with
dissolved gases and solids, dependent on the degree of permeability of the
protoplasm, we have next to inquire into the mode of absorption of materials
by the complex plant body. Examination of such a cell-complex as we meet
with in the higher Algae (Florideae, Fucaceae) or in a submerged or floating
Phanerogam (e. g. Lemna triscula), discloses a superficial cell layer whose
individual units are, so far as absorption of materials from without is concerned,
in all respects comparable to the solitary independent cells of which we have
already spoken. Beneath the superficial cells, however, we find internal cells
which are prevented from obtaining direct access to the external medium,
and are dependent on such materials as have been permitted to pass through
the cell-walls and protoplasm of the more peripheral cells. The external cell
layer, in the first instance, determines what substances shall pass into the
internal cells, although not all substances capable of entering the superficial
layer necessarily pass farther inwards. The internal cells may in a sense be
considered to be in direct relation with the external medium, since they are in
connexion with it through their cell-walls, in which, generally speaking, all
the substances in question — more especially water — can move.

In principle this is true of all land plants, and one might affirm that a
cell in the topmost bud or leaf of an oak-tree was in direct communication,
through the cell-walls, with the aqueous solution in the soil in which the roots
are imbedded, although thousands, or even millions of cells intervene between
it and the tips of the roots. In practice, however, this case is quite different
from the preceding one, since further exposition will show that any exchange
of materials is impossible by this means, owing to the enormous distance which
separates the units. Hence we must regard the absorption of materials in the
land plant as quite distinct in character from that occurring in a single sub-
merged cell. No physiological researches are requisite to prove that the two
regions of the terrestrial plant, manifestly different even to the non-botanist —
viz. the root imbedded in the soil and the shoot expanded in the air, are essentially
different in their methods of absorbing nutriment. The root absorbs the water
present in the soil and the substances dissolved in it in the way already
described, while the shoot absorbs the materials in the air in the gaseous
form essentially. Naturally, therefore, in the following pages we must con-
sider separately those constituents of the higher plant which are derived from
the soil and those which are absorbed from the air.

First of all the plant absorbs water from the soil, which, as is well known,
is indispensable to all organisms and to the vegetable kingdom in particular.
Apart altogether from the fact that the chemical elements which go to form
water, i. e. oxygen and hydrogen, form, in combination with carbon, the most
important constructive units in organic compounds, water itself is an indispen-
sable constituent of all cell membranes, which, as a matter of fact, are, in the

THE ABSORPTION OF WATER 25

living state, invariably saturated with water of imbibition ; in the second place,
the protoplasm of the living cell is also always saturated with water, and in
the third place, the vacuoles, which frequently constitute the greater part of
the cell, consist mainly of water. Again, chemical analysis (p. 5) has shown
that parts which appear to contain little or no water do, as a matter of fact,
contain it in considerable quantities. Were the plant to be as sparing in its
use of water as it is of nitrogen (Lect. XI), absorption of water would be pro-
portional to the addition of new members to the plant body ; but such economy
is by no means the rule. On the contrary, the plant is, at least under certain
conditions, lavish in its use of water. The large amount of water which the
plant has extracted from the soil by means of the roots, with the expenditure
of much energy, it sends back again into the air in the form of water vapour.
According to HABERLANDT (1877), a plant of maize in the course of the summer
gives off into the atmosphere 14 kg. ; a hemp plant gives off 27 kg. ; and
a sunflower, 66 kg. ; i. e. in each case several times the weight of the plant. All
these plants are small ; what then must be the amount given off by a tree ?
The following data, which we owe to v. HOHNEL'S very careful calculations,
may be quoted in this relation. A large birch tree with, say, 200,000 leaves,
gives off in the course of the season 7,000 kg., or about 38 kg. per day. A 110-
year old beech tree gives off, in round numbers, 9,000 kg. in the course of the
summer, and, on the basis of 400 such trees to the hectare, a wood of that
size (two and a half acres) would give off 3,600,000 kg. of water in one season.
Although these numbers make no claim to absolute accuracy, still they
give us a rough idea of the very large amount of water concerned in this
process.

Our first task must be to investigate the means by which the plant is able
to absorb so vast an amount of water from the soil. With this subject is
intimately connected the transpiration of water vapour from the leaves. Since
the parts which are concerned in the absorption of water and those con-
cerned with its transpiration are situated far apart, to obtain a complete
picture of the whole process of water circulation in the land plant we have to
consider as well the mode of conduction of water.

The soil from which the normal terrestrial plant obtains its entire water
supply consists of a mixture of mineral detritus and the remains of organisms
(humus). The individual constituent particles are of very variable size, loosely
or compactly arranged, but in all cases so aggregated that interspaces occur
between them, spaces which we will assume, to begin with, are full of air.
If rain falls on such a soil, or water reaches it from somewhere else, the
air is completely driven out of the interspaces between the soil particles, which
then become filled with water. If the subsoil be impermeable to water, for
example if it consists of clay, this Condition of things becomes permanent
and a marsh arises, characterized at once by the abundance of water and the
scarcity of air. The abundance of water enables the plant to recoup itself
with ease for the water it has lost, and hence it might be imagined that such
a soil would form an ideal habitat for a plant. Experience teaches us that
this is far from being the case. Certain plants only, and amongst cultivated
plants only a few (e. g. rice) are able to thrive in marshy soils, or even tolerate
them for any length of time, whilst the majority of our economic plants are
killed in the presence of superabundance of water, and thrive only in soils with
a moderate supply (WOLLNY, 1897). The reason for this is, not that there is
too much water, but that certain accessory conditions are not fulfilled. That
the injurious effects associated with marshy conditions are due to the poisonous
influence of putrefying substances is a common belief, but no good evidence in
support of this view is as yet forthcoming (WACKER, 1898). The only other
possible reason for the injurious influence of a marsh on land plants is the
absence of oxygen. As a matter of fact, water-culture experiments teach us

26 METABOLISM

that there are very few land plants which are capable of developing normally
when their root-systems are submerged in such water-culture solutions (Lect.VII),
for under these conditions the only oxygen available is that dissolved in the
water — an amount far below that present in a well-aerated soil. In water-
cultures, moreover, the root is always able to find some free oxygen, whilst in
swampy situations the minutest traces of this gas are often used up. Swamp
plants overcome this difficulty by providing themselves with an elaborate
intercellular space-system by means of which the gas may enter the root from
above, and frequently by the formation of special respiratory roots which pro-
ject above the medium (GoEBEL, 1886, 1887 ; JOST, 1887 ; KARSTEN, 1892).

Let us now consider the case where the subsoil is permeable to water. The
water soaking into the superficial layers of the soil will partly, at least, drain
into the deeper layers and carry the air with it into the larger interspaces,
but all of it does not percolate downwards in this way. Some remains adherent
to the individual soil particles, while more collects in the minute cracks and
cavities in the particles and is firmly held there by capillarity. The amount
of water which remains in the soil as compared with the volume of the soil
is termed the soil's water capacity. Water capacity varies with the nature
of the soil, and also, and chiefly, with the number and size of the spaces existing
between the soil particles. This variation, within wide limits, is very con-
siderable. The following data will serve to illustrate this fact : —

Water capacity of different soils Water capacity of quartz soils

(AD. MAYER, 1901). (WOLLNY in RAMANN, 1893, p. 67).

Vol. % Vol. %

Humus soil . . . .55 Size of grain i .00-2-00 mm. 3-66

Clay soil .... 53 „ » 0-25-0-50 4-38

Fine sandy soil ... 30 ,, ., 0.11-0-17 6-03

Coarse „ . . . 10 ,, ., 0-01-0-07 35-5<>

These amounts of water are retained by soils only immediately after
a thorough soaking ; part is lost again in the process of evaporation. Plants,
however, during their vegetative period, when most water is required, are
frequently obliged to obtain it from a relatively dry soil, and hence they must
be provided with a greatly branched root-system with the utmost possible
absorbent surface.

The importance of the root for the purpose of water absorption is evidenced
by the fact that in the seedling the root is driven into the soil long before the
leaves unfold. In a word, water is what the seedling primarily needs, since
all the other substances required by it are supplied in abundance by the reserves
stored up in the cotyledons or in the endosperm. In many cases the primary
root derived from the radicle of the seedling remains active for many years,
or, it may be, for life ; it grows in length and may pierce the soil to a very
considerable depth, should the nature of the deeper layers permit. In desert
plants especially, tap-roots of enormous length may be developed, which are
primarily of service in the absorption of water from the deeper layers of the
soil. As a general rule, however, the primary root is not the only active
agent ; it is aided by a series of lateral roots, which arise from it in acropetal
succession, sometimes almost at right angles from the parent root and pene-
trating the soil horizontally, or curving downwards at angles of 70°, 60°, or 50°.
In many plants, e. g. in Vicia faba, according to HELLRIEGEL (1883), the main
root shows continued growth after the appearance of the lateral roots, whose de-
grees of development are approximately indicative of their age, those nearest the
apex being the shortest, those farthest away the longest, whilst the apices of all
of them lie approximately on the surface of an imaginary cone, whose apex is
the tip of the main root itself. Another type of root-system is exemplified by
the yellow lupin. Here the lateral roots are much fewer in number and more

THE ABSORPTION OF WATER

27

irregular, in the first instance forcing their way to a moderate depth beneath the
surface of the soil, while the older roots rapidly fall behind in rate of develop-
ment. Again we meet with a third type where, while the earlier stages in
development are similar to those already described, the tap-root later on
surrenders its prominent position, then ceases to grow, and finally becomes
altogether abortive. To this type of root-system, found in trees more espe-
cially, we will recur again presently. In the fourth type the primary root
is wanting from a very early stage, and is replaced by a tuft of lateral roots,,
equivalent in value, which arise from the base of the stem. Examples of
this type are furnished by grasses and bulbous plants. For descriptions of
the root-systems of herbaceous plants, a subject on which very little research
had hitherto been carried out, reference may be made to the comprehensive
work of FREIDENFELT (1902).

The root-systems of trees deserve special mention for two reasons ; first,
because these plants go on growing for many years, and because the enormous
amount of water used up by the crown of foliage imposes a specially heavy
demand on the activity of the root. Thanks to the elaborate researches of
NOBBE we are able to form a fairly good conception of the root-systems of
the pine, the fir, and the spruce. NOBBE (1875) cultivated seedlings of these
plants during one summer period in large glass vessels filled with sand, and,
when autumn arrived, estimated the number of roots and their collective
length after washing out the entire root-systems. Some of the data he ob-
tained are summarized in the subjoined table : —

i. Rank (chief root)
2. „
3- „
4- ?>
5- „

Total .
Proportional value .

Nu
Fir.

tnber of ro<
Spruce.

JtS.

Pine.

Total lei

Fir.

igth of root
Spruce.

s in ram.
Pine.

i
48
85

0
0

i

85
162

5

0

i
404
1955
749
26

300-0
636-0
56-0
•o

.0

290-0
1333-5
312-5
5-o
«o

873-0
4438-5
5491-5
H43-3
4i-5

134

253

3*35

992.0

1941-0

11987-8

i

2

24

I

2

12

These three one-year-old plants, grown under exactly similar conditions,
exhibit highly striking differences, both in the number of their branch roots
and also in the total length of their root-systems. The sum of the lengths
of all the roots is, in round numbers, in the fir, im., in the spruce, 2m., in the

Eine, 12 m. If estimated in surface measurement, we obtain areas of, in the
r, 49-52 sq. mm., in the spruce, 64-33 sq. mm., and in the pine, 142-23 sq. mm.
Similarly, when the absorbing surface is considered, we find that the pine
again stands far ahead of the fir and the spruce. The mass of soil entangled in
the roots of the pine forms, according to NOBBE, a cone, 80-90 cm. in depth,
and with a surface area of 2,000 sq. cm. If we divide this cone into layers,
each 10 cm. in thickness, we find 1,548 lateral roots in the uppermost layer,
and, successively downwards, 217, 446, 366, 121, and 38 lateral roots. The
pine is thus in contact with a very considerable mass of soil by means of its
root-system ; thus it is able to put the soil to greater account, and so may succeed
in growing in what is otherwise an unfavourable situation. Its alleged in-
difference to its surroundings is thus shown to be due to its great power of
making the best of things. The behaviour of the plant in later years differs
very considerably from that of the seedling. A vigorous outgrowth of lateral
roots near the surface of the soil, in the case of the pine seedling, is suggestive
of the subsequent feebler power of growth on the part of the main root and

28 METABOLISM

the formation of a root-system distributed widely in an almost horizontal
direction ; the tap-root, however, still remains in existence. The tap-root
of the spruce certainly penetrates the soil at first deeply, but after five years or
so it ceases to grow, so that the tree in later stages of development is quite
superficially rooted. The fir alone becomes a deeply-rooted tree with a pre-
dominant tap-root. An example may now be taken from deciduous trees.
The beech, according to HARTIG (as cited by C. KRAUS, 1892), bears in its first
year a simple tap-root with a few lateral branches. By the third year the
most superficial of these take on vigorous growth and develop into a richly
branched root-system close to the surface of the soil. By the fifth or sixth
year the tap-root, which has now attained a length of at most J m., ceases to
grow, and only the lateral roots go on developing. Up to an age of thirty
years, two or, more rarely, three of the deeper seated lateral roots develop pre-
eminently, pushing their way obliquely into the deeper layers of the soil. After
that, onwards, the more superficial roots overtake them as far as rate of growth
is concerned, and spread themselves out horizontally beneath the surface of
the soil, constituting the chief part of the root-system. Hence when the tree
is felled the root-system is found to form an unusually shallow layer in com-
parison with its horizontal extension, being at most 60 cm. deep.

Since the time of HALES (1748) many estimates have been made as to the
extent of the root-system of different plants as well as with regard to the amount
of soil laid under contribution by them. Thus NOBBE (1872) has shown that
the aggregate length of all the roots of a one-year-old wheat plant amounts
to 500-600 m. ; that of a pumpkin may reach 25 km. (SACHS, 1882, p. 19), while
SCHUMACHER (1867) has made measurements by weight of the root-system of
several cultivated plants. SACHS (1882, p. 19) estimated the space occupied by the
roots of a sunflower at one cubic metre, so that one may safely conclude that
the root-system of a large tree is distributed through hundreds of cubic metres
of soil. Inquiries of this kind, however, are, from the physiological point of view,
not of much service since, as is well known, all roots have not the same functions.
In perennial root-systems we may distinguish conducting and absorbing roots.
The former are the permanent parts of the root-system, soon becoming covered
with cork on their outer surfaces and taking no further part in the absorption
of water ; they serve to fix the plant firmly in the soil, however, and also to
carry the absorbent roots. The latter are thin and remain so, and after
a certain time disappear. These are the roots which absorb the water, although
by no means their entire surface subserves this purpose — only that of the
extreme apices where these are covered with hairs, or where hairs have not yet
developed (KNY, 1898). Roothairs are always wanting in some land plants, and
the general epidermis performs the function of water absorption in such cases.
Apart from such exceptional conditions we may designate the roothairs as the
special organs for the absorption of water. The roothairs are tubular pro-
longations of epidermal cells, sometimes of considerable length, which have the
effect of increasing very considerably the absorbent surface of the root.
F. SCHWARZ (1883) has reckoned that the surface of the root of the maize is
increased five and a half times by the formation of roothairs, of barley, twelve
times, and of Scindapsus, eighteen times. New roothairs are developed acro-
petally from day to day on the elongating root as the older hairs die off behind —
for the roothairs live for only a short period. Regions covered by dead hairs
absorb water only with difficulty, so that we must calculate the area of the
apical regions as well as the increase of surface due to roothairs, if we are to
arrive at a correct estimate of the region of functional activity of the root.
Such estimates have not as yet been made.

If we now inquire how the individual roothair absorbs water from the
soil it will aid us considerably if we study carefully SACHS'S statement on the
subject (1865) taken in conjunction with an examination of Fig. 5. This

THE ABSORPTION OF WATER

29

figure shows the surface cells (ee) of a root, from one of which a single root-
hair (hh) has grown out. ' The bodies deeply shaded are microscopically minute
particles of soil between which are shown airspaces left white. Each soil
particle is surrounded by a thin layer of water held fast by surface attraction ;
where the attraction of neighbouring particles of earth co-operate at their
re-entering angles these, otherwise thin, layers of water form thicker accumu-
lations. These aqueous spheres are indicated in the drawing by wavy lines.
The surface of the roothair is also (at a) covered by a thin layer of water, and
its walls are saturated with it. Let us now regard the roothair for a moment
as inactive, and assume that no disturbance at all is taking place in the soil ;
then all the aqueous spheres of the soil particles are not only in contact with
each other, but are also in equilibrium.'

' If we now assume that the roothair, hh, absorbs water at a, its surface layer
in that situation will have less water than corresponds to its power of attraction ;
it withdraws water first of all from its immediate neighbourhood and, in conse-
quence, the equilibrium in these situations will be disturbed. This disturbance
spreads outwards on all sides until
the molecular equilibrium of all the
aqueous spheres is re-established.
By this means they all become
thinner and thinner and the soil as
a whole drier. This desiccation,
however, may make itself evident
not merely in the immediate neigh-
bourhood of the roothair, but will
at the same time affect more distant
parts. Every roothair becomes
thus a centre of a current directed
towards it from all sides, and at
the surface of a small root covered
by thousands of roothairs a similar
movement results which directs
the aqueous particles in the soil
from all sides, but more especially
radially, towards the axis of the
root/ The root is thus capable of
making use of layers of soil although it may not actually be imbedded in
them. ' If we assume the aqueous envelope of a particle of soil to consist of
several very thin layers, then the aqueous molecules lying nearest to the
particle of soil will be attracted with maximum force, and this attrac-
tion becomes progressively less in the successive external layers until, in the
outermost layer, when the soil is saturated with water, the molecular attraction
is only just great enough to prevent the water from trickling away. When the
water disappears at a or at /?, y, &c., the outermost layer of the aqueous spheres,
more especially, moves first, because it is the one least firmly held and most
easily put in motion. The more water the roothair has already taken up the
thinner are the aqueous spheres of the entire system, and the greater is the force
with which the primary layer — now outside — is held ; so much the greater
must the force be which can pull the water into the wall of the roothair,
and the more difficult and slower the transmission of a disturbance from a to
ft y, 8. A condition of the aqueous envelopes may finally ensue where all the
primary layers are held so firmly by the soil particles that no more water
can enter the wall of the roothair.'

When this degree of drought in the soil is reached then the aerial parts of
the plant must obviously wither though transpiration be prevented as much as
possible. SACHS found withering took place in tobacco plants grown in different

Fig. 5. Roothair, hh, in the soil. Diagrammatic. For expla-
nation see text (simplified from SACHS'S Experimental Physio-
logy). In the figure the thickness of the adhesion layers is
greatly exaggerated ; they cannot be distinguished micro-
scopically.

METABOLISM

kinds of soil, with different amounts of water, even when the leaves were exposed in
the dark to a damp atmosphere. The following table shows this more in detail: —

Kind of soil.

Original amount of water in g.
in 100 g. of dry weight.

Amount of water in g. present
in soil when the plant
withered.

Mixture of sand and humus .
Loam . . . . .
Coarse quartz sand.

46-0
52-1
208

12.3
8.0
i-5

ioo g. of soil contained in the first case 12-3 g., in the second 8 g., and in

the third 1-5 g. of water,
which were not available
for the plant's use. These
quantities correspond ap-
proximately to those left
in the soil when it is air-
dried.

By what force then
does a roothair overcome
the adhesion of the water
to the soil particles ?
After what we have
learned as to the osmotic
characters of a single cell,
we can have no hesita-
tion in ascribing this
power to osmotic acti-
vity. In fact, the plas-
molytic method enables
us to demonstrate with
ease an osmotic pressure
in the roothairs. Owing
to this osmotic pressure,
as we have seen, the cell-
wall will be stretched
until its elastic expan-
sion equals the turgor,
and the water will be
sucked into the cell
cavity so enlarged by the
stretching of the wall, as
by a suction pump. The
cell-sap will first of all
withdraw the water from

Pi,. 6. Po.oo.eu, <^t^^^lM*«r.*»l ^itsTuSo^g^fe

power of imbibition, will endeavour to draw fresh supplies of water from the
membrane. The wall will then contain less water than it can hold in virtue of
its power of swelling, and in consequence will suck up the water which we have
spoken of as adhering to the soil particles.

If now the water osmotically absorbed by the roothair be retained, move-
ment of water, after a time, on the restoration of equilibrium, must come to
an end. In reality the movement of the water is not completely stopped,
a condition of equilibrium only is reached in which the osmotic entry of water
is balanced by the outflow effected by the pressure of the cell-wall. However,

THE ABSORPTION OF WATER 31

owing to the fact that the aerial parts are transpiring, as well as to other causes,
the epidermal cells of the root in ordinary land plants are always absorbing
water, and hence a continuous inflow of water is kept up. If the supply is
sufficient to replace loss by transpiration, then the amount of water in the
plant is thus approximately a constant quantity. As the soil becomes drier
the absorption of water becomes, as we have seen, increasingly difficult and
the plant begins to wilt. The absorption of water by the root is influenced
to a very considerable extent not only by the amount of water present in the
soil but by other external factors as well. Thus it has been long known that
low temperatures, from + 4° to + 2° C., cause certain plants, e. g. tobacco and
pumpkin (SACHS, 1860), to wither, and even to die, if the exposure to such
temperatures be prolonged. These low temperatures act injuriously on the
plant, very often not directly, but by retarding the absorption of water (KiHL-
MANN, 1890). Strictly speaking, the withering is due not merely to a diminution
in the absorption, but possibly also there may be an interference with con-
duction somewhere ; at all events the direct influence of temperature on
absorption itself still wants elucidation. KOSAROFF (1897), to whom we owe
investigations on this subject, employed a simple apparatus, known as a poto-
meter, which we shall find of service in other investigations later on. The
principle of this apparatus is explained by Fig. 6. Into the end of the U-tube
a branch is inserted through a cork, K, care being taken that the junctions are
airtight ; into the other end is inserted, also through a cork, K', a capillary
glass tube, Gl, bent at right angles and having a graduated scale attached to it.
Each absorption of water by the plant manifests itself by a backward move-
ment of the thread of water in front of the scale. The apparatus may be im-
proved by placing the root-system of a growing plant into a large glass vessel,
holding a water-culture solution, instead of into a U-tube (Lecture VII). By
means of a funnel, suitably fixed, and provided with a stop cork, it is possible
to replace periodically the water lost, so that the experiment may be carried on
for a longer time.

When the root-system of Phaseolus multiflorus was placed in this apparatus
and kept at 20-8° C., KOSAROFF observed that in twenty minutes the meniscus
in the capillary tube had moved through 210 mm. At o° it moved only about
140 mm., and other experiments gave quite similar results. The amount of
water absorbed at o° C. was only three-quarters to two-thirds of that absorbed
at 20° C.

How is this to be explained ? If we assume transpiration interrupted,
a certain time will have to intervene before the cells of the root reach osmotic
equilibrium, that is to say, until they have absorbed as much water as corre-
sponds to their osmotic activity. The amount finally absorbed by the cells
when equilibrium is fully re-established will be practically the same whether the
temperature be o°C. or 20° C., but the time which elapses before the re-establish-
ment of equilibrium will depend very materially on the temperature. As a matter
of fact, the osmotic pressure alters according to temperature in the same way
as gas pressure ; but as this alteration does not amount to more than -^ per
degree we need not consider it any further, since it is of no physiological im-
portance. RYSSELBERGHE (1901), by observing plasmolysis and recovery, has
estimated the time taken by water in passing through the protoplasm in dif-
ferent cases, and has arrived at the following results : —

Temperature o° 6° 12° 16° 20° 25° 30°

Rate of movement of water .12 4.5 6 7 7-58

At 30° C. the movement of water was eight times as rapid as at o° C. At
first sight this result appears remarkable, and does not agree with the behaviour
of PFEFFER'S osmotic cell ; for the copper- ferrocyanide membrane exhibits no
such irregularities in behaviour at different temperatures. RYSSELBERGHE,
however, referred the matter to purely physical causes, and he remarks

32 METABOLISM

that, according to differences in temperature, gelatine also shows variations
which are very noticeable, but always less so than those exhibited by protoplasm
in its resistance to the passage of water. Nevertheless it appears to us that the
part played by the protoplasm in the absorption of water is not purely physical.
It is certainly remarkable that the rate of water transference increases only
slowly between 20° and 30°, and it is unfortunate that RYSSELBERGHE did
not employ still higher temperatures in his researches, seeing that, as we shall
have frequent occasion to note in the course of these lectures, the vital activity
of the plant increases in intensity proportionally as the temperature increases,
until a degree is reached somewhere between 30° and 45°, when it again de-
creases. Certain observations of KOSAROFF pertinent to this question tend to
support this view. Withering soon takes place if a stream of carbon-dioxide
or hydrogen be passed through soil in which healthy pot plants are growing ;
absorption of water is therefore retarded by this means as well as by low tem-
peratures. This result may be observed within an hour after the initiation
of the experiment, and the carbon-dioxide itself could scarcely have caused
death in that time. The hydrogen is not so rapid in its action ; we know for
certain that it is itself harmless, but operates only by displacing the oxygen.
It would appear, therefore, from the evidence afforded by these researches,
that the absorption of water is retarded by suppressing the supply of oxygen
to the root. Oxygen, as we shall see later, is an indispensable factor in a large
number of vital processes, although of no account in the diffusion of water
through a dead membrane ; we are thus compelled to believe that living
protoplasm plays a great, but as yet unknown, part in the absorption of water.
The fact established by KOSAROFF that dead roots take up less water does
not necessarily prove this, for death itself will doubtless induce purely physical
changes in the plasmatic layer.

Biologically it is of the greatest interest to note that all plants are not
injuriously affected to the same degree by low temperatures ; many, indeed,
can still absorb water from a frozen soil (KOSAROFF, 1897).

The root is the normal organ for absorption of water in ordinary land
plants, hence plants whose roots have been destroyed invariably die from
want of water, even though their shoots be frequently watered by rain or
dew. We must not, however, conclude from that, that aerial organs are al-
together unable to absorb water. The epidermal cells of the leaf, just as much
as those of the root, contain osmotically active substances in their vacuoles,
and must also be able to absorb water if only the outer wall be permeable
to it, and if sufficiently large quantities of water derived from rain or dew can
accumulate on the leaf. Not infrequently, however, the shape and arrange-
ment of leaves (SxAHL, 1893) are adapted to the rapid drainage of surplus
water from the leaf surface. Thus, according to STAHL'S researches (1897), the
position of leaves on the axis is such as to prevent or hinder the formation of
dew upon them (Lect. XXXIX) ; finally anatomical adaptations occur, espeAlly
coatings of wax, which render the leaves waterproof. Such peculiarities in
structure are of restricted occurrence, but aerial parts of plants differ in general
from subterranean parts in possessing a cuticle formed from the outermost
layer of the cell- wall. This cuticle is not only of maximum thickness on the
stems and leaves, but exhibits varied physical and, probably also, chemical
peculiarities. The cuticle of the shoot only has hitherto been studied in suffi-
cient detail, that of the root is still urgently in need of investigation. A
research of this kind has been carried out by KROMER (1903, Bibl. botan.,
Heft, 59) in the Marburg Institute, and he holds that the root epidermis has an
outer wall of very varied composition and structure, but that a genuine cuticle,
such as occurs in the epidermis of the stem and leaf, is entirely absent.

The cuticle has been found to consist of a material which shows a strong
resemblance to cork, agreeing with that substance in possessing the physical

THE ABSORPTION OF WATER

33

characteristic of being capable of swelling very slightly in water, and hence
of permitting very little water to pass through it. The cuticle of the root,
on the other hand, is easily permeable to water and swells up into a gelatinous
mass (SCHWARZ, 1883). Even the almost waterproof and strongly cutinized
cell-walls of the leaf of Sedum fabaria do not appear to be quite impermeable
to water, since WIESNER (1882) has observed experimentally that the leaves
of this plant, if immersed in water, increase in weight. By means of an old
experiment (HALES, 1748, p. 78), easily repeated without special appliances,
direct absorption of water by the leaf may be demonstrated. If part of an
amputated leafy twig be immersed in water, leaving the cut end and some of
the leaves exposed to air, the latter go on transpiring but remain turgid all day
long despite the fact that water is being given off from the exposed regions — so
demonstrating that the submerged leaves are able to absorb as much water as is
given off by the exposed leaves. The success of the experiment in any given plant
depends entirely on the relative numbers of the absorbing and transpiring leaves.
WIESNER (1882) dipped only the apices (bearing some young leaves) of ampu-
tated twigs of the vine in water, leaving most of the older leaves to transpire into
the air. The result was surprising. The exposed leaves remained turgescent,
whilst the apical leaves, although submerged, wilted. In this instance the
amount of water absorbed by the apical leaves was insufficient to cover the
loss sustained by transpiration. The older leaves withdrew water from the cells
of the apex of the branch, and caused them to wilt, even though in water all the
time. From what has been said it would appear that aerial parts of plants
are also capable of absorbing water, and it would not be difficult to bring
forward evidence from the literature on the subject to show that not only leaves
and young stems, but also bud-scales and older branches, whose cuticle has been
replaced by the still more impermeable cork, can absorb water (KNY, 1895). In
our ordinary terrestrial plants, however, during the rainy period, the amount of
water absorbed is quite insufficient to cover the loss due to transpiration, and
hence the absorption of water through the shoot may, for all practical pur-
poses, be ignored. In tropical regions, with a much greater precipitation of
moisture, with frequent downpours of rain and greater general dampness of
the atmosphere, innumerable plants exist which do not come into contact with
the soil at all, and hence can obtain their water supplies from the air only.
There are, for instance, the epiphytes, living on the tops of trees, whose bio-
logical peculiarities have been described for us in a most attractive manner
by SCHIMPER (1888) and GOEBEL (1889). While making a general reference
to the works of these authors, we must confine ourselves here to mentioning
only a few examples. In the case of many of these epiphytes, as for example,
the Araceae and Orchidaceae, long aerial roots are formed whose function it
is to absorb water from the air. These roots differ widely in structure from
ordinary subterranean roots. Instead of a single layered epidermis producing
roothairs, we find a many-layered cellular envelope, the units of which have
lost their protoplasm at an early period of life, and which now -form air chambers,
communicating with each other and with the exterior by means of pores.
When rain falls on this sheath, the drops sink into it as into a sponge, replacing
the air in the otherwise empty cells ; from thence the water readily penetrates
to the living cells of the root cortex beyond.

In other epiphytes the roots are reduced in size and serve merely as holdfasts,
whilst absorption of water is carried out by the leaves alone. This is seen best,
for example, in many Bromeliaceae, where the leaves a*e often arranged in a
rosette, their bases enclosing a funnel-shaped cavity in whj^h rain-water accumu-
lates as in a cistern. Hairs of special character, quite distinct from roothairs,
absorb the water in the pitcher. [These hairs have been lately studied by ME Y
(I9°4» Jahrb. f. wiss. Bot. 40, 157) and STEINBRINCK (1905, Flora, 94, 464).]

JOST D

34 METABOLISM

SCHIMPER has proved that the amount of water absorbed from the funnel exactly
corresponds with that lost in these plants from transpiration, whilst their roots
are quite unable to provide water in quantity sufficient for their needs. On the
other hand, there are some forms provided with special holdfasts and which
have lost their roots altogether. The best known of these rootless Brome-
liaceae is Tillandsia usneoides, whose long, grey, tail-like masses occur in tropical
and sub-tropical America in such quantities that they actually obscure the
foliage of the trees on which they are epiphytic. ' The first beginning
of a tuft is, as a rule, the separation of a solitary twig which twists round
another branch of the tree ; from it arise numerous lateral twigs, some of which
become themselves propagative shoots, although most of them develop quite
freely into the air.' The leaves of this Tillandsia form no collecting funnels, they
are certainly not arranged in a rosette, but come off individually from the stem
and are small and inconspicuous. The whole plant is covered with water-
absorbing hairs such as occur on the leaf bases of other forms, and these give
it its grey colour. In general appearance, as indeed its specific name, ' usneoides/
indicates, it resembles an indigenous lichen, also a pendent epiphyte from trees.
This recalls to us the fact that epiphytes occur in our own climate also, although
these are almost entirely confined to plants of low grade, viz. mosses and
lichens. The feature in which these plants have an advantage over higher
forms and which qualifies them to withstand our dry seasons, is not any
specially economical management of the water absorbed, but a capacity for
being able to withstand desiccation, a capacity, however, by no means confined
to epiphytic forms. These plants may often become so dry that they may be
actually crumbled into dust, and that, too, without losing their vitality. As
soon as the first drops of rain fall on them and they have absorbed as much
as they require, they start life afresh. Perhaps the best examples of this
peculiar mode of life are to be found among the crustaceous lichens which
grow over the walls of old houses or on bare rock. Such forms often obtain
in a few hours or days only, during the course of months, all the water they
need for carrying on their vital functions, and in the interval are completely dried
up by the sun's heat. Not only is this capacity for resisting desiccation of
the very greatest importance to these plants, but the ability they also possess
of absorbing the first traces of water after long drought is of deep significance.
Their cell-walls in the dry condition remain capable of being easily wetted,
and rapidly take up water once more, differing in this respect from the dust
of our streets, which in losing water loses also its capacity for quickly reab-
sorbing it. Owing to this characteristic, mosses and lichens play a very
important part in the economy of nature, inasmuch as they are able to store
up rain, forming living water reservoirs whose contents are for a long time
of benefit to other organisms. Without going into further details we may, in
conclusion, merely allude to the fact that other epiphytes which are unable
to withstand desiccation, are at first entirely absent from regions subject to
periodic deficiency of water. Further they are compelled to exercise great
economy with the store of water which they have collected during the rainy
period, i. e. they must limit their transpiration greatly or provide themselves
with special water reservoirs. Many varieties of such reservoirs have been
described by SCHIMPER and GOEBEL, as well as by other earlier writers.

Bibliography to Lecture III.

FREIDENFELT. 1902. Flora, 91, 115.

GOEBEL. 1886. Ber. d. bot. Gesell. 4, 249. 1887. Bot. Ztg. 45, 717.

GOEBEL. 1889. Biologische Schilderungen. Marburg.

HABERLANDT, F. 1877. Wiss.-prakt. Unters. auf d. Gebiete. d. Pflanzenbaues,

2, 158.
HALES. 1748. Statik der Gewachse. Halle.,

THE ABSORPTION OF WATER

35

HELLRIEGEL. 1883. Beitr. z. d. naturw. Grundlagen des Ackerbaues. Braun-
schweig.

HOHNEL. 1879. Wollny's Forsch. auf d. Geb. d. Agrikulturphysik, 2, 398.

JOST. 1887. Bot. Ztg. 45, 60 1.

KARSTEN. 1892. Bibliotheca botanica, Heft 22.

KIHLMANN. 1890. Pflanzenbiol. Studien aus Russisch Lappland.

KNY. 1895. Ber. d. bot. Gesell. 13, 361.

KNY. 1898. Ibid. 16, 216.

KOSAROFF. 1897. Einfl. auss. Faktoren auf d. Wasseraufnahme. Diss., Leipzig.

KRAUS, C. 1892. Wollny's Forsch. auf. d. Geb. d. Agrikulturphysik, 15.

MAYER, Ad. 1901. Agrikulturchemie, 5th ed., II, i, 154.

NOBBE. 1872. Landw. Versuchsstationen, 15, 391.

NOBBE. 1875. Tharandter forstl. Jahrb. 201.

RAMANN, E. 1893. Forstl. Bodenkunde u. Standortslehre. Berlin.

RYSSELBERGHE. 1901. Bull. Acad. Belg. (Sciences), 1901, Nr. 3. (Recueil Inst.
bot. d. Brux. 5, 209.)

SACHS. 1860. Bot. Ztg. 18, 123.

SACHS. 1865. Handb. d. Exp.-Physiol. (Hofmeister, Handb. d. phys. Bot. 4). Leipzig.

SACHS. 1882. Vorlesungen iiber Pflanzenphysiologie. Leipzig.

SCHIMPER. 1888. Die epiphytische Vegetation Amerikas. Jena.

SCHUMACHER. 1867. Jahresb. f. Agrik.-Chemie, 83.

SCHWARZ, F. 1883. Unters. aus d. bot. Inst. Tubingen, i, 135.

STAHL. 1893. Annales Jard. Buitenzorg, n, 98.

STAHL. 1897. Bot. Ztg. 55, 71.

WACKER. 1898. Jahrb. f. wiss. Bot. 32, 71.

WIESNER. 1882. Sitzungsber. Wiener Akad. 86, 40.

WOLLNY. 1897. Forsch. auf d. Geb. d. Agrikulturphysik, 20, 52.

LECTURE IV

TRANSPIRATION

AFTER this brief reference to epiphytes we may now return to the con-
sideration of ordinary land plants, of which our trees and cultivated plants
may be taken as examples. The soil, we have seen, supplies them with the
water they require, and that they absorb by the root ; we must now study
the reverse process, viz. the giving off of water by transpiration from parts
above ground, more especially from the leaves.

No special methods are required to demonstrate this phenomenon, for
just as a free water surface, a sponge saturated with water or damp soil, gives
off water vapour into the air, provided the latter be not itself saturated, so
too must also the plant, containing as it normally does abundance of water.
And just as under natural conditions the water evaporated is not always re-
placed at once, so, too, transpiring plants exhibit great variations in the
amount of water they contain, variations often so obvious as to be noticeable to
the naked eye. Who has not noted herbaceous plants and even trees with limp
leaves or flowers on a hot day in July ? The wilting is simply the indication
of the suppression of osmotic distention of the cell-walls and, consequently,
of tissue tensions, owing to the loss of water. So long as the loss of water keeps
within certain limits, a renewed supply of water can once more induce normal
turgescence ; consequently, we notice that not infrequently during the night,
when transpiration is reduced by lowering of the temperature, the leaves again
become rigid. Not only from such everyday experiences, but also from the
fact that we can prevent wilting by placing the plant in the shade, or by water-
ing it at the right time, even the ' man in the street ' can appreciate the im-
portance of external conditions in determining the amount of transpiration.
Before passing to the consideration of the question as to how far the plants
themselves and how far external factors influence transpiration, we may glance

D 2

36 METABOLISM

at the ways in which the existence of this phenomenon may be proved, and
the more exact methods employed to estimate minute losses of water, not
merely the grosser evidence presented by the process of withering.

Thanks to the large number of experimental researches that have been
carried out on the subject from the days of HALES (1748) up to the most recent
times (e. g. BURGENSTEIN, 1887-1901), we have become acquainted with so
many methods that we must limit ourselves in their enumeration. Evapora-
tion from a plant may be demonstrated in the clearest and simplest way
by observing the dampness deposited on a bell jar placed over it and kept at
a low temperature. The reason for this deposition of moisture is the same as
that for the dimming of a window pane when one breathes on it ; it is nothing
more or less than the deposition of dew on a cold surface. The most exact,
uniform, and quantitative method of proving the existence of transpiration
is to employ a balance. If proper precautions be taken to permit of water
being given off from the plant only, and not at the same time from the
earth in which it is rooted, it may be shown that the decrease in weight
from hour to hour is due to loss of water. It is true that there are other pro-
cesses taking place in the plant which lead to change in weight, still, quantita-
tively speaking, they are insignificant when compared with the change in weight
due to movements of water. The data given on p. 25 as to the amount of tran-
spiration have been obtained by weighing. A third method, extremely con-
venient and useful for demonstration, consists in making use of the alteration in
colour which many substances undergo when they absorb water. STAHL (1894),
to whom we owe the application of this excellent method of investigation, used
strips of filter paper soaked in cobalt-chloride. ' Cobalt paper ' is deep blue
when dry, but becomes red when wet. The method of use is to place a small
piece of the blue paper on the subject of investigation — say a leaf — covering it
up with a glass plate so as to eliminate the influence of atmospheric moisture.
According as the leaf gives off much or little water the paper changes in colour,
after a few seconds, hours, or days. In place of change of colour we may employ
bodies which exhibit hygroscopic movements, such as gelatine (BENECKE, 1899) or
awns of Erodium (DARWIN, 1898), for the demonstration of the same phenomenon.

Many authors, e. g. VESQUE (1877), MOLL (1884), BONNIER and MANGIN
(1884), and KOHL (1886), have employed the potometer figured on p. 30 for
demonstrating transpiration. It will be remembered that with this apparatus
it was the amount of water absorbed — not the amount given off — that was
measured. If transpiration be kept within moderate bounds, however, one
can make out that the two amounts are equal — that the loss due to evaporation
is covered by the amount absorbed. The potometer method has many ad-
vantages ; it is very easily demonstrable, more so if coloured water be employed
in the capillary tube ; it is very convenient, especially if the influence of external
factors on transpiration have to be studied ; it does not necessitate the presence
of roots on the plant — isolated branches are quite sufficient for the purpose.

With the aid of one of these methods let us, first of all, study the effect which
the plant's structure has on transpiration. Observation alone teaches us that the
external walls of the epidermal cells are the parts of the plant first concerned
in the giving off of water vapour. Like all other cell-walls these contain water
of imbibition, and this water is retained with a certain amount of force. Every
particle of water lost by evaporation is replaced by another which the wall
attracts from the protoplasm. The protoplasm in turn abstracts water from
the cell-sap. But the cell-sap also holds the water firmly, and, in consequence
of its osmotic properties, together with the imbibitive energy of the cell-wall
and protoplasm, the outer surface of the plant gives off less water vapour than
an aqueous surface of equal extent under the same conditions. AUBERT
(1892) found that, taking the evaporation from a water surface, per unit of

TRANSPIRATION

37

time, as 1,000, the evaporation from an equal surface of a dilute solution of
gum was 843, of malic acid 837, of glucose 773. An Opuntia, on the other
hand, gave off only ten units of water per unit of time. A special factor must,
however, be taken into consideration, viz. the cuticle, already described in
speaking of the absorption of water, whose influence is in the direction of
retarding transpiration. Since the cuticle imbibes little or no water, it acts
like a film of oil spread over an aqueous surface. The differences between
varieties of cuticle have been already referred to, and these are of the greatest
importance in relation both to the absorption and giving off of water. The
thin and gelatinous external walls of the root and of submerged plants are very
permeable to water, so that these parts readilydry up and wither when exposed to
air, and between this condition and the other extreme, where the cuticle is thick
and practically impervious to moisture, as in hard leathery leaves, every possible
transition occurs. Some numerical idea of the action of the cuticle may be
obtained from a study of some of BOUSSINGAULT'S results (1878). He experi-
mented on apples which were in part provided with a normal cuticle and in part
had the cuticle removed.
A square centimetre of
normal apple surface lost
0-005 g. of water per
hour, whilst the skinned
apple lost 0-277 £•> or
fifty-five times as much.
Such investigations,
however, take for granted
that the cuticle over the
exposed part is a con-
tinuous layer and desti-
tute of all apertures, but
this is by no means true of
all cuticles. In very many
cases the cuticle is pierced
by microscopically mi-
nute but extremely numerous holes ; the otherwise continuous epidermis is
interrupted by special organs, the stomata. Each stoma (Fig. 7) consists of two
cells (guard-cells) which differ from other epidermal cells in their curved
form. Owing to the fact that these cells have their concave sides turned toward
each other, a small slit is left between them, opening on the one side to the air and
on the other into a large intercellular space, known as the ' respiratory cavity '
(see Fig. 7, B}9 standing in direct communication with the general intercellular
space-system in the body of the plant. The spaces found between the cells in
the plant's interior are not, however, completely shut off from each other, but
form an intercommunicating system of chambers and canals, constituting the
aeriferous system of the plant. By means of the stomata this system is put
in direct communication with the atmosphere.

The stomata, the exits of the aeriferous system, permit gases of all kinds
to enter the plant as well as to pass out, and direct gaseous exchange may, by
this means, take place between cells deep in the interior of the plant and the
air. We can easily convince ourselves of the value of the stomata and inter-
cellular space-system for such exchange by placing a leaf from an appropriate
plant in water and arranging that the atmospheric pressure on the end of the
submerged petiole is less than that on the leaf blade. As a result of a quite
insignificant difference of pressure — mouth suction is often sufficient for the
purpose — a continuous stream of air-bubbles may be seen escaping from
the petiole. It can be at once demonstrated that this air has entered by the

Lower epidermis of Tradescantia virginica.

Fig. 7.

) from above ;

two guard-cells centrally placed : /?, in section, showing below the guard-
cells the respiratory cavity, on which the chlorophyll-bearing parenchyma
abuts, x 240. (From the Bonn Textbook.)

38 METABOLISM

stomata, for if these be blocked up by vaseline or tallow the stream of
bubbles at once ceases. It must also be noted that other exits from the
aeriferous system, in addition to stomata, are known to occur in plants.
(' Pneumachodes ' ; compare HABERLANDT, 1896).

At the present moment the only gaseous exchange taking place through
the stomata which we need consider is the giving off of water vapour. It
follows from the structure of the plant as described above that, in addition to
the transpiration taking place from epidermal cells, there must also be a certain
amount of ' internal transpiration ', since each cell wherever it borders on an
intercellular space will give off water vapour, the immediate result being the
saturation of the air in the intercellular spaces. The whole plant thus loses water
whenever water vapour escapes through the stomata from the intercellular
spaces. Obviously only a minute quantity of water can pass to the exterior
through a single stoma since the slit is so small. The diameter of the largest
of them (e. g. those of Amaryllis) is only 0-01-0-02 mm., and the openings are
so minute that a needle prick appears as a huge hole in comparison. The
significance of the stomata in the vital economy of the plant, apart from charac-
teristics which will be studied later, depends on their immense number. In
the situations in which they are most abundant, e. g. the underside of the
foliage leaf, we find on an average from 40 to 300 per sq. mm., but in extreme
cases there may be as many as 625 (Olea) or 716 (Brassica rapa) in the same
area. According to NOLL (1902) an average-sized leaf of Brassica rapa has
no fewer than eleven millions of stomata, while a leaf of the sunflower has
thirteen millions. It must be remembered in this connexion that BROWN and
ESCOMBE (1900) have shown that diffusion of gases through a plate pierced
by numerous fine pores takes place as rapidly as if the spaces between the
pores were non-existent.

We must also differentiate between an epidermal and an intercellular, or, in
other words, between a cuticular and stomatal transpiration, and the difference
may often be recognized by contrasting the behaviour of opposite sides of the
leaf. Many foliage leaves bear stomata only on their undersides, and if the
cuticle be alike on both sides, we may assume that we have a cuticular tran-
spiration taking place from the upper side, and a stomatal transpiration
taking place from the lower as well. Several methods of investigation, most
conveniently perhaps the cobalt-chloride method, demonstrate, however,
that cuticular transpiration is frequently so small that it may be taken as
practically nil. If a piece of blue cobalt-chloride paper be placed on the under-
side of a leaf of Liriodendron tulipifera it becomes red in a few seconds, while
a piece placed on the upper side remains blue for several hours, the general
conditions being the same in each case. Plants which live in damp air, e. g.
the Hymenophyllaceae, possess much thinner cuticles than those which live
in drier air, hence in their case cuticular transpiration is very obvious and
readily capable of demonstration by cobalt paper. The extreme case is shown
by submerged plants and by roots where the permeability of the cuticle (in the
absence of stomata) to water is shown at once by the rapidity with which
wilting takes place.

Even assuming that the nature of the cuticle, and the number and dimen-
sions of the stomata in any plant are known, the absolute amount of transpira-
tion cannot be determined without knowing as well what the external conditions
are, since the amount of transpiration varies extremely with alterations in these.
The way in which many of these external factors affect transpiration is obvious,
for they may be observed just as readily in purely physical experiments with
a substance capable of absorbing water, e. g. glue or filter paper ; in the plant, on
the other hand, the influence of external factors introduces remarkable compli-
cations, due to the fact that the organization of the plant is, in the first instance,

TRANSPIRATION

39

altered by them, and this in turn affects the amount of transpiration. Among
physical influences the dampness of the air holds a foremost place ; its effects
are so obvious that further explanation is unnecessary. Similarly with tem-
perature ; every increase in temperature must cause an increase in transpiration
— every decrease must retard it. If the plant be at a higher temperature than
the environment it can still give off water vapour into the air even though the
latter be saturated ; this higher temperature is attained by respiration or by the
absorption of light and heat rays, of ten aided by the presence especially of colour-
ing matters ($TAHL, 1896). Transpiration is also increased by oscillation, for the
plant is thus taken out of the saturated atmosphere produced over its surface in
consequence of transpiration, and brought into a new region not yet saturated.
The same effect is produced by moving the air rather than the plant, and hence
every breath of wind aids transpiration. The dampness of the soil has an influence
which is somewhat less obvious. Dry soil hinders transpiration because it
retards absorption of water ; owing to the lack of a reserve of water the cell-sap
becomes more concentrated in the transpiring organs, and hence is less ready to
give off water vapour into the air. Concentrated salt-solutions, if the root has
to absorb water from them,
act in the same way as a
dry soil, although dilute
solutions also have an in-
fluence on transpiration
which has not as yet been
fully explained. Dilute
acids retard, and dilute
alkalis accelerate, transpira-
tion. Probably in these
cases the explanation is not
purely physical, but must
be sought for in alterations
in the characters of the
plant itself, having their
cause especially in the acti-
vities of the guard-cells of
the stomata.

We have not as yet discussed this question of the activity of the guard cells,
and so far, our treatment of the subject would suggest that the stomatal aper-
ture was always of the same size. That is, however, by no means the case.
On the contrary, the guard-cells are capable of opening and closing the stomatal
slit according to conditions, and thus of allowing of the most varying amounts
of transpiration, from nothing upwards. Variations in the size of the pore are
attained by a very simple method, viz. by varying the degree of curvature of
the guard-cells. To understand the mechanism of the process it will be
necessary for us to study the structure of the stoma somewhat more in detail
than we have already done. We may select for detailed study the stoma of
Amaryllis, whose structure has been elucidated most thoroughly by SCHWEN-
DENER (1881). Other plants exhibit other adaptations than those seen in
Amaryllis, but the mechanical principles involved are fundamentally the same for
all (compare HABERLANDT, 1896, and COPELAND, 1902). Fig. 8 shows a stoma
of Amaryllis, both in the open and in the closed condition, in surface view
and in transverse section. The latter (Fig. 8, 7) shows the asymmetrical form
of the guard-cells in relation to the line 8, which separates the concave from
the convex side. While the convex half forms almost exactly a half circle, the
outer contour of the concave side is much more complicated, and as a conse-
quence the intercellular space between the concave edges of the guard-cells also

Fig. 8. I-IV, stomata of Amaryllis forntosissinta (after SCHWEN-
DENER). /, in section ; 77, surface view of half-stoma; ///, surface view
of a closed, and IV, of an open stoma ; V, VI, for explanation see text.

40 METABOLISM

presents special peculiarities. On the outside, the lumen is constricted by
horn-like ridges (H) ; then follows an enlargement known as the vestibule of
the stoma. The vestibule again narrows to form the slit proper, followed by a
further widening inwardly — the rear vestibule — once more contracted by a second
pair of ridges. The inner contour does not, however, run parallel with the outer
one, but forms almost a half circle. The concave wall is thus not of the same thick-
ness throughout ; it is relatively thin in the middle and thicker above and below
where the ridges are (see Fig. 8, 7). The occurrence of abundant chlorophyll
in the guard-cells must be specially noted, the ordinary epidermal cells being
as a rule destitute of green pigment. Further, the protoplasm of the guard
cell encloses a large vacuole which is the seat of great osmotic activity. Under
the influence of osmotic pressure the cell-walls are stretched, but the concave
sides, owing to their greater thickness, are more able to resist this extension
than are the convex sides, so that the stretching is more obvious on the latter
than the former. The effect of this differential stretching is best illustrated
by a model. If one takes a caoutchouc tube, completely closed, and having
a strengthening layer pasted along one side, and forces into it air or water,
though straight at first, it becomes bent (Fig. 8, V, VI). Now imagine two
such tubes with the strengthened sides facing each other, united by their ends,
but with the central region free; if turgor be produced in these tubes they will be
seen to separate from each other in the middle. Much the same sort of thing
takes place in the guard-cells of the stoma ; increase of pressure induces the
slit to open, as much as is shown in comparing Fig. 8, /// and IV. We speak of
the stoma in the first case as closed, in the second as open; as a matter of fact,
however, when turgor is at a minimum, the two guard-cells do not lie so closely
together as to hermetically seal the opening, although it is sufficiently obliterated
to make the amount of water vapour which passes through scarcely worth speak-
ing about ; in other words, stomatal transpiration is almost as good as sup-
pressed. By altering the pressure in the guard cells the plant is able to vary
the width of the slit very considerably, and in this way the stomata become
regulators of transpiration to a degree which would scarcely be anticipated.

The width of the slit does not depend entirely on the osmotic pressure in
the guard-cells, but is also influenced by pressure in the cells surrounding
them. If this pressure be eliminated, e.g. by pricking these cells, it will be
seen that the stoma opens at once, although the pressure in the guard cells
is not at the same time increased. Conversely also, increase in turgescence
of the surrounding epidermal cells may bring about a passive closure of the
stoma. Authorities differ in their estimates of the degree to which this activity
in the general epidermal cells is functional in nature. SCHWENDENER (1881)
held that it was of no importance, LEITGEB (1886) believed that it played a
great part in the process, while DARWIN (1898) takes an intermediate position.
So far as we are concerned we may take it that the active movements of the
guard-cells alone are quite sufficient to account for the opening and closing of
the slit. The effect of variations in pressure within the guard-cells may be
readily demonstrated under the microscope. If the preparation shows the
stomata open, addition of a plasmolytic solution will quickly cause them to
close, and they may be made to open again at once by replacing the plasmolytic
solution with water. In nature, variations in pressure so extreme as to lead
to complete abolition of turgor, such as is effected by plasmolysis, do not
occur ; on the contrary a pressure of several atmospheres is maintained even
when the stoma is quite closed.

The conditions under which opening and closing of the stomata take place
are very different in different plants, still, in general, we may say that stomata
in appropriate ways regulate the amount of transpiration, and that the plant
is thus able to protect itself from wilting. Conditions connected with this

TRANSPIRATION 41

regulation of transpiration may undoubtedly occur which are dangerous to
the plant, for we must not forget that the stomata are not merely the organs
of transpiration, but are primarily the means by which carbon-dioxide enters
the plant. Everything that retards transpiration must at the same time
hinder the absorption of carbon-dioxide, and hence clashings may occur be-
tween the two processes, when the plant must naturally suffer from one cause
or the other. Conflicting conditions of this kind are to be met with whenever
we investigate natural phenomena closely.

We have now to study separately the more important factors which play
a part in the movements of the stomata, and foremost among these we may place
the vapour tension of the atmosphere. For purely physical reasons, abundance
of moisture in the air must, as has already been shown, retard transpiration ;
but at the same time it will induce the stomata to open widely, and thus the
physical condition will be in a sense neutralized. In dry air on the other
hand, evaporation is increased, while transpiration is reduced because many
plants close their stomata at once when wilting begins. It is easily seen how the
degree of humidity of the air will act on the guard-cells sooner than on other
cells ; in damp air they attain the utmost osmotic turgor which the cell-sap
is able to exert when water absorption is at a maximum ; when loss of water
takes place owing to increasing dryness of the atmosphere, if at the same time
an abundant supply of water is not forthcoming, a rapid reduction in the turgor
ensues, accompanied by closing of the stoma. Closure of the stoma is, how-
ever, by no means always a concomitant of wilting ; in many plants the wilted
leaf has its stomata wide open. Such leaves continue to give off water, and
shrivelling of the leaf takes place with great rapidity. Plants of this kind can
exist only in regions where the moisture is excessive, as, for example, among
our native plants, those inhabiting marshes (Alisma, Acorus, Menyanthes, &c.)
and shady places (Osmunda regalis). The cobalt-paper method will be found to
be convenient in investigations of this sort, for by this method we may convince
ourselves, for example, of a vigorous transpiration from the half-dried leaf in
the plants above mentioned, where the stomata remain open, while in Tro-
paeolum majus this cannot be observed owing to the rapid closure of the stomata
on wilting. Further, actual wetting will induce wider opening of the stomata
than a damp atmosphere. This explains the result obtained by WIESNER
(1882), who found that there was a marked increase in transpiration after the
leaves had been dipped in water. Under certain conditions a quite contrary
effect may be produced, as, for instance, when the neighbouring cells absorb
water and the stomata are in consequence passively compressed (KOHL, 1886),
or when the pores are blocked by capillary water.

The stomata of different plants do not always react in the same way to
the second factor, which we may allude to here — namely, light. Frequently, e.g.
in Amaryllis, Aspidistra, &c., we may observe that the slits open when the leaves
are more brightly illuminated. Here again, since for purely physical reasons light
furthers evaporation, there is the danger of excessive transpiration. In many
cases withering of the leaves may be prevented in spite of the brilliant illumi-
nation, if the stomata close at the first indication of wilting. STAHL (1894) has
proved that this result does not always follow, however, and it may be shown
that if the leaf of Tropaeolum, slightly wilted, be exposed with closed stomata
to direct sunlight, it does not wither any further even after some hours,
although a fresh leaf exposed under the same conditions dries up rapidly, just
because it does not close its stomata. We may conclude, therefore, that the
action of light on the guard-cells is dependent for its effect on the chlorophyll
contained in these cells. In fact, as we shall see later, the chlorophyll is able
under the influence of sunlight to generate osmotically active substances, and
hence, in a sense, to bring about opening of the stomata. Assuredly, however,

42 METABOLISM

light operates still more in an indirect way, i. e. as a stimulus to the guard-
cells. In many plants closing of the stomata has been observed to take place
on darkening. Darkening must, therefore, also be recognized as a stimulus,
since after osmotically active bodies have been manufactured in light, they
cannot be removed so quickly, after exclusion of light, as to account for the
rapidity with which the stomata again close. According to LEITGEB (1886)
the closing of the stomata in the dark is passive, owing to the guard-cells being
pressed together by the turgor in the neighbouring cells. An increase in tur-
gidity in darkness is a very general and easily explicable phenomenon.

In addition to the effect of light and atmospheric moisture on the width
of the stomatal aperture that of changes of temperature also has received
attention, but into that and related subjects we need not enter, those already
discussed being the most important. In spite of the voluminous literature
on stomata, detailed researches on these organs in representatives of the chief
biological groups of plants are still wanting. We know enough, however, to
be able to affirm without fear of contradiction that in stomata plants possess
a remarkably valuable apparatus for regulating transpiration, provided the
external conditions for absorption and transpiration be approximately normal,
that is to say, such as do not approach the extremes to which the plant
cannot adjust itself. Any attempt to cultivate some of our economic plants,
such as cereals or tobacco, under the conditions prevalent in a desert, or in
the saturated atmosphere of a tropical forest would, undoubtedly prove
a failure. At the same time, a study of plants which are naturally fitted to
live under such extreme conditions discloses to us a variety of adaptations
calculated, on the one hand, to limit transpiration as much as possible, and, on
the other, to further it to the utmost. A brief sketch of these adaptations may
be appropriately introduced here ; the reader is referred for details to the
works of HABERLANDT (1896), SCHIMPER (1898), and STAHL (1893, 1896).

Reduction in the amount of transpiration may be effected by a reduction
in the number of stomata or by an alteration in their anatomical structure.
Further, plants which live in dry regions usually have well-developed cuticles,
whose power of retarding transpiration is increased by the deposition in, or on,
them of wax ; moreover, the capacity for imbibing water may be reduced in such
parts of the plant as are exposed directly to air. Development of hairs full of
air can also effectively retard transpiration, since such a covering protects
the plant from the effects of air currents, producing a superficial region free from
atmospheric movement. In addition to such preventatives dependent on cell
structure we find also adaptations dependent on the form and arrangement
of the parts concerned. A flat extension of the foliage leaf greatly favours
transpiration, more especially if the whole leaf surface be exposed to the
sun's rays ; but plants are known to exist which avoid strong insolation by
presenting the edges of their leaves to the sun (compass plants, Eucalyptus,
&c., Lecture XXXVI), and especially, as need arises, by changing the position
of their leaves with relation to the direction of the incident ray (Lecture XXX VI).
More effective still is the power which certain plants possess of reducing their,
surface by rolling up leaves which are normally extended, or the extended
form may be avoided altogether and a spherical shape assumed (Cactaceae,
Euphorbiaceae, &c.).

On the other hand, in plants which inhabit very damp situations we find
adaptations for furthering transpiration ; as, for example, the special form and
arrangement of leaves for aiding in the rapid drainage of water from their
apices ; coloured cell-sap and consequent increased temperature of the cells
concerned ; thin, easily permeable cuticle ; increase of the epidermal surface ;
complete exposure of the stomatal guard-cells, &c.

It is of the utmost importance that the plant should be able, according to

TRANSPIRATION

43

variations in external conditions, to modify those structural adaptations which
aim at aiding or retarding transpiration. This capacity the plant possesses during
its development only, it is true, within certain definite limits (Lecture XXX).

From what has been said it will be seen that the amount of transpiration
must vary greatly in different plants, and in the same plant under different
conditions. Further, transpiration in individual organs varies very greatly.
[Under ordinary circumstances transpiration from the foliage leaves is so pre-
eminently greater than from other parts that we may really regard them
as the organs of transpiration, and in consideration of the extent of surface
they exhibit and the numbers of stomata they bear, this function is at once
obvious.] Transpiration in the individual organ varies also with its develop-
mental condition. Into these problems we need not enter here, but confine
ourselves rather to inquiring whether such variations are bound up with the
nature of the plant or whether they are accidental, in other words, whether
the enormous exhalation of water vapour seen in certain plants is useful and
essential to their welfare, seeing that others, especially submerged plants, can
do without transpiration altogether. The answers given to this question have
been by no means unanimous ; indeed some have been diametrically opposed
to each other — one author holding that transpiration is a necessary evil (VoL-
KENS, 1887), another that it is vitally indispensable. Our experiences in these
latter days have taught us, however, that there is nothing more likely to lead
to error in the realms of physiology than the making of generalizations, since
in more than one aspect differences in the most fundamental vital conditions
have been shown to exist amongst organisms which outwardly give no hint
of them. Without anticipating a detailed exposition later on, reference may
be made here to certain lower plants, the condition of whose existence is a
medium free from oxygen and which stand out in sharpest contrast to ordinary
organisms which require free oxygen. It would certainly be quite wrong for
us to conclude that, because individual plants can get on without transpira-
tion, therefore transpiration was not essential to any. One thing is clear ; the
entire structure of the land plant necessitates transpiration, since, were it to
cease, the absorption and excretion of other gases would be impossible, and
the life of the plant would come to an end. Plants inhabiting dry climates
show us how far such a limitation of gaseous exchange can go. If we fail to
find such protective adaptations against excessive transpiration in the majority
of plants it must not be concluded that it would be impossible for these plants
to develop such adaptations ; on the contrary, we must hold that they have
no use for them. Finally, if we meet with plants which manifestly possess
adaptations for accelerating transpiration, we ought to conclude that tran-
spiration is a fundamentally important process in such plants. As a matter
of fact, this view may be substantiated in several ways. There can be no
doubt that transpiration markedly aids in the absorption from the soil, in large
quantities, of the salts whose significance we shall have to discuss in an early
lecture. [The influence of transpiration on the absorption of salts in the soil
is very clearly demonstrated by TREUB'S researches on the absorption of
potassium nitrate (1905, Annales Jardin bot. Buitenzorg, 2nd ser. 4, 119).]
These salts are presented to the roots in very dilute solution, and it would
take a very long time to transfer them to the highest branches of a tree by
diffusion only. In reality these salt solutions are transported by way of the
vascular bundles, and are carried right up to the cells of the leaf. Here eva-
poration takes place and a consequent concentration of sap and accumulation
of salts. Moreover, there is another effect of transpiration which must not be
overlooked. The leaves are exposed to sunlight and, since they are able to
absorb light rays by means of their chlorophyll and, in certain cases, by means
of other colouring matters also, they must become sensibly warmer. Obser-

44 METABOLISM

vations show that the temperature of the plant is nevertheless on the whole
about that of the air, and that can be possible only if cooling takes place after
the increase in temperature due to the absorption of light rays. Heat, however,
is necessary for evaporation of water, and hence transpiration must be accom-
panied by reduction of temperature. It should be remembered in this relation
that we make use of this cooling effect of evaporation in everyday life when
we sprinkle water about during a hot summer day, or when we store water
in an unglazed earthenware vessel. If evaporation be thus a regulator of
temperature in the plant, a considerable rise in temperature must be observable
in feebly transpiring plants when exposed to the sun. ASKENASY (1876) was
able to observe very high temperatures in certain oily plants. [URSPRUNG'S
researches (1903, Bibliotheca botanica, Heft 60) may be compared with those
of ASKENASY, which on the whole they confirm.]

Temperature of plant. Temperature of air.
Sempervtvum alpinum 49-3° C.)

51-2° C.I . . . 31-0° C.

Aubretia deltoides
Semperuivum alpinum

arenartutn

Opuntia raffinesquiana
Gentiana cruciata

35-o° C.
52-0° C.
49-o° C. |
43-o° C.
35-o° C.

28-1° C.

While plants like Aubretia and Gentiana reach a temperature very little
above that of the air, oily plants attain a temperature of over 50° C. This
observation is of special interest when taken in conjunction with the fact that
by far the majority of plants cannot endure so high a temperature. We see
also how the power of resisting high temperatures must be a characteristic of
such plants as are capable of living in dry climates, and that by no means all
plants can adapt themselves to such vital conditions.

Bibliography to Lecture IV.

AUBERT. 1892. Annales d. sc. nat. vu, 16.

ASKENASY. 1875. Bot. Ztg. 33, 441.

BENECKE. 899. Bot. Ztg. 57, Abt. II, 130, Note.

BONNIER and MANGIN. 1884. Annales d. sc. nat. vi, 17, 288.

BOUSSINGAULT. 1878. Agronomic, 6, 349.

BROWN and ESCOMBE. 1900. Phil. Trans. R. Soc. B. 193, 223.

BURGERSTEIN. 1887-1901. Materialien zu einer Monographic d. Transpiration, I,

II, III, Verh. d. zool.-bot. Gesell. Wien, 47, 691 ; 49, 399 ; 51, 49.
COPELAND. 1902. Annals of Bot. 16, 327.
DARWIN. 1898. Phil. Trans. R. Soc. B. 190, 531.

HABERLANDT. 1896. Physiol. Pflanzenanatomie, sect. IX. 2nd ed. Leipzig.
HALES. 1748. Statik der Gewachse. Halle.
KOHL. 1886. Die Transpiration d. Pflanzen, etc. Braunschweig.
LEITGEB. 1896. Mitt. a. d. bot. Institut Graz, i, 123.
MOLL. 1884. Archives neerlandaises, 18.

NOLL. 1902. Bonner Lehrbuch d. Botanik, 5thed. p. 157. Jena.
SCHIMPER. 1898. Pflanzengeographie auf biolog. Grundlage. Jena.
SCHWENDENER. 1881. Monatsber. Berl. Akad. p. 833 ; Ges. Abh. 1898, i, 33.
STAHL. 1893. Ann. Jardin Buitenzorg, 11, 98.
STAHL. 1894. Bot. Ztg. 52, I. Abt., 117.
STAHL. 1896. Ann. Jardin Buitenzorg, 13, 137.
VESQUE. 1878. Annales d. sc. nat. vi, 6, 183.
VOLKENS. 1887. Flora d. agypt.-arab. Wiiste, p. 51. Berlin.
WIESNER. 1882. Sitzungsber. Wiener Akad. 86.

[A very complete discussion of all the questions relating to transpiration will
be found in A. BURGENSTEIN'S (1904) Die Transpiration der Pflanzen ; eine physio-
logische Monographic. Jena.]

THE CONDUCTION OF WATER. I

45

LECTURE V
THE CONDUCTION OF WATER. I

SINCE certain parts of the plant give off water and other parts absorb it,
conduction of water must of necessity take place in the regions lying between.
Absorption, evaporation, and conduction of water may be carried out, under
certain circumstances, in different parts of a single cell. On clay soils there
is occasionally to be found an alga, Botrydium granulatum, which consists of
a spherical green body, about the size of a pin-head, attached to the substratum
by a number of colourless branches (Fig. 9). The spherical green region may
be compared to the shoot of a higher plant, and the
branched colourless region to the root, but the whole
structure, being destitute of partition walls, may be
looked upon in a sense as a single cell. Similarly
the unicellular fungus, Pilobolus, has a root-system
distributed throughout the substratum, and a club-
shaped aerial portion at the termination of which
the fruit is ultimately formed. Let us now sup-
pose that transpiration is set up in one of those cells,
which we will assume to be saturated with water ;
obviously the molecules of water will in the first
instance be withdrawn from the membrane covering
the aerial region ; the membrane, in other words, loses
its water of imbibition. In consequence of this, forces
will be generated in the membrane which will induce
a withdrawal of the water in the neighbourhood held
firmly by the protoplasm. The protoplasm in its turn
will seek to make good its loss by absorbing water
from the vacuole, thus bringing about a higher con-
centration in the contents of the upper parts of the
vacuole. Owing to diffusion, however, uniformity in
concentration in the contents of both ends of the
cell is re-established and the disturbance in equili-
brium is transferred to the regions where renewed ab-
sorption of water from the soil may take place.

Let us now consider a somewhat more complicated case by assuming that
the Botrydium has been subdivided into two parts by a partition wall separating
a green transpiring region from a colourless absorbing region. The first effect
of transpiration will, just as above, lead to the concentration of the cell-
sap in the green cell. This cell, however, abuts, not directly on water but on
the colourless cell ; from it, in obedience to the law of osmosis, the green cell
will absorb water, and will continue to do so until the fluids in both cells
attain the same degree of concentration. Equilibrium, however, is never
reached so long as the green cell continues to transpire and so long as the
colourless cell is in contact with water ; for every withdrawal of water from
the colourless cell is followed by absorption of water from the soil, and every
entrance of water into the green cell is succeeded by renewed transpiration. In
this illustration the osmotic attraction arising from the evaporation from one
cell is simply transferred to the other, and thus there is really no essential
difference between the first example we studied and this one. The case be-
comes slightly more complicated, if we take into consideration transpiration
as exhibited by a multicellular fungus, such as Penicillium, a plant which is
partly rooted in the substratum and partly exposed to the air.

Fig g Botrydium
tum. x 25. (After ROSTAFINSKI

es<> first ed"

46

METABOLISM

Fig. 10 represents a small specimen of Penicillium with its horizontally
placed hyphae imbedded in the substratum, and its erect conidiophores pro-
jecting into the air. For our present purpose we may imagine the organism
simplified so as to be represented by the model illustrated at Fig. n, where
the cells of the filament, marked A, B, C, lie in the substratum, whilst the
others, a, b, c, &c., are aerial. If now the cell a loses water in consequence of
transpiration, it will at once endeavour to obtain a fresh supply from cell b.
But b is also transpiring and will, in turn, abstract water from c. That another
cell higher up than b is also endeavouring to obtain water is equivalent to
imagining that b is transpiring more vigorously ; in other words, to the abstract-
ing power of b exerted on c we must add the abstracting power of a also, and
so on until we reach the cells which absorb the water from the medium. In
these cells the converse process takes place to that exhibited by the transpiring
cells. The sucking forces of all the cells, a, b, c, &c., are acting on A, while
A replaces the water which it, in consequence, loses by abstracting water not
only directly from the medium but also from the cell B. In this way the suck-

Fig. 10. Small plant oUPenicilliuiH. (After
BREFELD.) Contr., conidiophores; Co.%
conidia.

Fig. ii.

ing action of transpiration is transmitted backwards to all the cells which
perform the function of roots.

As yet we have spoken only of those water currents in the organism which
arise from the disturbance of osmotic equilibrium, and which continue to exist
so long as there are differences in degree of concentration between the individual
cells. If a large number of cells become united to form a simple or a branched
cell thread or cell mass, we might conclude that any loss of water in consequence
of transpiration from the superficial cells would be made good by water absorbed
from the cells lying below. Looked at by itself this assumption would be correct
if only the rapidity of the water currents due to osmotic activity were sufficiently
great. Even although the pits found in the cell-walls aid in the passage of
water, this is true only under special conditions and in relatively small cell-
systems such as, for example, in Penicillium. Indeed the mode of occurrence
of moulds in nature affords us most valuable suggestions on this subject, for we
know that these organisms are confined to situations possessing a certain degree
of atmospheric humidity ; we find that they prefer to live in small enclosed
areas with stagnant atmospheres, and are quite unable to thrive where there
is abundant aeration, even although all the other conditions necessary for their
well-being are fulfilled. In these plants the absorption of water by osmosis takes
place so slowly, that if the evaporation of water be increased even very slightly
the transpiring cells are unable to obtain from below sufficient supplies to
compensate them for what they have lost, and the plants in consequence perish
from drought.

Under these circumstances one can very well understand that movement

THE CONDUCTION OF WATER. I

47

of water from cell to cell in the higher plant cannot possibly suffice to replace
the loss due to transpiration, and experiments have confirmed this conclusion.
WESTERMAIER (1884) arranged strips of somewhat flaccid parenchyma taken
from the water-bearing tissue of Peperomia and Tradescantia in such a way
that one side only was in contact with water, and observed to what distance
from the surface of the water the cells regained their turgescence. Although
the nature of the external conditions made transpiration almost impossible,
the height to which the water ascended never amounted to more than a few
centimetres. Even the cells which were more than from about two to four
centimetres distant from the surface of the water were unable, by means of
osmotic suction, to attain their normal water capacity. From experiments of
a similar nature, REINKE (1902) concluded that parenchymatous tissues of
submerged plants, if one side be immersed in water, become dried up about two
millimetres above the water level. Such experiments as these, coupled with
observations on small multicellular plants, force us to the conclusion that in
larger plants, trees especially, where the height of ascent of water is to be
measured not by centimetres but by metres, the movement of water cannot
possibly be effected by its transference from one living cell to another, but that
conducting organs with special structure and activity must be forthcoming by
means of which transport of water in quantity may be carried out.

These organs are the vessels. That this is the case may in the first place be
concluded on purely anatomical grounds. The contents of the vessels consist, at
least in part, of water ; their elongated form and the continuity of their lumina
for long distances point emphatically to the conduction of water as their func-
tion. Not less suggestive is their distribution. They begin to appear in the
centre of each root about the same distance from the apex as the roothairs
do on the exterior ; and these roothairs are, as we have already seen, the
water-absorbing organs. They stretch from this point upwards, uniting in
their progress with side conduits from every lateral root, increasing in number
and in total transverse section as they progress from apex to base. On reaching
the stem they pass outwards into every branch, every twig, and every petiole.
In all these organs the vessels are collected together into a few strands, but in
the foliage leaves, the organs of transpiration, we find these cords resolving
themselves into stouter or weaker subdivisions which are distributed through-
out the leaf-blade in the form of a complicated network, so that every trans-
piring cell is in contact with a vessel either directly, or indirectly through the
medium of a few parenchymatous cells. We thus arrive at the important con-
clusion, already recognized as fundamental on other grounds, viz. that trans-
port of water from cell to cell is extremely limited, if indeed the cell bordering
on a vessel can abstract water from it.

Physiological evidence as to the function of vessels may be most readily
found in trees, since, owing to their often great height heavy demands are
made on the vessels in such plants. Further, it is not unusually the case that
both on the main trunk and on the branches there are regions of considerable
extent which possess no lateral organs of transpiration and which are pre-
vented from transpiring by their corky coverings. In such situations transport
of water only, and no evaporation, can take place ; moreover, by breaking the
continuity of the tissues in succession, we can determine with certainty which
anatomical system is especially concerned in water conduction. Comparative
examination shows us that the pith has nothing to do with water transport,
seeing that in many cases it is absent altogether or consists of dried-up cells
filled with air, or of parenchyma, with whose feeble power to act as a trans-
porter of water we are already acquainted. It is quite otherwise with the
cortex and phloem ; there is no want of elongated elements here running con-
tinuously for long distances, e. g. collenchymatous and sclerenchymatous tissue

48 METABOLISM

and sieve tubes. That these elements have nothing to do with the transport
of water in the stem is shown by a simple experiment, first carried out by HALES
(1748) and repeated on numberless occasions since his time and always with
the same result. In order to break the continuity two circular incisions are
made round the stem right into the wood, and the intervening ring of tissue
is removed. If this ' ringing ' be not carried out too extensively, and if due
care be taken that the stem does not become dried up or rotten at the region
of ringing, the leafy crown will remain fresh for a long time, and the transport
of water will not be interrupted to any appreciable extent by the ringing.
We may conclude, therefore, that the conduction of the water is effected by the
wood. In the long run, of course, it is impossible to prevent injuries to the
exposed wood, when its capacity for conducting water will decrease and the
leaf -bearing region above the ring dies after a few years, unless it has contrived
meanwhile to make itself independent of the original root-system by the forma-
tion of a special new root-system above the excised ring. Observations made by
TRECUL (1855) have shown how long a tree can, in spite of such treatment, re-
main alive above the point of ringing. A lime tree at Fontainbleau, for example,
showed signs of life in its apex forty years after ringing had been performed.
From an examination of amputated branches, which, as every one knows,
remain for a long time fresh, and are therefore capable of transporting water,
one is easily able to conclude, by the method of exclusion, that the carriage
of water takes place only by means of the woody tissue. If at the lower end
a branch be cut in such a way that only the cortex, the pith, or the wood
comes in contact with water, it will be found that in the first two cases the twig
rapidly withers, but that it retains its vitality for a long period if the wood only
be immersed. Examination of such branches further enables us to determine
which part of the woody tissue is more especially concerned in water transport.
Although a priori it would appear unlikely that the wood fibres or parenchyma
should be the tissues specially concerned in the transport of water, still direct
evidence that the vessels are the real agents is not at once attainable. The
fact that water does ascend by the vessels, and more especially by their lumina,
may be demonstrated (though the mode of proof is scarcely scientifically exact)
by placing cuttings in a solution of an appropriate colouring matter (e. g. eosin)
and permitting transpiration to take place. From the coloration of the walls
of the vascular elements we may deduce the rapid ascent of the solution in
the vessels, and all the more readily and clearly if we employ transparent white
petals, the vascular network in which appears deeply stained by the pigment
after transpiration has gone on for only a short time. Such investigations
cannot, however, be carried out on the entire plant owing to the fact that the
roots refuse to absorb colouring matters. Further, certain precautions have
to be taken in experimenting with cuttings, of which we shall speak later.
Obviously, then, the fact that the living wood elements are unable to absorb
colouring matters leaves a doubt in our minds as to the validity of such ex-
periments ; for although under these conditions a movement of the colour
solution is on the whole possible only in vessels which are destitute of proto-
plasm, and although its entrance proves more especially that it can ascend
in the vessels, still these facts do not prove that in the uninjured plant it uti-
lizes the lumina of the vessels exclusively for its ascent. Experiments where
the lumina are occluded by the infiltration of foreign substances and thus made
impassable for water are, for this reason, much more convincing. Thus ELFVING
(1882) was the first to place cuttings in cacao-butter, which has a low melting
point, and allowed them to transpire whilst standing in that medium, so that
the butter rose in the cavities of the vessels. ERRERA (1886) employed melted
gelatine for the same purpose. The infiltrated substances were thereafter
coagulated by cooling, and complete occlusion of the lumina of the vessels was

THE CONDUCTION OF WATER. I 49

thus obtained. Plants treated in this way, though once more placed in water,
wilted with great rapidity ; they were no longer capable of absorbing water,
and the conducting power of the stem was destroyed. At the low tempera-
tures at which gelatine and cacao-butter can be used, injury to the living cells
is out of the question, so that it may be considered as certain that the vessels
are essential to the transport of water and that that transport takes place by
the lumen of the vessel, not as SACHS (1879) believed, by its wall. We must
draw attention to the fact, however, that the walls of the vessels as well as the
neighbouring parenchyma cells may also be concerned in the process ; for our
experiment only teaches us that the lumina of the vessels are essential,
but gives us no information as to the participation of other elements in
the conduction. By far the most convincing experiment is that which was
made, first by VESQUE (1883), and later by KOHL (1886) and STRASBURGER
(1891). It is possible, by means of a screw clamp, to compress the lumina of
the vessels and so occlude them, the parenchyma being at the same time almost
entirely crushed. So long as the vessels are squeezed in this way, the stem is
unable to conduct water, but its capacity for doing so returns at once when, by
removal of the pressure, the vessels regain their original shape by elastic recoil
and once more exhibit distinct lumina. This experiment is most effective
if water-cultures be employed, or if it be carried out on cuttings, with the aid of
a potometer. Constriction of the lumina of the vessels at once makes itself
felt, for the absorption of water rapidly sinks to nil ; on the other hand, re-
laxation of pressure is immediately followed by the inrush of water into the
vessels with increased rapidity. This tightening and slackening of the screw-
clamp may be repeated many times, but always with the same result.

Before we make any attempt to examine more closely the ascent of water,
we must inquire how the water succeeds in entering the vessels in the
first instance. In the familiar experiments with amputated branches the
water enters the cut ends of the vessels, very much in the same way as it
enters a capillary glass tube whose open end has been submerged. In the
normal plant, however, the vessels are both at their lower ends, and
laterally as well, completely enclosed by living cellular tissue, and abut upon
other vessels higher up, so that any water that succeeds in entering them must
first of all have penetrated the living tissue by which they are surrounded.
Now we have already seen in Lecture III how the cells of the root epidermis
absorb water osmotically from the soil, and at the commencement of the present
lecture we have determined under what circumstances water travels from cell
to cell. The water absorbed by the epidermis is transferred to the centre of the
root since the cell-sap is in a state of greater concentration there than it is in the
epidermis, and it will continue to be so transferred until a similar osmotic pressure
prevails throughout all the cells of the transverse section. Water in the same
way will pass osmotically into segments of young vessels while these are still
in an embryonic state and possessed of normal cell contents. When, however,
a segment fuses with the next older segment an immediate dilution of its
osmotically active cell-sap must take place since it is essentially water that is
found in adult vessels. The question then comes to be how can water be
abstracted from the cell-sap of a parenchymatous cell and transferred to the
lumen of a vessel ; one would expect the precisely converse process to take
place. Before we attempt to answer this question it will be advisable to make
ourselves thoroughly acquainted with the details of the process.

It is by no means difficult to demonstrate that water is given up to the
vessels by the parenchyma cells. In many cases all that need be done is to cut
off the aerial shoots of a herbaceous plant, when at once, or after a short time,
a quantity of sap may be seen escaping from the wound. Owing to the
turgidity of surrounding parenchyma, sap may also be squeezed out of the

JOST E

5°

METABOLISM

cut ends of latex tubes, sieve tubes, and similar elongated elements, which,
in consequence, lose their turgescence. This particular instance, however, is not
apposite in this relation. Similarly the contents of the vessels may be squeezed
out by the pressure of surrounding parenchyma, more especially if the vessels be
young ; at a later stage the secondary thickenings in their walls renders this im-
possible. Nevertheless it is obvious that only a small amount of sap can pass out
in this way. The following are the amounts of water which HOFMEISTER (1862)
found were ejected from the root-systems of Urtica urens and Solatium nigrum : —

Plant.
Urtica urens

j> »
Solanum nigrutn

Time in hours. Sap given off in cmm. Volume of root in cmm.

99
40
48
65

3025

11260

1800

4275

1350
1450
1530
1900

These observations are specially valuable in consequence of the fact that
the volume of the roots from which the water was
excreted was noted as well. The table shows that
in a few days the root gives off several times its own
volume of water. During that time it must have
been absorbing new supplies of water from the soil,
and it can, after even a longer period of activity,
still maintain its original amount of liquid contents.
Similar excretions occur elsewhere, not only
directly from roots, but also from stems and
branches, if these be cut off from the plant or
if borings be made down to the woody cylin-
der. The phenomenon of ' weeping ' or ' bleed-
ing', as the gardener terms it, which occurs in
spring-time, notably in the vine, when the stem is
cut, is well known to every one. This phenomenon
we may also designate by the term ' bleeding '. It
has long been known that the sap is ejected from
the plant often with very considerable force (' bleed-
ing-pressure ' or ' root-pressure '). Long ago,
physiologists, e. g. HALES (1748), measured this pres-
sure in essentially the same way as we do to-day.
A glass tube, bent twice in the form of a U, is
fitted to the stump of a root (Fig. 12) and filled
with water just above the cut surface and then
closed with mercury. By noting the height to
which the mercury is driven in the open leg of the
tube it is possible to estimate the amount of root-
pressure.

Our next task must be to attempt a more exact
estimate of the quantity, quality, and pressure of the sap expressed from such
wounds.

On submitting the sap which has been collected to analysis, we find it to
consist of water in which are always dissolved organic and inorganic substances
although in very varying proportions. The extremely watery sap of the potato,
the sunflower, and the vine, contains from I per cent, to 3 per cent, of solid in
solution, of which two-thirds in the case of the vine, half in the sunflower, and
one-third in the case of the potato is organic in its nature. The inorganic salts
of the sap are exactly the same as those which we have already met with in the
plant, while among the organic compounds, acids, proteids, and especially
sugars occur. In concentrated sap the various sugars markedly predominate;

Fig. 12.
by means of rub

The glass tube,

bEer tubing, c} t<
stump of a Dahlia, s, and filled with

is fixed
to the

water (W) and mercury (Q). (From
the Bonn Textbook.)

THE CONDUCTION OF WATER. I 51

thus 1-4-1-9 per cent, of sugar has been found in the birch, in Acer plata-
noides 1-2-3-2 per cent., in Acer saccharinum 3-6 per cent., in Agave americana
as much as 8-8 per cent. (SCHRODER, 1869).

The amount of sap excreted from day to day varies very considerably ;
sometimes it amounts only to a few drops, at other times it reaches several
litres. It may not be without interest to present here, in tabular form, some
of the maxima which have been observed (compare PFEFFER, Phys. I, p. 240 ;

WlELER, 1893 ; MOLISCH, 1898).

Plant. Observer. Amount per day in litres.

Vitis aestivalis . . . . (CLARK) ...... 0-227

Vitis vinifera .... (CANSTEIN) ..... i-o

• • • .; • 8*

Birch ...... (WlELER) . . . . V 5-1

Ostrya ...... (CLARK) . . * '','*. . 5-6

Birch ...... (CLARK) . . . > „ .6-8

Agave americana .... (HUMBOLDT) * , . 7.5

Phoenix dactylifera . . . (SEMLER). ...';. 8-0-10-0 (a)

Musanga ..... (LECOMTE) . . . -' * . 17-0(6)

Caryota urens . . . . (SEMLER). . . *•'.»''. 50-0 (a)

(a) Whether SEMLER'S results are to be depended upon must remain an open question.
MOLISCH did not obtain such large amounts.

(b} Calculated on the results obtained during 10 hours.

The maximum outflow is not generally attained at once upon making the
incision ; usually there is at first a gradual increase in the amount excreted,
followed later by decrease ; we are unable, however, to attribute these varia-
tions to any definite external causes. BARANETSKY (1873) has shown this rise
and fall very clearly in numerous tables which he has compiled, and MOLISCK
(1898) also has given us the following numbers as applicable to Arenga sacchari-
fera, where the amounts excreted in ccm. during fourteen days are indicated : —

i 2 3 4 56 7 8 9 10 ii 12 13 14
Day . . . 440 500 1500 1400 1300 2050 1640 — — — — —
Night. . . 675 1080 2175 29°° 335° J35o — —

Total in 24 hours 1115 1580 3675 4300 4650 3400 - 1440 3600 2500 1140 700 175 o

From an examination of the table it will be seen that, taking into considera-
tion the amount of sap excreted in the twenty- four hours, there is a gradual
increase up to a maximum which is reached on the fifth day, followed by a de-
crease until the fourteenth day, when excretion ceases. It is also worthy of note
that the decrease is by no means gradual and regular, but that a second maxi-
mum occurs on the ninth day. It is possible that the second rise may have
been induced by external conditions, but in any case irregularities such as these
exhibit themselves repeatedly and even more strikingly in experiments carried
out in the laboratory under the most equable conditions, so that we are justified
in assuming that the plant works irregularly owing to internal causes. There
is yet another fact which the table teaches us, and that is, that the amount
excreted by day is considerably less than that excreted by night.

Great variations also exhibit themselves in the duration of the outflow
after the infliction of the wound. In palms the excretion often continues for
two or three months ; in Arenga it lasts for several years, and in Agave ameri-
cana, whose sap (as in the case of palms) is used for the preparation of an
alcoholic drink, the bleeding may continue, according to HUMBOLDT, for four or
five months. The outflow continues for a shorter period in our indigenous trees
(one month), and is most limited in small plants, where it may last for a few days
only. Generally speaking, however, the lower limits of the duration of bleeding
have not as yet been accurately determined. As a matter of fact, changes,
which bring about an occlusion of the lumina of the vessels and a consequent

E 2

52 METABOLISM

stoppage of the outflow, frequently occur on the cut surface, due not only to
the activity of the plant itself, but also to the action of Bacteria. If a fresh
surface be exposed, a renewal of the bleeding may not infrequently be observed,
but this precaution has certainly not been taken in all experiments. Finally, it
must be remembered that sooner or later excretion from a root-stump must
come to an end, since the root itself dies when deprived of the nourishment
which it would normally receive from the leaves.

Since, as we have seen, the duration of the bleeding and the amount of
sap excreted daily show very marked variations, both specific and individual,
it follows that the amount of sap excreted during the entire bleeding period
must also vary to a very considerable extent. In the case of palms and Agave,
where the bleeding is both excessive and long continued, the amounts which
have been recorded are enormous. Thus Agave can, according to HUMBOLDT,
give off in round numbers 1000 lit., a single axis of inflorescence of Arenga,
according to SEMLER, produces 250 lit., although MOLISCH (1898) obtained from
the same plant only 18-29 lit. Equal or even greater amounts have been
obtained from our native trees. WIELER (1893) obtained, for example, 36 lit.
from a birch tree in 8 days. Finally, with reference to the pressure exerted,
WIELER (1892, 122) has given us a summary of a large number of determina-
tions which he obtained on the subject of maximum pressures. At the present
moment we cannot do more than quote a few selected examples from his tables.
Low pressures occur in herbaceous plants ; thus Petunia gives a pressure of
7 mm. of mercury, Chenopodium, 16 mm., Ricinus, 334 mm., Urtica dioica,
462 mm., the vine from 900-1100 mm., and finally the birch, 1390 mm.,
while CLARK obtained in Betula lenta as much as 1924 mm. These heights of
the mercury column may be expressed in atmospheres, by saying that in
Ricinus the pressure is equal to half an atmosphere, and in Betula lenta to
two and a half atmospheres, and these appear to be the greatest pressures
which can be attained as a result of normal root-bleeding. Under certain con-
ditions, of which we shall have to speak presently, much higher pressures
than these may be reached. Thus FIGDOR (1898) has recorded in the stems
of certain tropical trees pressures of 6-8 atmospheres, and BOHM (1892) and
MOLISCH (1902) have obtained respectively pressures equal to 8-6 and 6-4
atmospheres in our native trees.

Just as the amount of sap excreted increases gradually, so also we find
that the pressure does not reach a maximum all at once ; at first it gradually
rises and, later on, gradually falls. The periodic variations in the amount of out-
flow are indicative of correspondingly periodic fluctuations in pressure. In
addition to daily and yearly fluctuations, irregular variations have also
been observed. Although we must assume that the variations in the amount
of outflow are due to the same causes as variations in pressure, still it does not
follow that other and closer connexions exist between them. A large amount
of water may be excreted though the pressure be quite low, and conversely
high pressures may be accompanied by the excretion of very little water. This
latter condition is exemplified in the case of the specially high pressures just
cited (MOLISCH, 1902), where obviously only a few cells are concerned in the
excretion of water, because these cells are separated off from their surroundings
by layers of cells which are impermeable to water, and in such a state of affairs
a high pressure may be readily attained. It is not at all improbable that there
are individual cells in the ordinary root which allow water to filter through
equally energetically ; since other cells exposed to this pressure also permit water
to pass, what we obtain by aid of the manometer is not the maximum activity
of the individual cells but the resultant of secretion plus nitration.

It is particularly striking to note that when several manometers are con-
nected with a stem at different levels (BRUCKE, 1844), the decrease in pressure

THE CONDUCTION OF WATER. I

53

from below upwards is by no means always regular, and further that the pres-
sure variations in the individual manometers are often quite independent of
each other. This phenomenon is explained especially by the fact that bleeding-
pressure under consideration may exist not merely in the root, but also in a number
of other places, in the stem, in the leaves, in the axis of inflorescence, and so on.
Although the regions where bleeding occurs may be quite near to each other, it
by no means follows that they are in uninterrupted communication with each
other, and this condition is applicable to two nearly related regions in the
xylem of a tree. Why intercommunication in this latter case is not unrestricted
we shall discover when we consider the external conditions of bleeding.

The first and most general condition of bleeding is the occurrence of living
cells in the neighbourhood of vessels. Death of the plant stops all further
bleeding, and certain stimuli, which diminish its vitality without actually pro-
ducing death, temporarily, at least, retard the phenomenon. For instance,
WIELER (1893) brought the bleeding at once to a standstill by stopping the
supply of oxygen, that is to say, by inhibiting respiration, and chloroform has
the same effect. We may deduce, therefore, from these facts that bleeding
is a vital phenomenon.

A second important condition of bleeding is that the cells which exhibit
this phenomenon must be abundantly supplied with water, and this is effected by
aiding absorption and retarding evaporation. The soil in which the roots lie must
be well moistened, and the air kept saturated with the object of reducing tran-
spiration. In our native trees bleeding may be observed most conveniently
in early spring before the leaves come out, because, owing to the activity of
the root, all the cells are saturated, and also because loss arising from transpira-
tion is at a miminum. If, on the other hand, the tree be felled in summer, we
find that, even after the soil has been well watered, not only does the cut
surface exude no fluid, but that water poured upon it is at once greedily ab-
sorbed. But if, finally, a plentiful supply of water is collected by the root-
system, then bleeding shows itself, and a positive root-pressure may be induced
where previously a negative pressure, that is to say, a pressure less than that
of the atmosphere existed.

A third condition of bleeding is a certain temperature, differing markedly
of course in different plants. At o° C. very few plants exhibit bleeding ; others,
such as the gourd, begin to bleed at 7°-o,0 C., whilst in all plants an increase
in the amount of fluid given off accompanies an increase in temperature.
Detailed investigations on this subject are, however, still wanting.

In addition to the three factors referred to above, there is yet another
which is of some significance, viz. light, but this factor we need not discuss.
Variations in these conditions are accompanied by corresponding variations
in the amount of fluid excreted, and in the pressure exerted by the sap,
and it would be natural to attribute the periodic fluctuations described above
to the influence of these external factors. The researches of BARANETSKY
appear to support this view, but those of later- investigators scarcely confirm it.

The individual eccentricities are indeed quite incomprehensible. One of
the most striking illustrations of this is given by WIELER, who has recorded
diametrically opposite results arrived at from experimental observations made
on two plants of Alnus glutinosa, which were not only of the same age but were
similarly treated and studied under the same external conditions. While the
one exhibited a minimum exudation of sap in the forenoon and a maximum
in the afternoon, the other gave precisely converse results. When we recollect
also that the same investigator was unable to demonstrate any periodicity at
all in the birch, we can have no hesitation in concluding that we are still far
from having reached a satisfactory explanation of this phenomenon.

To the factors above mentioned it is customary in many cases to add yet

54 METABOLISM

another, viz. the wound inflicted on the plant. Very frequently the bleeding
commences at once after the incision has been made, and then it is mani-
fest that the vessels were already charged with water, and that the incision
merely provided an opening for the exit of the fluid ; but in other cases it is not
so, for the exudation may not begin to take place until some time after the
wound has been made ; in such cases the actual wounding itself must have
been the cause of the exudation. Until quite recently it was customary to
attribute the exudation of sap containing large quantities of sugar from in-
cisions made into the young axes of inflorescence of certain palms (Cocos nucifera,
Arenga saccharifera, &c.) to the influence of root-pressure. According to
MOLISCH (1898), however, no such root-pressure exists in these palms, and no
sap exudes either from the stumps of the trees when felled, nor from auger
holes bored in their stems ; nor is there any excretion from the inflores-
cence itself, if only a simple incision be made in it. In Cocos nucifera bleed-
ing first begins to take place after the apices of the inflorescence have been
cut for several days in succession, and in Arenga a much greater effect is
produced if, during the four or five weeks previous to flowering, repeated
bruises be inflicted on the base of the boll by blows from a wooden hammer
(a practice followed by the Malays) ; then, when the inflorescence is cut off,
secretion begins at once. Nor are these altogether isolated observations.
BOHM (1892) was the first to observe the high pressures mentioned above
(more than eight atmospheres), which might be obtained from our native
trees by means of manometers fixed for a long period in auger holes bored
in their stems ; and MOLISCH (1902) has drawn the conclusion that these
pressures have nothing to do with root-pressure, seeing that the trees at
the time of the experiment were covered with leaves, and when ex-
perimented upon with fresh auger holes gave, as indeed might have been
expected, no positive pressure at all, sometimes even a negative one. Here
also the secretion was, in the first instance, traceable to the actual wound
itself and produced gradually and in its immediate vicinity. It originated,
in all probability, from cells which arise or are stimulated to further growth in
consequence of the wounding. At the same time various infiltration products
enter the lumina of the vessels in the neighbourhood of the wound and render
that part of the wood quite impermeable to water. It is quite evident, there-
fore, that a marked pressure may be produced purely locally, although a scarcity
of water prevails in the immediate vicinity. MOLISCH speaks of such cases as
local pressures, and it is very probable that local bleeding may take place not
merely in palms and other trees above mentioned, but generally in all cases
of amputated branches and leaves, where the existence of bleeding has been
established (PiTRA, 1878), whilst what may be termed normal bleeding is associ-
ated, perhaps, with the root only.

That bleeding, i. e. the unilateral excretion of water from parenchyma
into vessels, is an osmotic phenomenon, has been generally accepted since the
time of DUTROCHET. In order to obtain an explanation of this unilateral
excretion, however, we must now study examples of plants which exude sap
of very weak concentration, and we may assume that the cells concerned are
lined by protoplasm which is completely impermeable to the substances dis-
solved in the vacuole. Then comes the question, how can a unilateral expres-
sion of water take place from such a turgescent cell ? In an ordinary cell the
all-round endosmosis of water is balanced by the exosmosis caused by the
pressure from within ; as in one place more water will exude than enter, so in
another place more v/ater will flow in than flow out. In order to explain the
varying behaviour of the different sides of the cell it has been the custom
hitherto to ascribe it to the different qualities of the plasmatic membrane,
since it was believed that the degree of osmotic pressure was dependent
on the character of the plasmatic membrane. If one half of the cell possesses

THE CONDUCTION OF WATER. 1 55

a membrane which gives a lower osmotic pressure than the other half, then on
that side undoubtedly there will be a continuous outflow of water. This view
has been, however, demonstrated to be fundamentally erroneous. As PFEFFER
(1890, p. 303) has shown, and as is itself obvious from the kinetic theory of
osmotic pressure, the degree of pressure depends only on the number of mole-
cules plus free ions, and not in the least on the quality of the protoplasm ; for
a precipitation membrane, no matter how variable it may be, chemically and
physically, no matter whether it be thin or thick, must always give the same
pressure so long as it is impermeable. PFEFFER' s (1877) explanation, on the
other hand, may be looked upon as the physically correct one. If at different
points in the cell different degrees of concentration of the cell-sap arise, the
inflow on the side of greater concentration must still exceed the outflow at
the moment when equilibrium between the two has been already established on
the other side ; the result is a unilateral outflow of water under a pressure
corresponding to the difference in concentration on the two sides of the cell.
Such a difference of concentration cannot obtain in physical experiments,
since, in consequence of diffusion, readjustment of the balance must necessarily
take place. If it occur and be also maintained in the plant, however, an expen-
diture of energy is clearly essential, such as the living cell can always furnish,
but which a physical apparatus ( PFEFFER' s osmotic cell) does not possess.
This is quite in accordance with what we have already learned, viz. that uni-
lateral secretion of water is at once inhibited by the withdrawal of oxygen or
by the action of chloroform, whereby the cells are transformed into non-living,
purely physical pieces of apparatus.

An entirely different theory as to the cause of unilateral excretion has been
advanced by GODLEWSKI (1884). He postulates rhythmic and continuous varia-
tions in osmotic pressure, during which the osmotically active substance is
always breaking down and as constantly being built up afresh. At each lower-
ing of the osmotic pressure there is an excretion of water, owing to the con-
traction of the elastically distended wall, and should these contractions follow
each other at longer or shorter intervals, it may be said that the cell exhibits
pulsations. Although there is much to be said in favour of this idea, looked
at by itself, its absolute correctness may be called in question since, in the
first place, there seems to be no good reason why the water, in these pulsations,
.should always be forced out on one side only, and, in the second place, there
would seem to be every reason for the re-absorption of the water secreted
immediately on the re-formation of the osmotically active substance.

A third explanation which has been advanced lays special emphasis on
the concentration of the sap. The assumption is that outside the cell which is
excreting water there arises, either in its membrane or in the membrane of the
vessel, an osmotically active substance which withdraws water by osmotic suc-
tion from the cell ; the cell itself would, according to this view, act in an entirely
passive manner during the process. That such a phenomenon does take place
in nectaries has been definitely proved, but whether it also plays a part in the
process of bleeding is very doubtful. WIELER has estimated that the osmotic
pressure of bleeding may rise in the birch to two and a half atmospheres, so that
it is quite legitimate to consider the pressure as due in this case to osmotic
action ; and it is still more feasible to make this assumption in the case of
the much more sugary sap of Acer, Agave, and Palmaceae. But it is quite
impossible to conceive that bleeding-pressures in general are in this sense
osmotic, since no definite relation can be observed between the amount of the
pressure and the concentration of the sap. In the vine, for example, high
pressures may be accompanied by a low concentration of sap. Moreover,
WIELER has conducted a series of experiments on the subject, and has found
that when he allowed osmotically active solutions to soak into the vessels of
the root-stump the bleeding was not increased at all.

56 METABOLISM

Then again, it is very difficult to believe that an amount of sugar as great
as that which may be obtained from palms, Agave., and other plants, comes
from the cell-walls ; obviously it must have been formed in the interior of the
cell, and, inasmuch as it passes through the protoplasm in order to reach the
exterior, the plasma cannot be so impermeable as we have hitherto been
led to believe. So soon as we postulate a unilateral permeability for the proto-
plasm, however, the conditions necessary for unilateral excretion of sap are
fulfilled, because in this way a permanent difference in the concentration of
cell-sap on the different sides of the cell is established. If the qualitative difference
in the plasmatic membrane at different situations in the cell consists in its
being impermeable on one side, on the other partially so, unilateral excretion of
water becomes possible ; but the fluid that niters through is always cell-sap,
however dilute it may be, and never pure water.

We are not in a position at present to advance conclusive evidence in favour
of one rather than another of the explanations which have been put forward
to account for bleeding ; but after due consideration we are inclined to favour
the view that bleeding is to be accounted for by differences in the concentration
of cell-sap on two sides of the cell, differences which are often produced and
maintained by expenditure of energy on the part of the cell itself, often also by
unilateral permeability of the protoplasm.

Bleeding, so far as we have considered it, though of the greatest impor-
tance to the plant physiologist, cannot be regarded in any other light than as
an injurious and even pathological process to the plant itself. No matter
whether it be pure water or a concentrated sugar solution which escapes from
the wound, the plant is always suffering a loss of material without receiving
any compensation. In the uninjured plant we find that an excretion of
water takes place under pressure into the vascular strand. It is true we are,
as a rule, unable to prove this fact directly, since the exudation of water is
manifested only after the infliction of the wound ; but the fact that at certain
times and in certain plants excretion of water takes place immediately upon
the cutting off of a branch, proves indubitably that a bleeding pressure exists
in the plant, although no incision has been made. T. HARTIG (1853, 1862) has
already observed that in spring, before the unfolding of the leaves, sap exudes
from the buds of the hornbeam and other trees, although no lesions are visible.
STRASBURGER (1891, p. 840) has also more recently demonstrated that this
extravasation of drops is a result of bleeding pressure, and that drops exude
from the apices of the leaves of the previous year, whose cuticle has been burst
open. This feature is by no means an annual one, nor is it observable in all
hornbeams, hence we must conclude that an especially high pressure is neces-
sary, not only, in the first place, to force the water up to the apices of the
branches, but also, in the second place, to overcome the opposition which the
leaf apices present to the outflow.

What is in arboreal plants the exception is the rule in many herbaceous
plants ; for under favourable conditions, especially excessive dampness of the
soil and reduced transpiration, conditions found during the night, water is
forced into the whole vascular system of such plants, mainly through the activity
of the root, with such force that an exudation of drops takes place wherever
opposition to filtration is reduced. A noteworthy example of this phenomenon
is furnished by the leaves of Colocasia antiquorum, and other Aroidaceae, e. g.
Remusatia vivipara show it also. The drops in these cases exude exclusively
from the apices of the leaves and succeed each other very rapidly. In Colo-
casia, DUCHARTRE (1859) counted ten to fifteen, and, in extreme cases, as many
as thirty, drops falling from the leaf-apex every minute. Each drop, more-
over, was formed from the coalescence of five to six smaller drops expressed
simultaneously from the leaf, so that the leaf gave off a maximum of 180 drops
per minute, or three per second, and the total quantity of fluid which could be

THE CONDUCTION OF WATER. I

57

collected in the course of a single night amounted, generally speaking, to about
10 g. and might reach as much as 22 g. The sap contains very few substances
in solution, for BERTHELOT, who at DUCHARTRE'S suggestion undertook an ana-
lysis of the fluid, was unable to do more than demonstrate the presence of the
merest traces of organic and inorganic materials in as much as 400 g. ; we may,
therefore, speak of the fluid, for our present purpose, as practically pure water.
The same phenomenon, differing from that seen in Colocasia only in degree,
occurs in many of our indigenous and cultivated plants. After a warm night,
small drops resembling dew are to be seen at the apices of leaves, on the leaf
teeth, and, more rarely, on other parts of the leaf. It is quite easy to show
that these are in no sense dewdrops, but are the result of the activity of the
plant itself, for they occur frequently only on the young leaves, while there
is no reason why dewdrops should not appear on the older leaves as well. The
drops increase gradually in size, fall off, and are once more replaced, but they
never aggregate into such quantities of fluid as are given off by Colocasia.
Well-known examples of the excretion of drops are furnished by the leaf apices
of grasses, the leaf teeth of Fuchsia, Alchemilla, Brassica, and the potato ;
while Tropaeolum and many Urticaceae and Moraceae give off water not only
from the edges of their leaves but from the surfaces as well.

Fig. 13. Water stoma of Vicia faba
in section, sp. the stoma ; g, vessels
abutting on the intercellular spaces.
(After HABERLANDT, 1895, pi. 3.)

Fig. 14. Longitudinal sec-
tion through a leaf tooth of
Primula stnensis. Sp^ stoma;
Ep^ epithem; G, vessels. (After
HABERLANDT, 1^95, pi. 4.)

This excretion of water is effected by special organs, the so-called ' hyda-
thodes '. These differ, as a rule, from ordinary stomata in being larger and in
having immobile guard-cells ; for these reasons they have received the special
name of ' water-stomata '. They occur singly or in groups in the situations
where excretion of water takes place, and it is through them that the water
escapes which has accumulated in the underlying space corresponding to the
respiratory cavity of the ordinary stoma (compare p. 37). Usually the
ends of the vascular bundles only lie in close relation to these cavities.
In the simplest cases (grasses and Vicia sepium, Fig. 13) the ultimate tracheids
run immediately under the respiratory cavity, and frequently small superficial
tracheids border directly on this large intercellular space, or are separated
from it only by loosely arranged parenchymatous tissue ; the parenchyma
which immediately invests the vascular elements is practically destitute of
intercellular spaces. In the more highly developed organs which occur in
Fuchsia and Primula (Fig. 14), the tracheids open out at the ends of the bundle
in a brush-like manner, and the spaces between the individual tracheids and
also the not inconsiderable gap between the ends of the bundles and the water-
stomata are filled by parenchymatous cells. This parenchyma (epithem : Ep,
Fig. 14) is composed of cells which are much smaller than those of the meso-
phyll, and is not infrequently deliminated from it by a cuticularized sheath.

58 METABOLISM

There are marked intercellular spaces between these cells, spaces which appear
to be always full of water, even though no secretion is taking place.

In the plants which have been referred to (the Urticaceae and Artocar-
paceae, perhaps, excepted) the epithem plays the part of a filter. It is only
when a positive pressure has been developed in the vessels that an excretion
of water begins, and naturally this excretion takes place at the region
of least resistance ; the water leaves the tracheids, enters the intercellular
spaces of the epithem, and escapes thence to the exterior. This process is
facilitated by the same external factors which aid in the raising of the
bleeding-pressure and the exudation of drops, and hence we are able to
induce this exudation by elevating the temperature sufficiently, and by
moistening the soil and the air, at times when, under normal conditions, the
plant exhibits no such phenomena. It has long since been demonstrated
that the phenomenon we have been considering is due to the pressure of sap
in the vascular system, and that if water be forced into the cut end of
a branch by means of a column of mercury, say 20 cm. in height, an
excretion of drops may be at once induced. If instead of employing water
we use a watery solution of a colouring matter incapable of penetrating the
protoplasm (MoLiscn, 1880), the coloured solution is observed to issue from
the leaf teeth unaltered, indicating that the epithem, in so far as it consists
of living cells, takes no part in the filtration. HABERLANDT (1894) has demon-
strated this fact perhaps even more clearly by killing the epithem by means of
corrosive sublimate, and showing that the excretion of water continues not-
withstanding. On the other hand, he showed that in the Artocarpaceae and
Urticaceae the excretion of water ceased on the death of the epithem cells,
and for that reason he attributed to them in these cases a special capacity for
actively excreting water. This has not, however, been confirmed by other
investigators (e. g. SPANJER, 1898), so that at present it may be considered as
doubtful whether two types of epithem occur in plants, a filtering epithem and
an actively secreting epithem.

The existence of ' actively ' secreting hydathodes is, however, well estab-
lished. Many epidermal cells, i. e. such as grow out into epidermal hairs, are
known as hydathodes. Examples of these occur in the hairs found in the hollow
leaves of Lathraea (GoEBEL, 1897, HABERLANDT, 1897), and especially in the
secretory hairs of many insectivorous plants. The fact that there is, as a rule,
no vascular bundle directly connected with these hairs renders it apparently
impossible that the excretion of water can take place in these cases by simple
filtration. It is obvious that these hairs obtain their water supply osmotically,
and that their cells are possessed of an osmotic activity comparable in all respects
to that exhibited by those parenchymatous cells of the root which excrete water
into the vessels in that organ. The difference is only one of situation, and we will
describe as water glands all organs which exhibit unilateral excretion of water.

Owing to its chemical characters we have previously described the sap, which
exudes from the leaf apices of Colocasia, simply as ' water '. Although this no-
menclature may be appropriate in the case of Colocasia, it is by no means always
so in the case of other hydathodes, whether these be active or passive. Very
frequently the fluid contains carbonate of lime, which makes itself apparent
in the form of separate crystals or complete incrustations, after the evaporation
of the water. Examples are well known to occur in the filtering hydathodes
of species of Saxifragaceae, where the occurrence of scales of carbonate of lime
in the depressions into which the water filters is well known. The hairs of
Lathraea, as already mentioned, are especially good illustrations of actively
secreting hydathodes. In these cases the protoplasm must undoubtedly be
permeable to lime salts, just as it is permeable in other cases to other salts.
Thus in many Tamaricaceae and Plumbaginaceae peculiar glands are found,
by the activity of which these plants become covered over with a greyish

THE CONDUCTION OF WATER. I 59

incrustation of a saline nature which has the power of absorbing hygroscopic
water (MARLOTH, 1887). Hairs which give off acid secretions have frequently
been described, e. g. by STAHL (1888), in Cicer arietinum, Circaea lutetiana, and
Epilobium hirsutum, while they are widely distributed in insectivorous plants,
where, in addition to an acid, a proteolytic enzyme is also present (compare
Lecture XV). In many Fungi also, both unicellular types, such as Pilobolus,
and multicellular (Penicillium, Peziza sclerotiorum, Merulius lachrymans, Clavi-
ceps purpurea, &c.), drops of fluid have been observed to be excreted, which on
analysis are often found to be rich in organic materials, such as oxalic acid and
sugars of various kinds. The excretion of sugar is a frequent phenomenon in
the higher plants in the nectaries which, although they occur more especially in
the floral organs, are found on vegetative organs as well. There is practically
no difference in fundamental character between the water glands we have hitherto
been discussing and these nectariferous cells, so that, beyond referring to the
literature on the subject (WILSON, 1881, PFEFFER, 1892, BUSGEN, 1891, HAUPT,
1902) we need not go further into the question, more especially as it would
lead us too far away from our present subject.

The general survey of all the phenomena, which we may group together
under the head of exudation of water, shows us how varied these processes are,
looked at from a purely physiological point of view ; it can scarcely be
expected, therefore, that they will all fulfil the same or even similar functions
in the plant. The significance of nectaries is best known ; they induce insects
to visit the plant, and thus in very many cases facilitate the transport of pollen
to the stigma. Quite as familiar is the significance of the secretion which
occurs in insectivorous plants, a secretion which we shall have to study in
detail later on. In this case the secretion aids in the digestion of captured
insects, and in most cases indeed it is produced only when the opportunity
for such digestion occurs.

It is not so easy to ascribe biological meanings to some of the other secre-
tions mentioned in the preceding pages. If, along with the water, large amounts
of common salt or of carbonate of lime are given off, we may assume that the
plant is in this way ridding itself of superfluous or injurious substances. The
lime, it is true, may be found in many plants deposited within the body either
as oxalate in the cell-cavity, or as carbonate in the cell-wall ; but it would
scarcely be correct to conclude that when it is excreted to the exterior its actual
excretion serves any special purpose, the more so as silicic acid does not lend
itself to such excretion. Common salt, on the other hand, as we shall have
occasion to see later on, may cause direct injury to the plant, and since, in its
case, the precipitation of the fundamentally potent element chlorine cannot
be effected by the formation of an insoluble compound, there is no difficulty
in apprehending the advantage the plant obtains from its definite excretion.

It is quite otherwise with the excretion of pure, or almost pure water. In
this case the actual removal of the water as such from the plant cannot be the
object to be attained, any more than the removal of water vapour can be the
most important aim in the process of transpiration. If we have rightly appre-
hended the significance to be attached to the rapid movement of nutritive salts,
it will be at once apparent that this exudation of water in the form of drops
must be regarded as a process which takes the place of transpiration in
situations where transpiration is, for other reasons, out of the question. Con-
tinuous transpiration in aquatic plants is impossible, and in their case excre-
tion of liquid water has been often observed (WEINROWSKY, 1899). [Compare
also POND, 1905 (Biological relations of aquatic plants to the substratum.
U.S.A. Fish Comm. Reports, 1903. Washington, 1905).] Moreover, transpiration
will be retarded temporarily in many land plants both during the night and
in the early morning, owing to the saturation of the air. It is just at these
times that the exudation of drops is conspicuous in such plants.

60 METABOLISM

Only plants that have a great development of water tissue can afford to be
lavish in their use of this water. Amongst our native plants, save the willow,
herbaceous forms only exhibit excretions of water when in full leaf, but in
tropical countries, where the water supply is abundant, it is especially noticeable,
e. g. in the long-stemmed lianas. If the excretion of fluid water by hydathodes
be rendered impossible in appropriate ways, the injection of the intercellular
spaces of the leaf takes place. In spite of the statements of LEPESCHKIN
(1902) that the activity of leaves is not affected by the infiltration of water
into them, we cannot but believe that the prevention of such an infiltration
by the activity of hydathodes must always be of service.

Since probably all hydathodes are capable, under certain circumstances,
of absorbing water, they may be of service in this respect also.

In cases where an actual excretion is not attained, bleeding-pressure may
be useful to the plant ; thus it is possible to demonstrate that such pressure
aids in the unfolding of buds in spring.

The chief problem left for us now to consider is how does root-pressure aid
the ascent of sap in the plant ? We have already established the fact that by its
means water is pumped into the vessels ; whether it aids to any extent in the
transport of water the next lecture will show.

Bibliography to Lecture V.

BARANETZKY. 1873. Abh. Naturf. Gesell. Halle, 13, 3.

BOHM. 1892. Ber. d. bot. Gesell. 10, 539.

BRUCKE. 1844. Annalen d. Physik u. Chemie, 63, 193. Ostwald's Klassiker, Nr. 95.

BOHM. 1892. Ber. d. bot. Gesell. 10, 539.

u. Chemie, 63,
BUSGEN. 1891. Der Honigtau. (Jen. Ztschr. f. Naturw.)

DUCHARTRE. 1859. Ann. sc. nat. iv, 12, 267.

ELFVING. 1882. Bot. Ztg. 40, 714.

ERRERA. 1886. Bot. Ztg. 42, 16.

FIGDOR. 1898. Sitzungsber. Wien. Akad. 107, I, 641.

GODLEWSKI. 1884. Jahrb. f. wiss. Bot. 15, 602.

GOEBEL. 1897. Flora, 83, 444.

HABERLANDT. 1894 and 1895. Sitzungsber. Wien. Akad., M/N Kl., 103, I, 489 ;

104, 1, 55-

HABERLANDT. 1897. Jahrb. f. wiss. Bot. 30, 511.
HALES. 1748. Statik der Gewachse. Halle.
HARTIG, T. 1853. Bot. Ztg. n, 478.
HARTIG, T. 1862. Ibid. 20, 85.
HAUPT. 1902. Flora, 90, i.
HOFMEISTER. 1 862. Flora, 45, 97.
HUMBOLDT, cited in Meyen's Pflanzenphysiologie, 2, 85.
KOHL. 1886. Die Transpiration, etc. Braunschweig.
LEPESCHKIN. 1902. Flora, 90, 42.
MARLOTH. 1887. Ber. d. bot. Gesell. 5, 319.

MOLISCH. 1898. Sitzungsber. Wien. Akad., M/N KL, 107, I, 1247.
MOLISCH. 1902. Bot. Ztg. 60, 45.

MOLL. 1880. Verslagen u. Meded. Akad. d. Wet. Natuurk., R. 2, Deel 15.
PFEFFER. 1877. Osmotische Untersuchungen. Leipzig.

PFEFFER. 1890. Plasmahaut u. Vacuolen. Abh. Kgl. Gesell. d. Wiss. Leipzig, 1 6.
PFEFFER. 1892. Studien z. Energetik. Abh. Kgl. Gesell. d. Wiss. Leipzig, 18.
PITRA. 1878. Jahrb. f. wiss. Bot. n, 437.
REINKE. 1902. Ber. d. bot. Gesell. 20 (97).
SACHS, J. 1879. -Arb. d. Wiirzburg. Instituts, 2, 291.
SCHRODER. 1869. Jahrb. f. wiss. Bot. 7, 261.
SEMLER. 1886. Handbuch d. trop. Agrikultur, i. Wismar.
SPANJER. 1898. Bot. Ztg. 56, 35 (compare Bot. Ztg. 56, II, 177, 241, 315).
STAHL. 1888. Pflanzen u. Schnecken (Jen. Ztschr. f. Naturw.), p. 42.
STRASBURGER. 1891. Bau u. Verrichtungen d. Leitungsbahnen. Jena.
TRECUL. 1855. Annal. sc. nat. iv, 3, 343.
VESQUE. 1883. Compt. rend. 97.

THE CONDUCTION OF WATER. I 61

WEINROWSKY. 1899. Fimfstiick's Beitr. 3.
WESTERMAIER. 1884. Sitzungsber. Berlin. Akad., p. mo.
WIELER. 1893. Cohn's Beitr. z. Biol. 6, i.
WILSON. 1 88 1. Unters. aus d. bot. Inst. Tubingen, i, 8.

LECTURE VI
THE CONDUCTION OF WATER. II

HAVING seen how water enters the vessels, we have now to inquire as to
the forces which bring about its ascent to the tops of lofty trees. In order to
obtain some idea of the amount and direction of these forces it will be necessary
for us first of all to get a clear conception as to the course, the quantity, and
the rapidity of carriage of the water, and of the height to which it is carried.
As to the direction followed by the water, there is no question, at least in all
ordinary cases, viz. from below upwards, from the absorbing root to the tran-
spiring leaf. It is important to note, however, that a current may be formed
in the reverse direction, there being no special appliances in the interior of
the vessels for guiding the flow in one direction only. Indeed the older physio-
logists who studied the subject held strongly that water could move as easily
from the apices of the branches down to the root as in the normal direction.
More recently, TH. HARTIG (1861) showed that solutions of certain substances
could travel through the wood of felled trees and of isolated branches in the
reverse direction, if the upper parts were submerged instead of the lower.
Much the most convincing, however, was the experiment performed by STRAS-
BURGER on the beech (1891, p. 938). He employed a stem which had fused
high up with a neighbouring stem, the latter being abundantly provided with
leafy branches right down to the ground. He severed this stem at its lower
end so that both it and its branches were entirely dependent on the water which
the other stem had absorbed, although in order to reach the lowest branches
the water had to flow from above downwards. These branches, however,
remained quite fresh for several years (STRASBURGER, 1893). The experiment
showed, in addition, that the amount of water which flowed in the reverse
direction to the normal was amply sufficient to maintain turgidity in the leaves
of the lowest branches.

The amount of water transpired gives us some indication as to the quantity
which normally travels up a tree trunk. A glance at the flaccid leaves of
plants, as they appear in the evening of a hot summer day, shows clearly that
more water is evaporated from the leaves than is carried up to them. But since
the leaves by the following morning have again become rigid, it follows that
they must have recouped themselves during the night for what they have lost by
day. Generally speaking, it may be said that the amount of water transpired
during the twenty-four hours is roughly equivalent to what is lifted in the
same time. These amounts might be found identical were it possible to prove
that the amount of water contained in the wood showed no variations, but that
is not very probable. We owe our knowledge of the amount of water contained
in tree trunks at different seasons of the year to the laborious researches of
R. HARTIG (1882). Unfortunately each determination necessitated the felling
of the whole tree, so that it was impossible to estimate how much of the result
was due to individual variations and how much to the season of the year.
When one remembers that research has shown that, in the case of the Scotch
pine, 50 per cent, of the entire volume of the wood, according to one estimate,
and only 25 per cent, according to another, was water, we are compelled to
assume the existence of variations in water capacity in each stem, and to look

62 METABOLISM

on the wood as an immense water reservoir, which in times of abundance (e. g.
when transpiration is retarded) is full, and in times of scarcity (e. g. under
continuous drought) is depleted. For this reason it is impossible to draw
any reliable conclusions as to the amount of water carried by the stem from
a calculation of the amount of transpiration taking place during a single day.

The amount of transpiration taking place in a tree can be estimated only
with an approximation to accuracy, and even though we knew the number and
average diameter of the vessels we would still be ignorant as to how many of
them took part in the transport of water. The fact is that the whole of the
wood in the transverse section of a tree behaves by no means uniformly in this
respect. To begin with, all the so-called heart wood may be neglected, because
the lumina of the vessels in this region are quite unable to conduct water owing
to thyloses and other obstructions. In typical heart wood trees, such as, for
example, the oak, an incision, sufficiently deep to interrupt the continuity of
the sap wood, is enough to arrest the conduction of water. Indeed in many
cases this layer of sap wood is so thin that mere ringing of the bark impairs,
and even interrupts, the supply of water (Rhus typhina}. On the other hand,
there are trees which form scarcely any heart wood and whose older wood
retains the power of conduction. To this class the lime belongs, and it will
be remembered how it was shown in the last lecture that this plant is capable
of withstanding for many years the injurious effects of ringing. In an ordinary
heart wood tree, on the other hand, the sap wood beneath the place where the
operation of ringing has been performed is rapidly destroyed and the apex
withers. If a sharp line of demarcation exists between the heart and
sap wood, and if we assume, on the one hand, that the whole of the sap
wood is of equal value for conducting purposes and that the whole of the
heart wood is valueless, still only approximate estimates of the area of the part
of the wood actually concerned in the transport of water can be given. The
transition between heart and sap wood is, however, quite gradual, and not in-
frequently the lumina of the vessels formed in the second and third years of
growth are blocked up by thyloses (Robinia, WIELER, 1888). Generally
speaking, the youngest annual ring is the most effective water conduit, and the
capacity for conducting water gradually decreases as the centre is approached,
where possibly the tissue plays the part of a water reservoir only.

An estimate of the rapidity of movement of water based on the transverse
area of the conducting system and on the amount of water flowing through
it is, likewise, out of the question. A rough idea of the amount, however,
may be arrived at by employing the method perfected by SACHS (1878), but
based on the experiments of McNAB (1871) and PFITZER (1877). SACHS allowed
the plant to absorb a solution of lithium nitrate through the root, after finding
that this substance penetrated the protoplasm very rapidly without doing it any
injury and passed into the vessels as quickly as did the water itself. Since
lithium does not as a rule occur in the plant, and further, since the minutest
trace of that substance can readily be detected by the spectroscope, the rate
of its ascent could be easily determined. Some of the data with regard to the
hourly ascents which SACHS obtained are summarized in the following table : —

Acacia lophantha ..... (aver.) 154-0 cm.

Nicotiana tabacum 118-0 „

Musa sapientum (aver.) 100-0 „

Cucurbita pepo 63-0 „

Podocarpus macrophylla . . . . . , 18-7 ,,

It must not be supposed that these numbers indicate either the highest
or the lowest extremes of rapidity with which the fluid may ascend the tree.

The height to which water must in the long run be raised is, in the case of
some plants, very remarkable. Eucalyptus amygdalina, with a height of

THE CONDUCTION OF WATER. II 63

140-152 m., and Sequoia gigantea, with a height of 79-142 m., stand out as
giants in the plant world ; Abies pectinata (75 m.), Picea excelsa (60 m.), Fagus
sylvatica (44 m.), Platanus and Fraxinus (30 m.), may also be cited as examples
of trees of great height.

These observations, unfortunately, lead us to a very perplexing result.
The ascent of water cannot be treated merely as a physical problem, for some
of the most critical data required for the solution of the question are wanting ;
nor is it possible for the moment to speak of a theory of the movement of water
in the plant, since not one of the numerous investigations to which the problem
has given rise has up till now provided us with an estimate, proving that the
amount of water raised by the process assumed is actually commensurate with
that raised in the plant. So long as such quantitative evidence is wanting we
may, in our opinion, speak of hypotheses only, not of theories.

With these preliminary remarks we may now attempt to study more
closely the nature of the forces which operate in the ascent of sap. We are
unable to do more than present a critical analysis, which itself makes no pre-
tension at completeness. An historical enumeration of the voluminous researches
which have been made on this problem since the time of HALES would be indeed
of interest, but for this we cannot afford space (compare COPELAND, 1902).

In the first place we might imagine that the water was forced up by root-
pressure, and it is apparent that bleeding-pressure must operate to some extent
in this way. But the question at once arises as to whether this force is suffi-
cient to force water to the tree top, and whether the amount of water supplied
by the root is approximately enough to replace what is lost in transpiration.
Some experiments carried out by SACHS (1873) are worthy of consideration on
this latter point. He compared the amount of sap given off in a definite time
from the root of a herbaceous plant with the amount sucked up by a shoot
whose cut end had been submerged. A root-stock of Nicotiana latissima gave
off about 16 ccm. of sap in five days, but its shoot absorbed 200 ccm. A simi-
lar disparity was exhibited in other cases also. Further, it is very improbable
that the secretory capacity of the root is sufficient of itself to compensate for the
loss of water due to transpiration. It must be remembered, however, that varia-
tions may be set up in the root owing to the amputation of the shoot. [Compare
also DARBISHIRE, 1905 (Bot. Gaz. 39, 356).] So far as the pressure which causes
bleeding is concerned (apart altogether from local pressures, which must ob-
viously be entirely disregarded in this connexion), we may accept the statement
of WIELER (1893) as indicating the extreme pressure which has been demon-
strated with certainty. WIELER found that in the case of the birch a pressure
of 139 cm. of mercury, or about two atmospheres, was developed. If we
disregard the frictional resistance presented by the vessels, such a pressure
might force water up to a height of 20 m., but it could not raise water to
the top of a growing tree 25 m. high. Let us take another example. The
silver fir may reach a height of 75 m., and if water is to be elevated to the top
of such a tree by means of bleeding-pressure only, that pressure must amount
to at least seven and a half atmospheres. Bleeding-pressure, however, in all
Coniferae is extremely feeble, as very many estimates clearly prove. HOF-
MEISTER (1862), indeed, states that, as a general rule, Coniferae do not ex-
hibit the phenomenon of bleeding ; and though WIELER (1893), at a later date,
was able to show that bleeding could be demonstrated in these plants, still it
is impossible to draw any other conclusion than that it was quite insignifi-
cant in amount. Quite apart from Coniferae there are many other plants
whose maximum root-pressure is quite inadequate to bring about the filling of
the vessels of the leaves and branch apices. Thus, according to WIELER' s
statements (1893, p. 122), the root-pressure in Morus reaches only to 12 mm.,
in Fraxinus to 21 mm., and in Acer pseudoplatanus from 169-313 mm. of

64 METABOLISM

mercury. It must further be specially remembered that this maximum
pressure shows itself, in general, in all our native plants only under conditions
when the evaporation of water is much reduced. Bleeding may be observed
in the birch and vine in springtime, before the appearance of the leaves, but
in the middle of summer special precautions have to be taken to retard tran-
spiration before the phenomenon can be demonstrated. When transpiration
is most vigorous no water exudes from the stump of a felled tree, indeed it is
greedily absorbed, and if a manometer be fixed laterally in the wood of a leafy
stem, it shows a negative rather than a positive pressure, that is to say,
a pressure in the interior of the stem less than that of the atmosphere.
This negative pressure, to which we have previously drawn attention, is
especially well seen in leafy branches, so that it is obvious that the bleeding-
pressure which makes itself apparent in twigs, branches, and in the stem
itself, does not exist at all during transpiration, or is so inconspicuous that it
is not worth considering as a means whereby water may be supplied to the
leaves. This tallies with V. HOHNEL'S observations (1879) on herbaceous
plants, for he observed that grasses gave off water in the form of drops in the
morning owing to the repletion of their vessels, and exhibited a conspicuous
negative pressure in the afternoon when transpiration became vigorous. For
these reasons it is impossible to ascribe to bleeding-pressure any fundamental
significance apropos of the conduction of water, although, perhaps, when it
does occur, it may to some extent aid in the ascent.

If, then, water be not forced up by the agency of a pressure acting from
below, we are compelled to cast about for a force which must have its seat of
activity at the upper end of the plant, and which is capable of sucking the water
up. We have already learned of the existence of such a force, viz. transpira-
tion ; our experiments with the potometer made this clear to us, but it is
possible to demonstrate its existence much more definitely if we fix a transpiring
shoot, air-tight, in a long glass tube, fill the tube with water, and stand it verti-
cally with its lower end immersed in mercury (Fig. 15). The branch continues to
absorb water by its cut surface, and mercury rises to take its place in propor-
tion as the water disappears from the tube. The height to which the mercury
ascends serves as a direct indication of the force exerted by the suction: This
sucking process is, however, a purely physical phenomenon, and for that reason
it will be advisable for us to study it in the first instance with the aid of a phy-
sical apparatus. We employ for this purpose a thistle tube, closed at the ex-
panded end by parchment and filled with water (Fig. 16), and with its lower
narrow end plunged into mercury. The parchment loses its imbibed water
by evaporation and, in consequence, sucks water out of the tube. This water
is in turn replaced by mercury. Since the atmospheric pressure on the outside
of the membrane exceeds the atmospheric pressure on the inside by the weight
of the column of water and mercury, the air easily passes through the parchment
into the interior and occupies the space between the membrane and the water,
and any further ascent of mercury ceases. For this reason, ASKENASY (1895)
replaced the parchment by a block of gypsum, which, when wet, is less per-
meable to air. Since the pressure in the interior of the apparatus becomes
less and less as the mercury rises higher and higher, the air escapes from
the water just as in an air-pump and stops any further ascent. If one uses
boiled water, however, very considerable heights are obtained. In ASKENASY' s
(1896) experiments, for example, the mercury reached a height of 82 cm. with
the barometer standing at 76-2 cm., and, in another case, 89 cm., when the
atmospheric pressure was 75-3 cm. It is obvious that still higher values could
have been obtained had the gypsum cork been able to resist the passage of air
entirely.

These heights reached by the mercury, exceeding as they do that of the

THE CONDUCTION OF WATER. II

mercury in the barometer, are at first sight surprising, because they appear
to contradict the lessons learnt from the Torricellian vacuum. How can these
facts be explained, and such heights be theoretically accounted for ?

By employing an air-pump in place of the evaporating block of gypsum, and
filling the glass tube with air from the beginning, it would certainly be possible
to produce a vacuum on attaining a height comparable to that produced by
atmospheric pressure. In order to obtain this result in our experiment, how-
ever, the adhesion between water and plaster of Paris, between the water
and the wall of the tube, as well as the cohesion of the water particles them-
selves, must, first of all, be overcome. It is well enough known that the force

Figs. 15, 16. From DETMER'S Smaller Practical Physiology (Figs. 77 and 78).

of adhesion is very great, but, taking as a basis the older physical experiments
on the subject, it is quite obvious that we have considerably underestimated
the force with which the water particles cohere. ASKENASY and DIXON have
performed a very great service in showing how immense that force really is.
Detailed estimates of the force of cohesion are as yet wanting, still we may for
the present be content with the results arrived at by DIXON and JOLY (1895, b,
p. 570), according to whom a pull equivalent to at least seven atmospheres
is necessary to tear asunder a column of water. In all probability this is an
under- rather than an over-estimate orthe force of cohesion. If it can only
be arranged that no air passes into the gypsum, a column of water 70 m., or
even more, or a column of mercury, 5j m., can be held in suspension by an
evaporating block of gypsum. According to the detailed statements of
REINGANUM (1896) and NERNST (1900) columns of water of considerably
greater length must be supported by transpiration.

JOST

66 METABOLISM

If in place of the block of gypsum used in ASKENASY'S experiment we
employ a PFEFFER'S cell (p. 13), it would appear probable that higher values will
be obtained, values which far exceed those of atmospheric pressure. In this
form the apparatus would present a greater likeness to the conditions obtaining
in the plant, for the PFEFFER'S cell would correspond to a parenchyma cell
of the leaf and the glass tube to a vessel. Let us assume that the cell borders
directly on an intercellular space on the one side, and that the vessel, on which the
other side abuts, is filled with water and has its lower end plunged into mercury,
and that its wall, as in the case of the glass tube, is impermeable to air. When
the cell-sap becomes concentrated owing to transpiration having commenced,
the water is withdrawn from the vessel and the mercury rises to take its place.
The question then is, how high will it rise ? Let us imagine, for instance,
that the mercury rises to a height of 150 cm., then there is no question but
that it must be supported by the cell ; but one must not imagine that the cell
is not affected by it. It must exert a suction on the cell, just as if we had
placed an osmotically active solution near one side of the cell. If the height
of the mercury still goes on increasing there comes at last a time when this
suction becomes equal to the osmotic pressure in the interior of the cell.
When this point is exceeded the mercury acts just like a plasmolysing solution.
At the same moment any further absorption of water on the part of the cell
comes to an end, the water passes backwards into the vessel, and the ascent
of the mercury ceases.

Such considerations as these compel us to conclude that continuous water
columns in the vessels appear unequal to the task of raising water easily, since
they are unable to overcome the great cohesion existing between the particles
of water. In proportion as the columns become longer the absorption of water
by the leaf-cells becomes more difficult, and must, in the long run, become
quite impossible. We are further, for many reasons, unable to calculate how
long the water columns may actually be. In the first place, we are ignorant
as to whether very lofty trees may not perhaps possess an especially high
osmotic pressure in their leaves (and in this relation it must be remembered
that DIXON'S (1896) determinations of osmotic pressures in the leaf are by no
means above criticism), and in the second place, the power of suction possessed
by the water column depends not only on its length but also on the friction
which it meets with, owing to the tendency to collapse on the part of the walls
of the vessels, as well as on the opposition to its passage through the living
cells of the root.

Should we desire to conduct a research with the view of finding out the
amount of f rictional opposition the water suffers (whether in ascent or descent)
in the wood of a tree, we must make it our business first of all to obtain a clear
conception of the intimate structure of the vessel, and, above all, to note in what
respects it differs from the glass tube which has hitherto served as our model ;
i.e. we have to study its length and breadth and the nature of its wall.

The vessel varies greatly in length according to its mode of develop-
ment. Two extreme conditions may be noted and designated respectively
tracheae and tracheids. The term ' vessel ' is generally taken as the equiva-
lent of ' trachea '. We shall follow in this work the suggestions of ROTHERT
(1899) in regard to nomenclature. The short and comprehensive title ' vessel ',
to include both tracheae and tracheids, is, we think, the more necessary, for
both morphological and physiological reasons, since very often, in individual
cases, it is by no means certain whether tracheae or tracheids are concerned
in the process.

Tracheids are nothing more nor less than much elongated cells which
remain entirely closed ; tracheae, on the other hand, are cell chains which run
through the plant in definite directions, and whose lumina have become united

THE CONDUCTION OF WATER. II 67

into one long cavity by absorption of the transverse partitions. In general,
tracheids are shorter and narrower than tracheae. The following table fur-
nishes us with data with regard to the dimensions of certain vessels : —

Width in mm. Length.

Tracheids of Dicotyledons O'i6-i mm.

,, Pinus up to 0-03 4 mm.

„ Musa and Canna o-i lomm.

„ Nelumbium 0-6 120 mm.

Tracheae of Mucuna sp. 0-6 —

,, Calamus draco 0-562

Wistaria sintnsis o.a up to 3 m.

Aristolochia 0-14 3m. or more

Oak 0.3-0-3 a m- °r more

Ash 0-14 ,

Beech 0-028

Ficus 1 0-66 cm.

When we remember that the water lost by the branch in the process of
transpiration streams along the vessels, much in the same way as it does in
the glass tube which forms part of ASKENASY'S apparatus, it will be at once
apparent to us that the tracheae are much better fitted for water transport
than the tracheids. In the first place, they are wider than the tracheids,
and a well-known law in physics tells us that the amounts of fluid which pass
through two tubes under the same pressure are proportional to the fourth power
of the radius. In the second place, the vessels are only rarely interrupted by
transverse walls, each transverse partition acting as an obstacle to the movement
of water. If the resistance to water conduction in the transporting conduits be
alone considered, it is obvious that the longest and widest vessels would be
the best fitted for the purpose. Since, however, we find that in the majority
of plants narrow and short tracheids occur in addition to long and broad
tracheae, we are compelled to conclude that this difference in the form of the
conducting elements corresponds to a difference in function ; wherein the
division of labour between the two elements consists, however, it is impossible
to say. It is known only that, to a certain extent, the tracheids may take the
place of the tracheae, a conclusion which may be arrived at from a knowledge
of the fact that the wood of many trees (e. g. Coniferae, Drimys, Trochoden-
dron, &c.) consists entirely of tracheids, and this may be experimentally
determined from deep incisions made right into the pith. The numerous experi-
ments on this subject were first correctly interpreted by STRASBURGER (1891).
If several incisions, at suitable distances apart, laterally and vertically, be
made in wood which contains only tracheae (e. g. Ficus), the wood is rendered
incapable of performing its functions, because all the conducting channels are
interrupted. When tracheids occur as well as tracheae, or when tracheids only
are present, the incisions do less damage, because, where the interruptions occur,
a lateral conduction through the tracheids is effected. This is especially evident
if solutions of colouring matters be used ; the tortuous path taken by the water
in its ascent is shown by the coloured fluid, while normally it may be demon-
strated that the water follows a straight course by means of the same method.

In addition to a vertical ascent a lateral passage of water in the wood is also
possible. Obviously in the case of lateral movement a far greater number of
walls must be passed through than in the case of longitudinal movement,
whether these be the walls of tracheae or tracheids. But, as we have stated
above, the walls present a certain amount of resistance to the movement.
Before going more closely into this matter we must study the structure of the
wall of the vessel ; a subject which is of interest from other points of view also.

The first thing noticeable with regard to the wall is its peculiar sculpturing.
There are no vessels whose walls are uniformly thickened for any great dis-

F 2

68

METABOLISM

tance ; thick and thin regions always alternate. The thickenings are annular,
spiral, or reticulate. In the spiral and annular vessels the thin portions are
of similar form to the thick parts and alternate regularly with them ; in the
reticulate type, however, the thin places are circular or elliptical and known
by the name of ' pits '. It may be logical, but scarcely customary, to call the
thin places in the first two cases ' pits ' also, all the more as there is no sharp
line of demarcation between the three types. This is not the place to enter
into a discussion of their other peculiarities, though an appearance of very
general occurrence may be noted, viz. the attachment of the thickened regions
to the original unthickened wall by a somewhat narrower base (Fig. 17), so
that a transverse section of the thickening presents a |- -shaped appearance.
This structure in ' annular ' and ' spiral ' vessels was for long overlooked, and
was first demonstrated by ROTHERT'S (1899) elaborate researches.

One form of reticulate thickening, on the other hand, has been often in-
vestigated and known by the name of ' bordered pits '. Looked at on surface
view (Fig. 18, 2) the circular or elliptical mouth of the pit appears surrounded
by two lines enclosing a ring-like area. It is easily understood that the cavity
is due to the deposit of thickening material on the outer rim of the pit, the
widening of which brings about the formation of the cavity ; pit canals of

Fig. 17. i. Spiral vessel of Cucurbiia
in longitudinal section. (After Ro-

THERT, 1899, pi. 6, fig. II. x 400.)

2. Reticulate vessel of Opuntia. (After
ROTHERT, 1899, pi. 6, fig. 30. X 400.)

Fig. 18. i. Bordered pit offinus
sylvestris in tangential section ; /,
the torus. 2. Surface view of the
pit ; corresponding regions are
united by dotted lines, j. As in /,
but the torus is pressed against the

Sit on the left. (After Russow,
iagrammatic.)

equal width throughout have no such cavity. Bordered pits occur especially
frequently on walls which are common to two vessels and are then (as in Fig. 18)
bilaterally symmetrical. The median closing membrane possesses the further
special peculiarity of having in the middle a lens-shaped swelling (torus) sharply
deliminated from the very thin remainder. The closing membrane is not always
found medianly situated, as is shown in Fig. 18, / ; it is capable of moving in the
pit cavity, and, in extreme cases, may be pressed up against one or other of
the exits of the cavity (Fig. 18, j).

The structure of the bordered pits, as well as the wall of the vessel in general,
as is easily seen, must be conducive to the transference of water into a vessel
from any element in the vicinity (parenchyma cell or vessel). The walls of a
vessel, saturated with water, like those of an ordinary cell, are swollen, and they
also allow water to pass through them, although they obviously present a certain
amount of resistance to the passage, a resistance which must, caeteris paribus,
be all the greater the thicker the wall. The closing membranes of the bordered
pits will also easily permit the passage of water, but the thickened regions will
present resistance. Experimental research amply confirms this a priori thesis.
Such researches are most conveniently carried out on Coniferae, because in these
plants the wood consists entirely of tracheids arranged with great regularity.

THE CONDUCTION OF WATER. II 69

The tracheids are on an average about I mm. long, 0-02 mm. broad, and as
much in depth. In transverse section they are nearly rectangular and so
arranged that their walls face radially and tangentially. Bordered pits occur
almost exclusively on the radial walls and the tapering ends, the tangential
walls being almost entirely free from them. If water be forced through coni-
ferous wood in a longitudinal direction it will meet with transverse walls which
are about a millimetre apart, but whose resistance to the flow is reduced by the
presence of pits in them. If water be forced through in a tangential direction
it will meet with fifty times as many walls as it will in the longitudinal course ;
finally, if it be forced through in a radial direction it will have to traverse
about the same number of walls as in the tangential course, but it will meet with
no pits in its journey. After this preliminary statement let us turn to STRAS-
BURGER'S experiments (1891) with the fresh wood of the silver fir, which may
be summarized as follows : —

1. A column of water, 50 cm. in height, filters completely through a piece
of wood, 8 cm. long, in a longitudinal direction in one hour : it encounters
no opposition worth mentioning.

2. A similar column of water passes in a tangential direction through
a piece of wood 1-3-5 cm- m length, at the rate of about 4-10 cm. in 20 hours.

3. The opposition is so great in the radial direction that passage of the
water cannot as a rule be noticed at all, and if the pressure be increased by
means of a column of mercury, it is found that 50 cm. of mercury will drive
water radially through a piece of wood 1-5 cm. long at the rate of only about
4 cm. in 24 hours, and about 6 cm. in 48 hours.

Similar results had been previously obtained by SACHS (1879, 297) and
ELFVING (1882). Thus we are able to understand clearly the significance of
the closing membranes of the pits, and may interpret the arrangement in this
way, that the margin of the membrane, when in a neutral position, behaves
like an ordinary closing membrane, while, when the torus is pressed to one side
against the pit opening, this thickened portion of the membrane, to a certain
extent, counteracts the activity of the pit. If, therefore, the torus becomes
sucked against the opening, owing to extensive abstraction of water and the
negative pressure of the air in the vessel resulting therefrom, then any further
withdrawal from this vessel must come to an end until this negative pressure
becomes general in the vicinity, and the closing membrane of the pit again
takes up a neutral position. This explanation, advanced by STRASBURGER
(1891), is the most satisfactory hypothesis of the function of the bordered pit,
and hence must be cited here, although it has not been conclusively demon-
strated (compare SCHWENDENER, 1892, Ges. Abh. i, 288).

As this torus-formation is not found in the pits of all vessels the h- -like
thickening of the wall must have a special significance. That this structure has
a meaning may be deduced from its very general occurrence ; but before we go
into this question we must examine more closely into the purpose of the thicken-
ing itself. Since the thin regions of the wall of the vessel are especially adapted
for the passage of water, why is the whole wall not of this uniform thickness ?
We can scarcely go wrong if we recognize the advantages which the plant
gains by the thickening from a mechanical point of view. Ordinary parenchy-
matous cells with thin walls are able to acquire very considerable rigidity by
means of osmotic pressure, but the water in the lumina of the vessel remains
either at atmospheric pressure or even less ; only rarely is the pressure
higher than that of the air. Further, if turgid cells surround a vessel, they
would be able to press the walls together and so obliterate the lumen, were
they not prevented from doing so by the great rigidity of the membrane
of the vessel. The membrane acquires this rigidity by means of the
thickening as well as in consequence of other physical characteristics, and

70 METABOLISM

the alternation of thick and thin regions may be interpreted as a compromise
whereby the wall permits water to pass through whilst at the same time
maintaining its rigidity ; compromises such as this are frequently met with
in the architecture of the plant. From this point of view the narrow con-
necting bands of thickening deposit are quite comprehensible ; they render the
existence of broader thin regions possible and, at the same time, do not
materially impair the rigidity of the vessel (SCHWENDENER, 1882 and 1892 ;
ROTHERT, 1899). We need not enter into a discussion here of the physiological
significance of the individual forms of thickening.

Having now appreciated the relation existing between the structure of
the wall of the vessel and the function which the vessel itself fulfils, the next

Question before us is the explanation of the chemical characteristics of the wall,
t is well known that the walls of all vessels are lignified, that is to say, their
original cellulose reaction is greatly altered by the infiltration of aromatic sub-
stances (hadromal, CZAPEK, 1899). But our knowledge of the subject is very
limited ; we do not know how the physical characters of the cell-wall are
altered, nor whether the vessels are rendered fitter for the performance of their
functions by lignification. That lignification is by no means necessary is shown
by the fact that it takes place not only in vessels, but also in other elements
which have nothing to do with the transport of water. Nor in the present
condition of our knowledge are we able to form a judgement on the interpre-
tation to be given to the fact that the closing membranes of the bordered pits
are different in their chemical nature from the rest of the wall.

Tracheae have received their name from the organs in animals known by
that title, because not only do they possess to a certain extent a similar struc-
ture, but also because they were supposed to have a similar function. For
long it was believed that they were the respiratory organs of the plant and
carried air in their interior. That this was an error was shown especially by the
researches of VON HOHNEL (1879) and BOHM (1879), and since that time it has
been conclusively shown that the lumen of the vessel always contains water.
I At certain times, e. g. during the growth of the vessels, and in trees during early
/ spring, when root-pressure is vigorous, the vessels are often completely filled
I with water; but as soon as transpiration sets in, air is plentifully present in them.
Whence comes this air ? There are two possible means of entrance. It could
enter the vessel as air dissolved in the water already in the root, or it could in the
first instance diffuse through its wall in higher regions of the plant (CLAUSSEN,
1901 ). In both cases the air would remain at first dissolved in the water which fills
the vessel. When, however, at the commencement of transpiration, the leaf -cells
demand more water from the vessel than it can supply, a vacuum must tend to be
formed in the vessel, and into that the air escapes from solution in the gaseous
i state. Such air-bubbles will have a lower pressure than one atmosphere, and
will therefore act in a sucking manner ; water will be withdrawn from the
neighbouring tracheid, and in it, in turn, air-bubbles under low pressure will
appear. This negative pressure of the air in the vessels has been fully demon-
strated by v. HOHNEL. He cut off branches under mercury from actively
transpiring trees and herbs, and observed how the metal was forced far up into
the lumina of the vessels by the external atmospheric pressure, overcoming
the very obvious capillary depression. The rarefaction of the air reaches
its maximum during the greatest transpiration ; but it may disappear again
entirely during the night owing to continuous entrance of water through the
root, whilst the vessels, when the air becomes dissolved once more, become full
of water. If the rarefaction of the air continues for a longer time, air from
outside enters through the walls of the vessels. When the air-pressure within and
without the vessel is by this means equalized, the vessels cannot be again com-
pletely filled with water, and further entrance of air, in the long run, will inter-

THE CONDUCTION OF WATER. II 71

fere with the capacity of the wood for transporting water. We shall refer later
on to the conditions which bring about a diminution in the air in the vessel.

Before considering in greater detail the distribution of air and water in
the vessels, we must briefly allude to one consequence of the reduced pressure
of the air in the vessels which is familiar to every one in the course of everyday
life, and which is also of great importance in physiological experiments.
If a branch be cut off, without taking any special precautions, and placed in
water, it very soon begins to wither, because, during the operation, air has entered
the open vessels and may ascend to a variable height in accordance with the
amount of negative pressure in the vessels. If, however, the branch be cut off
under water, the water is forced into the vessels by atmospheric pressure, and the
cutting does not wither. The entrance of water naturally equalizes the negative
pressure, but this may reappear, if the cut surface of the twig becomes im-
permeable to water owing to slime exuded from the plant accumulating on it,
or owing to the varied methods of blocking up of the vessels due to the plant's
own activity ( WIELER, 1888), or, finally, owing to the action of Bacteria. Con-
tinued transpiration again induces withering, but this we can easily overcome by
forming a freshly cut surface above the first made, of course also under water.

The negative pressure of the air in the vessels, as we have already seen,
must render the determination of the distribution of water and air in the vessel
very difficult. On undertaking an investigation on selected branches, as a rule
it will be found that the results arrived at are perfectly useless, because, in the
process of cutting, the basal parts of the branches have become filled with air.
It is preferable, therefore, to cut the branch off at two places at the same time,
or to isolate a cylinder from a tree trunk with a Pressler's growth borer: by
either means the air, oil, or mercury enters both sides at the same time, and hence
the air-bubbles originally present, mixed up with water, are forced into the
central region of the preparation, where not only the length of the parts with
water and air may be measured, but also where the initial rarefaction of air may
be easily determined. The rarefaction of the air is, according to SCHWENDENER
(1886), generally only about one-third of an atmosphere, rarely values of a
quarter to one-fifth of an atmosphere are reached, almost never less. Whilst only
one air-bubble can occur under such conditions in the middle of a tracheid, driven
there by the pressure of water from both ends, numerous air-bubbles always
occur in the tracheae, separated by water columns. The alternating air-bubbles
and water columns show not inconsiderable differences in size, still the follow-
ing averages, which SCHWENDENER (1886) obtained in case of Fagus sylvatica,
may stand for a fair representation of the usual distribution of water and air in
the vessel : —

Stem
May 30.

Stem
May 31.

Branch
2 years old
May 31.

Branch
10 years old
June 2.

Twig
2 cm. thick
June 23.

0-364
0-182

0-294
0-182

0-220

0-160

0-322
0-091

0-378
0-108

Air-bubbles (mm.)
Water column (mm.)

Total (mm.) . . 0-546 0-476 0.380 0-413 0-486

On an average a segment of the chain composed of air-bubbles and drops
of water is about 0-5 mm. in length. Such a series is known as a JAMIN'S
chain, and it is obvious that the movement of water in such a chain must take
place under conditions essentially different from those obtaining in a tube
completely filled with water.

SCHWENDENER does not say from what part of the wood the vessels upon
which he made his observations were taken, and, according to STRASBURGER'S
(1891) results, the amount of air in the vessels in the periphery of the wood,
as also in the younger parts generally, is much less than that in the older annual

72 METABOLISM

rings. Indeed, in many plants the youngest annual ring normally exhibits long,
continuous water columns entirely destitute of air-bubbles, while, further in,
the wood contains so much air that its conducting power is completely destroyed.

The physical apparatus — a transpiring osmotic cell with a glass tube attached
filled with water — which we used as a model of what is found in a tree,
differs in many essentials from the reality. The glass tube has a wall which is
impermeable to water and air, it forms a continuous channel, and is also full
of water throughout. The vessel in the plant is permeable both to water and air,
it has always here and there transverse walls which interpose a certain amount
of resistance to the movements of water, and, furthermore, it is generally not con-
tinuously filled with water but with a chain of alternating bubbles of water and
air. Now if, as DIXON and ASKENASY affirm to be the case, the cohesion of water
particles plays an important part in the ascent of sap, the vessels ought to con-
tain no such chains. It cannot be denied that, as NAGELI (1866), DIXON and
JOLY (1895), and ASKENASY (1895, 16) have shown, the entrance of air-bubbles
into the vessel may be prevented by certain factors, but in reality everything
goes to show that, as a rule, the entrance of air into a vessel transporting water
can no longer be doubted. For this reason alone the cohesion hypothesis must
be given up, or, at least, it must be admitted that it can play no very important
part in the process. [Compare STEINBRINCK on the cohesion hypothesis (Ber.
d. bot. Gesell. 22, 526, 1904).] Further, even though the columns of water in the
vessels were continuous, the cohesion hypo thesis could not be accepted as proved.
What we require in any theory of water movement is proof not only that the
water can be raised to a certain height, but also that it can rise to this level in
sufficient quantity.

The knowledge which we have previously acquired on the subject of the
ascent of water as a result of capillarity is instructive. It is well known that
the concave meniscus, which forms in a glass tube immersed in water, has less
surface tension than a corresponding level surface of water, and that, in con-
sequence, water rises in a capillary tube above the level of the surrounding
fluid. The height to which the water ascends depends upon the concavity of
the meniscus, and that, in turn, on the diameter of the tube, and it is easy to see
that, given a sufficiently narrow tube, any height may be attained. If we con-
sider the cavities of the vessels filled with water as capillary tubes in a microscopic
sense — and not much can be urged against this assumption — then it is con-
ceivable that very tall trees might be supplied with water by capillary attraction.
NAGELI (1866) and STRASBURGER (1891) have, however, shown that a capillary
ascent of this kind is quite insufficient to replace the water lost in transpiration.
Although the principle under discussion is, from a purely physical point of view,
perfectly correct, it does not come into play under the conditions existing in the
plant. The same might be affirmed of the cohesion hypothesis.

Taking into account all the facts which have been observed, we may
formulate our problem in this way : How can the ascent of sap be effected in the
plant if chains of water and air-bubbles ( JAMIN'S chains) occur in the vessels and
if transpiration is always effecting a suction at their upper ends ? On this question
we have to thank, more recently, SCHWENDENER (1893) and STEINBRINCK (1894)
for valuable information. It is possible that the ascent of water in a JAMIN'S
chain may take place in one of two ways, either the whole chain, or at least its
upper segments (including both air-bubbles and water drops), moves upwards, or
the water alone moves while the air-bubbles are stationary. Let us look first
at the movement of the whole chain. Let us imagine a long vessel or a glass
tube filled with air and water segments, each I mm. in length, with a suction
pump acting at the upper end ; by this means the air-bubbles will be stretched
out and the water columns will be pulled upwards. It is also obvious that the
topmost air-bubble will be extended most, and, in the long run, will show a pres-

THE CONDUCTION OF WATER. II 73

sure corresponding to the tension of water vapour. On the next air-bubble,
lying farther down, there will be the pressure of a water column of i mm. in
height, and hence it will not be stretched quite so much as the top one and so
on, each more deeply placed air-bubble being less stretched than the one next
above it. But there is another consideration to be taken into account. In
a horizontally placed tube, where the effect of the weight of the water columns
is eliminated, each separate segment of the chain opposes the movement in
a way not quite understood theoretically, but admitting of practical estimation.
SCHWENDENER found it to be approximately the weight of 4 mm. of water. On
the whole each small column of water opposes the upward ascent with a force
equal to the weight of 5 mm. of water. Under these conditions, and bearing
in mind that in the uppermost bubbles the tension is never that of water
vapour, but very much higher (one-third to one-fifth of an atmosphere),
SCHWENDENER estimated that the suctional power of the leaf was transmitted
backwards not quite 5 m. At this distance an air-bubble would exhibit only
atmospheric pressure.

The assumed length of the air-bubbles, viz. i mm., is, as a matter of fact,
too great ; but if one imagines them smaller, then their suctional efficacy is all
the less. On the other hand, the amount of the opposition to the movement of
the separate water columns has perhaps been placed too high, and thus the suction
would stretch them out more. On the whole, however, SCHWENDENER' s criticism
may be accepted as correct ; the suction of the air-bubbles which raises the water
can act only a few metres backwards from the top of the tree, perhaps to the
base of the branches, but certainly not into the stem. Other forces must, there-
fore, be forthcoming to effect the elevation of water in the stem. Again, negative
air pressure in the vessels, which can exist independently of transpiration (com-
pare p. 76), occurs in the trunk of a transpiring tree, and it has never yet been
observed that this pressure increases continuously from the apices of the
branches towards the base of the stem. We feel bound to conclude from this that
there is something in SCHWENDENER' s assumption not in accordance with fact.

There are other difficulties which stand in the way of our acceptance of
the idea of movement of the entire water and air chain. This theory assumes
that these vessels are continuous throughout the entire tree, while in reality
the vessels have always only a limited length. Furthermore it would be a
puzzle to determine where the air goes to when the water is absorbed by
the cells at the upper ends of the vessels. It is obvious that it would result in
the rapid formation of a cavity filled with air at low pressure, whose
removal later would be impossible. Under these conditions it will repay us to
keep the other possibility before us, and to inquire whether it may be possible
that a movement of the water in the chain takes place, though the air-bubbles
remain stationary. Such a hypothesis as this does not appear unjustified,
because in the tracheids, e. g. of coniferous wood, the conditions completely
forbid a movement of the air-bubbles for any great distance.

How then can suction, exerted on the upper end of the water column in
a long vessel, be transmitted downwards while the air-bubbles in the vessel
remain stationary ? Three ways are possible ; either the water flows between
the air-bubbles and the wall, or in the wall itself, or, finally, through the
neighbouring cells. In this last case, if no cell or vessel in the neighbour-
hood is capable of interfering in virtue of its vital activity, we may imagine
a system of several vessels lying one behind the other and acting together, re-
placed by a sinuous vessel subdivided frequently by transverse walls. This
assumption obviously has no advantages over a straight and continuous vessel
filled with water : there is nothing to be gained by discussing it further. The
difficulties in the way of a movement of water in the wall itself have been
already mentioned (p. 48), and these are sufficient to make the second possi-

74 METABOLISM

bility improbable. Only the first method remains, viz. that the water must
move between the air-bubbles and the wall of the vessel. This view was first
put forward by VESQUE (1883) and subsequently maintained by STRASBURGER
(1891). It must first of all be noted in this connexion that owing to the
sectional outline, which is far from being a circle, and owing to the frequent
occurrence of local thickenings on the wall — especially if these be of the spiral
variety — a close adhesion of the air-bubbles to the wall of the vessel is prevented,
and quite large spaces may exist between the bubble and the wall, in which
a movement of water would appear possible, and this would still be the case if
only the so-called adhesive water film between the air-bubble and the wall
were present. VESQUE has, as a matter of fact, seen a stream of water flow-
ing past the stationary air-bubbles, both in plant vessels and in capillary glass
tubes, and COPELAND has more recently (1902) confirmed VESQUE'S observa-
tions. Although there can be scarcely any doubt of this fact still it is not
possible at present to explain physically the ascent of sap in this way. By
this arrangement we have continuous columns of water which, every here and
there — at each air-bubble — have a diminished diameter ; this periodic lessen-
ing of the diameter decreases the tendency to sink on the part of the water
column, on account of the additional friction, and must, for the same reason,
hinder its ascent. Sinking of the column appears to be inevitable as soon as its
weight exceeds the resistance to filtration offered by the root-cells and the
friction in the vessels. If, however, these resistances are sufficiently great
to prevent this sinking, how a force arises which is capable of effecting
a lifting of the water is not apparent. One cannot resist the impression that
the more intimate physical conditions of the ascent of sap in the plant have
not as yet been clearly made out, and we must still wait for further elucidation
of these phenomena by experimenting with apparatus more nearly resembling
vessels than the glass tubes which we have hitherto employed to represent the
vessels. An apparatus of this sort has been devised by COPELAND (1902).
He has shown that if a tube, over 12 m. high, be filled with plaster of Paris,
water, and air, evaporation at the upper end is followed by an ascent of the
water. He has observed, further, that the pull of transpiration is active much
further down than SCHWENDENER thought, but he is unable to explain
physically the way in which his own apparatus works. It seems to us most
important to elucidate, first of all, the distribution of negative pressure in the
tree. According to STRASBURGER' s (1891) observations, and also according to
those of PAPPENHEIM (1892), the negative tension at the tips of the branches
does not appear to differ from that at the base, and a negative pressure ap-
parently exists at the bottom of the tree, as is proved by the sucking up of
water through the cut surface. Whether this is dependent on the suction
exerted by the leaves, or arises from other causes, it should not be difficult to
determine. If it be due, as appears to us likely, to the activity of the leaves,
then the osmotic suction must certainly reach down to the root (in opposition
to what SCHWENDENER found), and then one might truly say that transpiration
provides the force which causes the ascent of sap. But even then physical research
would require to show how it was that the air in the vessels at the base of the
stem was not compressed by the superincumbent water columns.

In addition to purely physical forces the activity of the living cells is fre-
quently brought forward as a cause of the ascent of sap. In a certain sense
their co-operation cannot be doubted, in so far, that is to say, as the living cells
build up the vascular system and develop into forms suitable for performing
this function in the plant. The vessels, when they begin to act as water
conduits are already filled with water. If the water be withdrawn from the
vessels they cannot be again filled by the plant's agency, their capacity is for
ever lost, and the plant dies unless water be injected artificially into the vessels.

THE CONDUCTION OF WATER. II

75

According to R. HARTIG (1883, p. 73) the spruce stem becomes incapable of convey-
ing water when the lumina of the tracheids are still more than half full of water.
Apart from this, however, living tissues, especially the parenchymatous cells,
which are almost always associated with the vessels, may take part in water
conduction directly or indirectly. WESTERMAIER (1884), GODLEWSKI (1884), and
JANSE (1887) have assumed that the parenchyma play a direct part in the process.
The essential point in all these theories is that parenchymatous cells abstract
water from one vessel and hand it on to one higher up. A complete discussion
and criticism of these views (ZIMMERMANN, 1885, SCHWENDENER, 1886) need not
be presented here, since, owing to the researches of STRASBURGER, all such
vital theories have received a severe blow, if indeed they have not been directly
disproved. Further, no positive evidence has been advanced in support of these
theories, and one accepted them because purely physical explanations appeared
to be inadequate.

Experiments on the ascent of sap in dead branches had been previously
carried out, but for the most part these were confined to killing short lengths
and establishing the fact that they were permeable to water. Naturally, it
cannot be concluded from such experiments whether, in stems of any length,
water conduction goes on after the death of the parenchyma. Such researches
on a large scale we owe to STRASBURGER (1891, 1893). He killed long
branches of Glycine, by placing the lower leafless part, which was 10-12 m.
in length, in boiling water, and observed that an eosin solution rose in them from
the cut end to a height of 16-8 m. ; still the leaves which were present at the un-
injured end of the stem remained alive for onlya few days, after which they dried
up and fell off. In all probability the deficiency in the supply of water to the apex
was not due to the death of the parenchyma but to the fact that masses of slime
and other obstructions entered the vessels, in the way we have already described.

STRASBURGER has also killed long reaches of plant stem otherwise,
e. g. by poisons, and proved that they are still capable of conducting water
in the dead condition. The following extract may be selected from the de-
scription of one of his numerous experiments (STRASBURGER, 1893, p. 10).

' The summer oak selected for experiment was 21-7 m. high, 27 cm. thick, at
a height of 10 cm. above ground, and 75 years old. On June 28, about 4 p.m.,
the tree was sawn off, obliquely, 10 cm. above ground, while water flowed
rapidly in the cut. The severed trunk was at once raised to a vertical position
and suspended in a tub of water. It remained in this tub about half an hour,
whilst its cut surface was cleaned and smoothed with a sharp knife. The tree
was then put into a vessel filled with a saturated solution of picric acid, which
is intensely poisonous to the plant. It was sunk about 20 cm. in this fluid/
The upper limit of the solution in the vessel was noted and the amount kept
constant by filling the vessel up to this level morning and evening each day.

The amounts of fluid absorbed by the stem were as follows : —

Hours.
Litres .

I.

p.m. N.

II.

D. N.

III.
D. N.

IV.

D. N.

V.

D. N.

VI.

D. N.

4 H
5 6-5

9 '5

6 5-5

9 15

575 4'2

8 14

6 4-7

ft 4

12 12

37 3

VIII.

D. N.

12 12

1-45 0-7

IX.

D. N.

12 12

0-3 i

It is clearly seen that considerably greater amounts of water were taken
up at first during the day than during the night, although the period of observa-
tion termed ' night ' for short was markedly longer than the 'day'. Later,
after the leaves had died, this periodicity ceased and the absorption of fluid
as a whole fell off very considerably. After the fourth day the picric acid had
risen up the stem to a height of 15 m. and had killed these parts ; when fuchsin
was added to the solution the rise of this colouring matter could be followed
in the dead stem. At the end of the experiment, on the tenth day, it was

76 METABOLISM

shown without doubt that fuchsin had been carried to the top of the tree
without the co-operation of living cells, because parts of the stem investigated as
high as 21-8 m. from the ground were tinged with the pigment.

The experiment just described is undoubtedly among the most interesting
of those which have been carried out on the subject of water conduction, but
it cannot be regarded as absolutely convincing, for it raises many doubtful
issues. First of all there is the question why the absorption of water decreases
so rapidly. Probably because evaporation from the leaves ceases, as a con-
sequence of withering ; but we do not know whether the withering of the
leaves is due to death by the poison or is a result of reduced water supply.
It is quite possible to assume that water still rises in the dead stem, but no longer
with the requisite rapidity, or in amount equal to what the transpiring leaves
demand. A completely convincing experiment must show that the leaves which
are supplied with water by a dead stem are able to remain for a long time alive.
[URSPRUNG (Beihf. z. bot. Centrbl. 1904, 18, 147) has advanced certain im-
portant criticisms tending to disprove STRASBURGER'S experiments, but his own
experiments do not convince us that living cells conduce to the ascent of sap.
Compare DIXON, 1905 (Proc. R. Dublin S., n, No. 2) ; URSPRUNG, 1905
(Bot. Ztg. 63, II Abt. 241), and JOST, 1905 (Bot. Ztg. 63, II Abt. 243).]

When we spoke above of an indirect action of the living cells on the con-
duction of water, we meant an action not dependent on a sucking and pumping
upwards of air, but referred, rather, to the influence of living cells on the air
in the vessel. NOLL (1897), in a preliminary treatise, has noted the fact
that gases which are injected into the vascular system undergo changes, and
DEVAUX (1902) has observed a negative pressure in the air of the vessel on
the stoppage of transpiration. This he attributes to the withdrawal of oxygen
from the vessels owing to the respiration taking place in the living cells. Fur-
ther investigations have now to show whether negative pressures in larger areas
are really produced in the wood by this means. If that be the case, then it
would be impossible to doubt the suctional action of the expanded air-bubbles,
and the living cells would then play an important part in the ascent of water.

The summary we have given above shows that our knowledge up to the
present of the causes of the ascent of water is in many respects very imperfect.
A complete exposition of the voluminous literature (COPELAND, 1902) is out
of the question here ; it would lead us to no definite results, for we are ignorant
even yet of the answers to the most elementary questions. It may be difficult,
perhaps, to clear up entirely the darkness which surrounds this question,
although we may still look for future researches to throw light on the problem,
if these, as we have often already emphasized, pay more attention than hitherto
to the quantity of the water to be raised as contrasted with the quantity
actually raised.

Bibliography to Lecture VI.

ASKENASY. 1895. Verhandl. d. naturhist.-med. Vereins Heidelberg, N. F. 5.

AsKENASY. 1896. Ibid.

BOHM. 1879. Bot. Ztg. 37, 225.

CLAUSSEN. 1901. Flora, 88, 422.

COPELAND. 1902. Bot. Gaz. 35, 161.

CZAPEK. 1899. Zeitschr. f. phys. Chemie, 17, 143.

DARWIN, VINES, JOLY. 1 896. Report of a discussion on the ascent of water. Annals

of Bot. 10, 630.

DEVAUX. 1902. Compt. rend. 134, 1366.
DIXON and JOLY. 1895 A. Proc. Roy. Soc. 57, 3.
DIXON and JOLY. 1895 B. Phil. Trans. B. 186, 563.
DIXON. 1896. Proc. R. Irish Acad. 4, 61.
ELFVING. 1882. Bot. Ztg. 40, 713.

THE CONDUCTION OF WATER. II 77

GODLEWSKI. 1884. Jahrb. f. wiss. Bot. 15, 602.

HALES. 1748. Statik der Gewachse. Halle.

HARTIG, R. 1882. Unters. aus d. forstbot. Instit. Munchen, 2, i.

HARTIG, R. 1883. Ibid. 3, 73.

HARTIG, Th. 1861. Bot. Ztg. 19, 22.

HOFMEISTER. 1 862. Flora, 45, 97.

HOHNEL. 1879. Jahrb. f. wiss. Bot. 12, 47.

JANSE. 1887. Jahrb. f. wiss. Bot. 18, i.

McNAB. 1871. Trans. Bot. Soc. Edinburgh, n, 45

NAGELI. 1866. Sitzungsber. Bayer. Akad. d. wiss. Bot., Mitt. 2, 369 and 429.

NERNST. 1900. Theoret. Chemie, 3rded., p. 165.

NOLL. 1897. Sitzungsber. Niederrhein. Gesell., November.

PAPPENHEIM. 1892. Bot. Centralb. 49, i.

PFEFFER. 1892. Stud. z. Energetik. (Abh. Kgl. Gesell. d. Wiss. Leipzig, 18.)

REINGANUM. 1896. Annalen d. Physik (Wiedemann), N. F. 59, 764.

PFITZER. 1877. Jahrb. wiss. Botan. n, 177.

ROTHERT. 1899. Bullet, de 1'Acad. de Cracovie, 34.

SACHS. 1873. Arb. Wiirzburgerbot. Inst. i, 288.

SACHS. 1878. Ibid., 2, 148.

SACHS. 1879. Ibid., 2, 291.

SCHWENDENER. 1 882. Die Schutzscheiden. Abh. d. Berl. Akad. (Ges. Abh. 2).

SCHWENDENER. 1 886. Sitzungsber. Berl. Akad. 561 (Ges. Abh. i, 207).

SCHWENDENER. 1892. Ibid. 911 (Ges. Abh. i, 256).

SCHWENDENER. 1893. Ibid. 835 (Ges. Abh. i, 298).

STEINBRINCK. 1894. Ber. d. bot. Gesell. 12, 120.

STRASBURGER. 1891. Bau und Verrichtungen der Leitungsbahnen (Histol.

Beitr. 3). Jena.

STRASBURGER. 1893. Ueber das Saftsteigen (Histol. Beitr. 5). Jena.
VESQUE. 1883. Ann. sc. nat. 15, 5
WESTERMAIER. 1884. Sitzungsber. Berl. Akad. 1105.
WIELER. 1888. Jahrb. f. wiss. Bot. 19, 82.
WIELER. 1893. Cohn's Beitr. z. Biologic, 6, i.
ZIMMERMANN. 1885. Berichte d. bot. Gesell. 3, 290.

[Many of the questions with which this lecture deals have been discussed by
EWART, 1905 (Phil. Trans. B. 198, 41-85 ; abstracted in Proc. R. S. 74 ; compare
Ann. Bot. 443). Into the results of his studies it is unfortunately impossible to go.]

LECTURE VII
ASH. I

ALL plants contain greater or less quantities of incombustible substances,
and small fragments of the cell-wall or starch granules leave behind them, on
combustion, demonstrable quantities of ash. The experiences of everyday life
confirm this. Every one in days gone by had an opportunity of seeing wood-
ash, but, perhaps, this opportunity does not occur quite so frequently in these
days of coal consumption ; still cigars are smoked everywhere, and they
illustrate the relatively large proportion of ash present in plant organs. No
modern investigator has any doubt that all these mineral constituents of the
plant must be obtained from without, and in the main from the soil. It is,
therefore, most instructive to note that in earlier times this, to us, self-evident
fact was thought to require definite proof, and that, even after the establish-
ment of the law of the indestructibility of matter, famous academies suggested
prize essays with the object of determining whether this law held also for
organic nature. For instance, in 1800, the Berlin Academy formulated this
question : —

' By what means are the earthy constituents obtained which, as a result
of chemical analysis, are found to be in the various indigenous cereals ? Do
they enter these plants in the same form as they are found in them, or are

78 METABOLISM

they produced by the agency of the vegetative organs themselves ? ' The
answer given by SCHRADER (1800) to this question ran as follows : — ' Plants
develop these ash constituents by their own vital force.' Almost forty years
later (1838), the Academy of Gottingen again formulated almost identically the
same question : — ' Whether the so-called inorganic elements which are found in
the ash of plants are still to be found there when they are not supplied to the
plant from without ? ' Such a question was certainly somewhat out of date.
DECANDOLLE'S Plant Physiology (1831), (German Edition, 1833, 1, p. 388), con-
tains a refutation of SCHRADER' s view.

The principles of chemistry had now, however, been established on a wider
basis, and the answer given to the question was totally different. WIEGMAN
and POLSTORFF (1842), by their researches, clearly proved the correctness of
the view now held.

Closer inquiry into the conditions governing absorption shows us clearly
that these substances pass through the external walls of plants in a state of
solution, since these walls are impermeable to solids. Not only the medium
of solution, water, but also, as a general rule, the materials of the ash are taken
up through the cells of the root ; only in rarer cases, e. g. in many epiphytes,
do the leaves take part in the absorption. The law of osmosis governs this
absorption and determines the nature and amount of the mineral matter
absorbed.

The nature of the salts depends, in the first instance, on the permeability
of the protoplasm, but exhaustive researches are still required to show to what
degree protoplasm is permeable to the inorganic salts which in nature come
into relation with the plant. So far it has been clearly established that the
ash of plants contains by no means all the mineral matters which occur in the
soil or in the water. Aluminium, for instance, is widely distributed in nature,
and many of its compounds are soluble in water, but, nevertheless, it is absent
entirely, or almost entirely, from the majority of plants, although it does occur
abundantly in a few species. On the other hand, we know that iodine is present
in sea-water only in such small amounts that it can scarcely be detected, yet
many Algae take it up in relatively large quantities. Generally speaking, the
mineral matters occur in the plant in proportions quite different to what they
do in the outside world. This is illustrated by WOLFF'S (1871, I, 132) analysis
of the ash of Lemna trisulca, as compared with that of the water in which the
plant was grown.

Minerals present in 100 parts of Ash.

K2O Na3O CaO MgO Fe2O3 PaOs SO3 SiO2 Cl

Water . 5.15 7-60 45.56 16.00 0-94 3-42 10.79 4-23 7-99

Lemna . 18-29 4'Q6 ai-86 6-60 9-57 11-35 7-91 l6-°5 5'55

From this summary it is impossible to say whether these substances occur
in the cell-wall or in the protoplasm ; if in the latter we are still ignorant as to
the permeability of protoplasm to these materials. Because a large amount
of iron is taken up it does not follow that the protoplasm is easily permeable
to it, and, conversely, the fact that lime is present in relatively less amount
in the plant than in the water must not be taken as indicating that protoplasm
is less permeable to that substance. We have already seen, in speaking of
osmosis, that, given that protoplasm is permeable to a substance, no storage
can take place in the plant unless the substance be altered in some way after
its absorption. The most obvious example is the coloration which is produced
by very dilute solutions of methylene blue. This is only possible if the methy-
lene blue becomes separated out in insoluble combinations. We are, however,
ignorant in individual cases wherein the changes in inorganic substances con-
sist which prevent their exosmosis and facilitate their accumulation. From

ASH. I

79

numberless analyses we learn that such accumulations take place in individual
plants specifically differently, and further, that different species may grow in the
same situation and yet show quite different ash constituents. As an example
we may take the analysis which GRANDEAU and BOUTON (1877) have made of
the mistletoe, and of the different media from which it obtains its ash : —

Percentage of
Ash in dry wt.

In 100 parts of pure Ash.

KaO

Na2O

CaO

MgO Mn20s

~~i?20

9.21
2.51
6.72
7.12
12-19 10-67

Fe203

P205

S03

Si02

Cl

Poplar ....
Poplar Mistletoe .
Robinia ....
Robinia Mistletoe
Fir

3-04
3-46
2-06
2-13
1-61
3-i4

6-56
16-09

a-35
15.90
8-40
30-79

2.82
2.04
0.47
2-58
2-03
trace

66.47
32-55
75-04
45-39
67-43
27.13

2-38
5-40
i-83

2-20
I-OI
1-52

4-76
26-29

3-45

12-02
7-89
IS'"

1-49
2-09
0-78
2-74
2-80
3-35

5-8i
4-79
n-77
6-41
2.03

1-22

1.64
1-47
1.72
2-01
1.27

trace

Fir Mistletoe . .

This table clearly shows that the mistletoe has markedly less silicic acid,
less lime, and much more potash and phosphoric acid than its host. Moreover,
the plants growing on these three different substrata are by no means similar in
chemical composition, and the differences between them cannot be in any way
referred to corresponding differences in the substrata. These differences are
peculiar to the individual and are at present inexplicable. It is self-apparent
that a definite species can, caeteris paribus, take up more of a substance from
a soil which contains it abundantly than from one which contains little, as is
shown by MALAGUTI and DUROCHER'S (1858) work on the analysis of lime as
a constituent of plant ash.

The total amount of ash in the example given above is very limited and
constitutes only a small percentage of the dry weight, but in other cases, the
amount is quite considerable. In addition to the large table given below, the
first column of which shows this fact prominently, we may extract the following
figures from WOLFF'S ' Ash Analysis ' (I, 137), dealing with several common
weeds grown on similar soils. The ash of Rumex acetosella amounted to
8-14 per cent., of Geranium dissectum to 9-98 per cent., of Sedum telephium to
11-96 per cent., and Myosotis arvensis to 17-85 per cent, of the dry weight in
each case. Far larger quantities of ash are found in seashore plants ; e. g.
16-51 per cent, in Aster tripolium, 17-91 per cent, in Artemisia maritima,
and 31-57 per cent, in Chenopodium maritimum. Although we must attribute
the large proportion of ash in these cases to the presence of large amounts of
common salt in the sand, still we find other conditions in other cases tending to
increase the amount of ash constituents. We need refer here only to transpira-
tion. On grounds which are easily understood, plants which transpire freely
are far richer in ash than those which transpire feebly, and the leaves, being the
organs of transpiration, appear to contain most. On that account transpiration
is of great value to the plant, because, as we shall see presently, the constituents
of the ash are essential to its well-being, and by no means superfluous or injurious.

The individual elements which occur regularly in the ash of all plants are only
eleven in number, i. e. : — chlorine, sulphur, phosphorus, silicon, and the metals
potassium, sodium, calcium, magnesium, iron, aluminium, and manganese.
As a rule the last two occur as traces only, the rest abundantly. The following
table ( WOLFF, 1880) shows the composition of the ash of certain plants, but only
the nine principal elements are noted ; the special abundance of an element is
emphasized by italics.

8o

METABOLISM

In

100 p

arts of

pure /

^sh.

%of

Ash.

K20

Na20

CaO

MgO

Fe.0,

P.O.

SO3

SiOa

Cl

i. Tobacco leaves
a. Potato tubers . .
3. Spinach ....

17-16

3-79
1648

29.09
60-06
i6.<;6

3-2i

2-96

3f'2O

36-02
2.64
11.88

7-36
4-93
6-^8

i-95

I-IO
q.oc

466
16.86

IO-2^

6.07
6-52
687

5-77
2.04

4*^2

6-71
3-46
6*20

4. Oak bark (150 years)
5. Beech in flower .
6. Almonds (seed) .
7. Italian clover . .
8. Wheat (fruit) . .
g. Horse radish (root)
10. Equisetum telmateia
H. Barley after flowering
la. Celery (root) . . .

7-ao
6-86
4.90
9.87
2-14

8-47
26.7;

6.47
11-04

4-36
32-39

2795
33-42
30-5I
38-96
8.01
25-44
4319

o-34
i-95
0.23
13.01
1.74
a 10
063
o-75

02-82
34.91
8-8i
12-73

282

10-10

8.63

5-77
13-"

1.19
10-90
17-66

6-53
11.96
3-66
1-81
3-03
5-8a

0*29
I- 08

o-55
j.86
0-51

i-S1

1.42
0-42
1.41

o-39
9.64

43-63
4-63
48-94
10-39

i-37
10.29
12-83

0-27
3-23
037
14.41
1-32
24-72
2-83
2-94
5-58

o-55
2-69

4-50
1-46
7-20

70.64
49-83
3-85

3^8

0-47
1-36
5-59
3-77
15-87

In addition to the problem as to how the constituents of the ash enter the
plant we have the further question as to whether they are of value to it or
merely accessories accidentally introduced along with the water. SENEBIER
(1800) and SAUSSURE (1804) have already shown that certain minerals are
essential to the plant as food-stuffs ; this view LIEBIG (1840) supported very
strongly, and owing to his authority it received general acceptance, although,
strictly speaking, it was not exactly proved till later. The methods employed
for this purpose are two in number. Both were intended, at the same time,
to show whether all or which of the ash constituents found in the plant were
essential. Important experiments for this purpose were first carried out by
Prince SALM-HORSTMAR (1856), who employed the first method. He cultivated
the plants, following the example set by WIEGMAN and POLSTORFF (1842),
in insoluble artificial soils, to which were added the materials which were to
be investigated. He used, e. g., soil composed of pulverized rock crystal and
carbon obtained from candy sugar, whilst WIEGMAN and POLSTORFF worked with
platinum filings and sand. SALM made out that silicon, phosphorus, sulphur,
potassium, calcium, magnesium, iron, and manganese were essential to the
normal development of oats, but he was doubtful as to the significance of
chlorine. Although his results could not be completely substantiated, still they
were very valuable in one point, inasmuch as they proved that sodium, though
never absent from the ash of plants, was not to be included among the
essential elements of oats (although SALM believed it to be essential in other
cases). This fact is all the more remarkable when one remembers that sodium
has important functions to perform in the higher animals.

The other method — the so-called water-culture method — is of the greatest
value for our purpose. Although SAUSSURE (1804) early in the century grew
Bidens and Polygonum in water, SACHS (1860) and KNOP (1860) were the first to
cultivate, experimentally, land plants in such a way that their roots, immersed
in a watery solution of various salts, could supply their requirements so far
as inorganic salts were concerned, and proved that the plants cultivated in
this way showed a large increase in their dry weight. An increase in dry
weight, especially a large increase, is a valuable criterion of the success of such
a culture, but we should fall into serious error were we to conclude from this
that a plant grows only if it be supplied with all the necessary food materials.
Growth can take place without increase in dry weight, and indeed without
the absorption of water. Again, we should be totally wrong were we to
conclude from the fact that plants, without taking up nitrogen, can reach
a weight three and a half times that of their seed (BoussiNGAULT, 1860), that
nitrogen was not necessary for growth. From the observations of many in-
vestigators it has been shown that maize can develop under the most favourable

i-og.

ASH. / 81

conditions from 60 to 370 times, and that buckwheat can increase 1,000 times
the weight of its seed ; an increase in dry weight of this amount may well be
described as abundant. Moreover, seedlings may show an increase in weight
without any absorption of ash constituents from the surroundings, and this is
explained quite simply by the not inconsiderable quantity of ash which the seeds
contain. Beans, for example, may, according to BOUSSINGAULT, develop up
to the flowering stage entirely without nutritive salts and, in consequence,
double or quadruple their dry weight. We see from such illustrations that it is
necessary to eliminate the reserves in the seed entirely, if we desire to prove the
essential character of an element which may be present in small amounts in the
seed, but which amount may, for the purposes of development, be large enough.

Without going into details we may mention only, with regard to the water-
culture method, that it is the custom, as a rule, to start from seeds which have
developed their principal roots in sawdust. The seedlings are then fixed in
the cork of a vessel of sufficient size, so that the stem is allowed to grow up-
wards and develop in light and air, while the root branches in the vessel contain-
ing the nutritive solution (Fig. 19). It is essential to prevent light from entering
the vessel, and this is done most effectively by sinking it in soil and at the
same time time keeping it at a uniform and not too high temperature.

The following substances are employed as nutritive salts dissolved in water
so as to form a solution of a concentration of a few parts per thousand :

I. II. in.

BIRNER and LUCANUS (1866). KNOP (1868, 606 ; 1884). SACHS (1882, 342).

Magnesium sulphate, about 0-5 g. . . 0-25 . . 0-5 g.

Calcium nitrate . . 1-5 g. . . i-oo Sodium chloride .

Acid potassium phosphate i-og. . . 0.25 Potassium nitrate

Ferric phosphate . . i-ig. Calcium sulphate .

Potassium chloride . 0.12 g. Calcium phosphate

Ferric chloride . . trace (finely ground)

Ferric chloride . trace

The first of these nutritive solutions is the simplest, and with its aid BIRNER
and LUCANUS were able to obtain a complete culture experiment with oats,
where the increase in dry weight was equivalent to 138 times the weight of the
seed. If we ignore for the present the nitrogen, which we will study later,
and which does not really belong to the ash constituents at all, we find that
the plant requires the following six elements : potassium, calcium, magnesium,
sulphur, phosphorus, iron. It can do without the other two elements, silicon
and manganese (which SALM-HORSTMAR believed to be essential), and also with-
out chlorine, as to whose indispensableness SALM was doubtful. All the six
elements first mentioned are absolutely essential, however, and if only one of them
be absent from the solution the increase in dry weight is greatly curtailed. In-
stead of increasing (in dry weight) from i to 138, the plant, in the absence
of magnesium, increases only to 5-1 ; without calcium, only to 1-3 ; without
potassium, only to 9-2 (compare Fig. 19, //) ; without iron only to 7*3 (in
another experiment only to 3-3) ; without phosphorus only to 6-5 ; without
sulphur only to 4-9 ; and only in a second experiment, where sulphur was
omitted, did the increase reach the relatively high figure of 35-4.

Numberless experiments have been carried out with such nutritive solu-
tions, all giving the same or similar results. [CRONE has obtained excellent
results with the following solution : potassium nitrate, I g. ; ferrous phosphate,
°'5 g.: gypsum, 0-25 g.; magnesium sulphate, 0-25 g.; made up to 1-2 lit. with water
(1004, Ergebnisse von Untersuch. iib. d. Wirkung der Phosphorsaure auf d.hohere
Pflanze, &c., Diss. Bonn). Compare BENECKE,i904 (Bot.Ztg.62, II Abt. p. 123).]

It has been found best to keep the solution slightly acid ; if it be alkaline
it is apt to react injuriously on the plant, save in the case of aquatic plants

JOST G

82

METABOLISM

both of higher and lower grade, which frequently thrive better in a weak alka-
line than in weak acid solutions (BENECKE, 1898). In addition to the reaction
the concentration and quantities are of primary import. In general we use
a solution of 1-5 per cent, of salts ; but NOBBE (1867) found that a 5 per cent,
solution was injurious to barley, and was forced to use solutions of weaker
concentration and to employ larger vessels. More recently, WORTMANN (1892)
experimented with culture vessels of much greater capacity, holding about
25 lit. In such vessels plants grow remarkably well, and it is unnecessary to
renew the culture fluids during the vegetative period.

Annual plants are especially well adapted for water-culture experiments,
since rapid results may be obtained from them.
Thus many grasses, Cruciferae, buckwheat, rape,
linseed, and Tradescantia, as well as potatoes, may
be employed with success. Indeed in the case of
buckwheat it has been affirmed (NoBBE, 1868)
that the plant grows better and shows a far greater
increase in dry weight (from I to 4786) under such
artificial conditions than when cultivated in soil
in the ordinary way. A similar remarkable develop-
ment is attained by oats, which, according to
WOLFF (1868) show an increase from i to 2359.

Many trees also, such as the oak, horse-chest-
nut, and alder, can be cultivated in aqueous solu-
tions. It is not surprising, however, to find that not
every plant proves a suitable subject for water-
culture, since the method assumes a capacity for
absorption on the part of the root under conditions
which are far from natural — a capacity which the
root of every plant does not possess. In those
cases in which the water-culture method is un-
favourable to the growth of the plant, the method
recommended, especially by HELLRIEGEL (1883),
may be adopted, viz. to employ a medium consist-
ing of a quantity of sand which has been thoroughly
cleaned by being heated to redness and boiled in
sulphuric acid, and to which has afterwards been
added the ingredients whose functions require
investigation.

The culture methods used for the lower plants,
Algae and Fungi, need not be studied here in
detail, because they are either self-apparent or
are, as in the case of seaweeds, still in want of
improvement.

Collecting together all the results which have
been arrived at as to which constituents of the ash
in different plants are indispensable, we find that
the six elements essential, according to BIRNER and LUCANUS, to the growth of
oats, are also essential to all other Phanerogams, or, at any rate, to the majority
of them, while to buckwheat (according to NOBBE, 1862) chlorine is also essen-
tial. Since, in other cases also, chlorine has been found to produce a favourable
effect it may be added to the nutritive solution. The problem as to what special
demands on inorganic materials individual plants may make will be discussed
later on. It has been shown by BENECKE (1894-8) and MOLISCH (1895-6)
that plants of low grade such as Fungi and Algae require fewer inorganic salts
than Phanerogams, for calcium is not essential in their case, so that only
five elements have to be considered as essential.

Fig. 19. Buckwheat grown in water-
culture solutions. /, normal ; //, with-
out potassium. (After NOBBE, from the
Bonn Textbook.)

ASH. / 83

The plant requires a certain definite amount of each essential element. If
too little of one element be present the plant is unable to develop healthily,
even if the others be present in excess. This fact is sometimes expressed thus : —
(AD. MAYER, 1902, I, 323) The nutrient present in minimum quantity gives a
standard for the amount of production as a whole. (' Law of the minimum.')

Despite the efforts of numerous investigators we are, unfortunately, still
very much in the dark as to wherein the need for these five or six elements lies,
and on taking stock of what little we do know we are brought face to face with
the further problem as to the combinations in which these elements are employed
by the plant. The significance of sulphur and phosphorus has been sufficiently well
established, for they are, as we have seen, as essential constituents of proteids
as carbon, hydrogen, oxygen, and nitrogen ; sulphur occurs generally in
such bodies, but phosphorus only in some of them, e. g. nucleins and many
globulins. Further, it is by no means a matter of indifference in what com-
binations these elements are presented to the plant ; it has been shown, on
the contrary, that they must be presented in highly oxidized forms, such as
sulphuric and phosphoric acids. Sulphurous and hyposulphurous are as use-
less as phosphorous and hypophosphorous acids, in fact, they are even poisonous
to many plants ; nor can sulphur and phosphorus be used in the elemental
form. It may be noted, in passing, that only one essential element is absorbed
by the plant in the elemental form, viz. oxygen. Further, sulphuric acid must
be presented to the plant, united with a metal, although it is apparently a matter
of indifference whether it be supplied as a sulphate of potassium, magnesium,
or calcium, thus offering simultaneously a necessary metal, or as a sulphate of
sodium or aluminium. We are, again, quite ignorant as to the region in the
plant where such sulphates and phosphates are transformed or 'assimilated'.
SCHIMPER (1890) has shown that plants which absorb abundance of sulphate
store it unaltered in many cells ; if, however, only a small quantity of sulphate
is available, it is, after entry, altered into a form which no longer gives the
sulphuric acid reaction ; no such reaction can be obtained from the young
cells of meristem or from buds or pollen-grains. The same is true of phosphoric
acid ; it may be presented as a salt of potassium or sodium, and these
compounds are more soluble than compounds with calcium, magnesium, \
or iron. In all situations where one fails to obtain the sulphuric acid »
reaction the phosphoric acid reaction is also wanting ; at the same time, \
certain plants, e. g. the horse-chestnut, Forsythia, the onion, &c., are well
known to store up large quantities of phosphate in old parenchymatous cells
of the leaf (compare also Lecture XI). In addition to the proteids it may be
noted that there are other sulphur-containing compounds in the plant
which, however, owing to their limited distribution, need not be referred to
here ; they will be discussed later on in Lecture XVIII, where the special
significance of sulphur in the vital economy of the sulphur-bacteria is treated of.

The third non-metallic element we have to consider is chlorine. It is em-
ployed only as hydrochloric acid and generally added to the solution as
potassium chloride. As already mentioned, chlorine cannot be ranked as of
the same importance as sulphur or phosphorus, for although of very general
service, it is rarely directly essential. In addition to being present in buckwheat,
as already mentioned, chlorine occurs (according to BEYER, 1869) in peas and
oats, but as to its special significance in these plants we are entirely in the dark.
It might be expected to play some part, more especially in plants which grow
in soils containing common salt, but some of these plants develop perfectly well
without any chlorine. We are ignorant also whether seaweeds can exist with-
out this element.

Among the metals, potassium is absolutely essential, and it is immaterial with
what acid it be united. Efforts have been made to replace potassium by one or
other of the related metals, lithium, sodium, rubidium, caesium, but all these are

G 2

84 METABOLISM

found to be quite unsuitable, and, save sodium, all are poisonous. Sodium, it is
true, may to a certain extent act as a substitute for potassium, e.g. if the latter be
present only in small amount. Under such circumstances the plant thrives
better if sodium be provided than if it be not ; a partial replacement of potas-
sium by sodium may be conceded, but we must not forget that this does not
apply in other cases, and certainly never when the principal functions are under
consideration. The case appears to be otherwise in the lower organisms. The
Cyanophyceae (according to BENECKE, 1898, p. 96) get on just as well with
sodium as with potassium ; in the lower Fungi potassium cannot be replaced
by sodium or lithium, but BENECKE found a marked increase in dry weight
when rubidium only was used, an increase which, when the rubidium was
present in a certain concentration, was as great as when potassium was present
in the nutritive solution. It is true that in this case there was a development
of vegetative organs only, and no spores were formed, hence one may conclude
that potassium cannot be entirely replaced by rubidium. Caesium behaves
like rubidium. It is possible that traces of potassium were present with these
metals as impurities, and so might influence the result; another explanation
of this remarkable result will be given later (p. 88). Apart from such doubtful
cases, we may say that potassium is absolutely essential. The function of
potassium in higher plants may be deduced from the effect of its exclusion
from water-cultures. SCHIMPER (1890) observed that in Tradescantia new
organs containing potassium were still produced at the growing point, although
potassium was excluded from the culture fluid. This is explained by the fact
that the new leaves obtained it from the older leaves, which died off when
potassium was removed. The new leaves were in every case, however, smaller
and thinner, and in the end attained only minute dimensions. As the amount
of potassium available from the dying tissues became less and less the growing
point began to die also. This research proves conclusively that potassium is
essential to the formation of the primordia of organs, whose size depends
within certain limits on the amount of potassium available. Although not
based on actual evidence it is still very probable that potassium plays an im-
portant part in the construction of the principal compounds which occur in
protoplasm, more especially the proteids. The evidence for this belief is not
as yet forthcoming in physiological chemistry, still any day may produce it.

What has been said of potassium is true also of magnesium. This metal
cannot be replaced by any related alkaline earth ; it is itself essential to every
member of the vegetable kingdom. Water-cultures, from which magnesium
has been excluded, give results similar to those which have no potassium. In
this case, also, we are driven to believe that magnesium takes part in the con-
struction of proteid, more especially since, according to SCHMIEDEBERG (1877),
the proteid crystals of the brazil-nut consist of a magnesium salt of vitellin, and
since GRUBLER (1881) has proved that the crystallizable proteid of the gourd
contains magnesium in no inconsiderable amount. Magnesium appears always
so to be a constituent of chlorophyll.

The case is quite otherwise with calcium. Though most Algae and Fungi
can do without it (according to BENECKE, 1898, it is essential in the cases of
Spirogyra and Vaucheria), we must not jump to the conclusion that it is an
important constituent of the proteids of protoplasm, although various im-
portant chemical compounds in individual groups of plants contain it. There
are other grounds, however, against this view as to the significance of calcium
in Phanerogams. To begin with, according to SCHIMPER, calcium is absent
from regions where protoplasm is being formed and where potassium and mag-
nesium are prominently present, e. g. at growing apices ; on the other hand,
it occurs in large quantities in older organs and especially in leaves. The ap-
pearance of Tradescantia when grown in a water-culture without calcium is

ASH. / 85

quite distinct from that in the absence of potassium or magnesium. During
the first few weeks the growth appears healthy and the newly formed leaves
are of normal size ; then they — but not the older leaves — begin to die off after
the appearance on them of brown spots. SCHIMPER has shown that these
brown spots are due to poisoning by oxalic acid, which cannot be neutralized
owing to the absence of calcium, and he has drawn the conclusion that, in
general, the neutralization of this acid is the function of calcium. PFEFFER
(Phys. I, 427) points out, on the other hand, that those plants which do not
form calcium oxalate, and they are by no means few in number, are as much
injured by free oxalic acid or potassium oxalate as the others. More recently,
PORTHEIM (1901) has succeeded in demonstrating that beans grown in a calcium-
free soil become diseased, but that they showed the presence neither of oxalic
nor of any other strong free acid. SCHIMPER' s hypothesis may be correct in
individual cases, but it does not explain the general necessity for calcium, and
we are bound to admit that its function has not as yet been discovered. In look-
ing for a substitute for calcium, strontium would naturally first occur to us, but
the researches of HASELHOFF (1893), undertaken to determine the capabilities of
replacement of calcium by this metal, are not very convincing. BENECKE (1895,
p. 521) found that strontium was poisonous to Fungi.

Our knowledge of the function of iron has been long believed to be much
better grounded than that of the other constituents of the ash. Absence of
iron in Phanerogams brings about the highly characteristic appearance known
as chlorosis. Chlorosis consists in the young organs taking on a pale yellow
or bleached appearance, as a consequence of which they soon die, owing to
their not possessing chlorophyll, which we shall find (Lecture IX) plays so
important a part in nutrition. For the development of the chlorotic condition
in plants water-culture solutions from which every trace of iron has been care-
fully excluded are necessary. Even then chlorosis appears at first only gradually.
The first leaves of the seedling are always green, because there is enough iron
present in the reserves to supply what is wanted. As a matter of fact some
plants with large cotyledons, such as the bean, are especially unsuitable for such
experiments, because the amount of iron present in them is sufficient for the
whole plant ; good results may, on the other hand, be obtained by using such
plants as maize, buckwheat, or sunflowers. Pea-seedlings grown in iron-free
nutritive solutions develop, according to MOLISCH (1892), three or four green
leaves first of all, then one yellowish-green leaf; the remaining leaves, as well as
the tendrils, are white. Such chlorotic plants may be again made green, as E.
GRIS (1843) first showed, by permitting them to absorb an iron salt through
the root, or by applying it directly to the chlorotic leaf. The cuticle of the leaf
must, for the success of the experiment, be possessed of a certain degree of per-
meability, such as is exhibited, according to MOLISCH (1892) by Helianthus.
Further, if greening is to take place the chlorotic organ must be young ; appli-
cation of an iron salt to old chlorotic leaves has no effect.

For a long time chlorophyll was held to contain iron, and the appearance
and disappearance of chlorosis in these experiments was accounted for by the
presence or absence of that metal. The question as to whether or not chloro-
phyll contains iron has been reinvestigated by MOLISCH (1892), who has con-
firmed an older research of RAULIN'S (1869), showing that Fungi which have
no chlorophyll, cannot do without iron ; BENECKE (1895) draws attention to the
same fact. Another function, therefore, must be found for iron. It seems
probable that, like potassium and magnesium, iron is necessary to the formation
of protoplasm, and that its absence is followed by chlorosis in the higher plants
as a secondary effect. Further, iron cannot be replaced by any other related
metal such as manganese.

One element present in the nutritive solution alone remains to be con-

86 METABOLISM

sidered, viz. nitrogen. When nitrogen in the form of nitric acid is absent
no noticeable increase in dry weight takes place even though all the other
salts be present, hence it should be mentioned here that nitric acid, in a form
capable of being absorbed from the soil in water, is essential, although,
not being found in the ash of plants, it need not be discussed here. It may
be noted that one of the characteristics of the substances which we have hitherto
been discussing is their capacity for resisting heat, but this characteristic is of
no consequence so far as the plant is concerned ; nor is it of any importance
whether, in ordinary combustion, the nitrogen is given off as a free gas or as
ammonia, since in the plant the nitrogen is firmly combined and only very
rarely escapes in the gaseous form. We will content ourselves with noting
that nitrogen, in the same sense as sulphur, potassium, phosphorus, calcium,
magnesium, and iron, is an essential food-stuff in every plant ; any further dis-
cussion of its characters would be premature (compare Lecture XI).

To sum up, we may say that we have clearly established a function for
nitrogen, sulphur, and phosphorus, viz. that they undoubtedly take part in
the formation of the living substance, and we have, further, good ground for the
belief that to these elements must be added potassium, magnesium, and iron ;
on the other hand, it may be said with certainty that this is, in general, not
true for calcium. Scattered through the very voluminous literature on the
subject, reaching from the time of LIEBIG to our own day, we find many state-
ments, suggestions, and hypotheses as to the function of the inorganic salts.
Thus, according to LIEBIG, the bases act as neutralizers of the acids — a fact
which cannot be doubted — but it is not so easy to say why special metals
should be required for this purpose. Again, it is stated that potassium is
required for the formation of osmotically active bodies, that other elements
render possible, or play a part in, the circulation of proteids, or in the construc-
tion of the cell membrane, of the nucleus, or the other organs of the cell. We
must rest content with this brief summary of the literature and leave over any
detailed criticism for the present, seeing that the various views above referred
to have not been sufficiently established.

In addi ion to the essential constituents of the ash the plant also absorbs
non-essentials from the soil, in greater or less quantity. Generalizations on
this question are, however, scarcely valid, and although, also, very few plants
are to be found which can subsist on the six elements mentioned above, there
are others again which make specific claims on the soil. Buckwheat, which,
according to NOBBE (1862), cannot fruit properly if chlorine be absent,
may be cited as a striking example of the existence of such specific differ-
ences. It is necessary, first of all, to examine substances which occur only in
certain plants. Thus it would not be surprising if it were proved that iodine
was an important nutrient for marine Algae, or if it turned out that aluminium,
which forms 22 to 39 per cent, of the ash of Lycopodium chamaecyparissus,
L. complanatum, and L. clavatum, and yet appears in the minutest traces in
most plants, including several other species of Lycopodium, has a special
function to perform in these plants. [Large quantities of aluminium occur in
species of Symplocos and Orites (CZAPEK II). JAMANO (Bot. Centrbl. 99, 2)
found that aluminium was of service in the development of barley.]

Similarly lithium, which is, generally speaking, not only redundant but
even poisonous, may be useful to those plants in which TSCHERMAK (1899)
demonstrated its constant occurrence in plants taken from various situations.

We do know, however, that certain substances which appear to be absorbed
in large quantity are yet actually superfluous, although they must not be looked
upon as entirely functionless. Sodium, for instance, appears in almost all analyses
quoted in the table on p. 80 in larger quantities than the indispensable iron. It
maybe assumed that this element has some duty to fulfil, though we cannot prove

ASH. I 87

it. It might serve, for instance, in place of another metal to neutralize acids
and in the form of a salt act as osmotically in the cell-sap. Again we may note
that silicon very rarely occurs in young organs or in seeds, although it is abun-
dantly present as silica in the shells of diatoms and in horsetails and grasses (see
p. 80, Nos. 10 and n), and in the majority of cases is localized in the cell-walls of
old stems and leaves. Its accumulation does not necessarily point to a use in
metabolism, since its appearance maybe due merely to withdrawal of the medium
of solution. SALM-HORSTMAR held that silica was an essential constituent of the
plant, but SACHS (1862) showed that maize could be grown satisfactorily in a
• silica-free water- culture. The evidence is, however, not quite conclusive, since the
ash of maize plants grown in the 'silica-free' solutions still contained 0-7 percent.
I of silicic acid (instead of 18-23 Per cent.), which it had absorbed from the glass
of the vessel in which the culture was made. Similarly, JODIN (1883) culti-
vated four generations of maize in silica-free solutions, one after the other,
[ f but he was not successful in completely excluding silicon, for in the
) f •' second generation there was more silica present than sulphuric acid. On the
: j other hand, some observers, e. g. SWIECICKI (1900) have endeavoured to show
that silicic acid has a favourable influence on the plant. At present, all we
can say is that the large quantities of silica present in grasses are certainly
unnecessary, but that it has not been proved that they can get on equally
well when silica is entirely absent. As to whether silica is of use or not in the
Equisetaceae and Diatomaceae we are quite ignorant. Again, it is worthy of
note that although silica may be quite superfluous from the chemical point
of view it may be of great service to the plant in the biological sense. Our
knowledge of these subjects, despite the amount of work which has been ex-
pended on them, is still very imperfect, and it is possible to defend the assertion
that all the ash constituents have definite functions to perform, although these
have not as yet been determined in all cases, and although these constituents
cannot be considered as taking part in metabolic changes.

Under the circumstances it is unnecessary to present a complete enumera-
tion of the 'non-essential* constituents of the ash. The occurrence of manganese
may, however, be specially noted, as leading to the consideration of a new series
of phenomena. It is not widely distributed in the earth, and yet is found, though
only in traces, in very many plants. RAULIN (1869) has shown it to be instru-
mental in the development of Mould-fungi. Nevertheless it is certain that these
organisms can exist for generations without manganese, and that it must not be
looked upon as a nutrient. [See GOSSL (Beihft. z. bot. Centrbl. 1905, 18, i, 119)
for a discussion of the distribution and functions of manganese.] Still more notice-
able are RAULIN' s discoveries with regard to zinc, which have been recently
completely confirmed by RICHARDS (1897). (Certain corrections of RICHARDS'S
results have been made by A. RICHTER, 1901.) RAULIN showed that the addition
1 of 0-0005 per cent, of zinc sulphate to a nutritive solution materially aided the
growth of Fungi, and that a 0-003 Per cent- solution of the same salt brought
] about a doubling of the plant's weight. The greatest effect was observed
i with this concentration, a further increase not only inhibited growth but
acted injuriously. There are quite a number of substances which behave
in a similar way, acting favourably in dilute solutions, but injuriously in
stronger. Cobalt sulphate gives an optimum effect with a concentration
of 0-002 per cent. ; nickel sulphate acts best in a 0-033 Per cent, solution.
ONO (1900) found that an acceleration of growth took place after the addi-
tion of minute quantities of ithium nitrate, potassium arsenite, and sodium
fluoride to Algae, and of mercuric chloride and copper sulphate to Fungi.
But a poisonous effect does not always take place immediately, certainly not
in the case of silicon, which, according to RAULIN and RICHARDS, acts bene-
ficially. On the other hand, there are poisons which are injurious in small

88 METABOLISM

doses and never accelerate growth, e. g. mercuric chloride, copper sulphate in
Algae (ONO), copper sulphate in Aspergillus (RICHTER, 1901). How are such
results to be interpreted ? RAULIN considered zinc and silicon as direct nu-
trients for Fungi, a view which cannot be accepted nowadays ; it is more
probable that RICHARDS (following on the preliminary investigations of PFEFFER,
1895) is correct in holding that these substances act as stimulants and not as
nutrients. Furthermore, on the presentation of the optimum amount of the
stimulant, development in the fungus is no longer normal, but an increase of
vegetative growth is induced and a retardation of the formation of conidia. The
normal correlation of growth in the organs is also interfered with, and the organism
as a whole ceases to thrive when such conditions are introduced. It is, as a
rule, quite possible to differentiate nutritive from stimulatory materials, for when
the nutritive substances, or only one of them, are carefully eliminated, the develop-
ment of the organism comes to a standstill ; when the stimulants are omitted,
growth is retarded but is otherwise normal. This distinction is not readily
made out in all cases ; iron, for example, is a difficult element to deal with, be-
cause it is essential only in the minutest traces, and is possibly both a nutrient and
a stimulant. Further, it must be remembered that such well-known food-stuffs as
salts of potassium are tolerated by the plant only when in very low concentration,
while in higher concentration they act injuriously owing to their osmotic action.

The facts which have now been put forward render intelligible a whole
series of observations which were previously obscure. For example, take
BENECKE'S observations on rubidium. We must bear in mind that the ru-
bidium presented to the plant may not have been quite free from potassium as
an impurity, so that we might regard the rubidium merely as a stimulant, while
the traces of potassium might be considered as nutritive. Rubidium acts, in
fact, like zinc sulphate ; in relatively small amounts it acts directly as a poison
and hinders the formation of conidia. Stimulus action further explains the
favourable influence often observed on the addition of silica to a water-culture,
perhaps also the good effect which carbon disulphide has on arable land
previous to the beginning of vegetative growth, by the action of sodium fluoride
on crops and possibly also that of copper on the higher plants. Copper is usually
very injurious to plants, and NAGELI has shown that it is a deadly poison to
Spirogyra even in a state of dilution so great that it cannot be chemically detected.
HATTORI has shown that 0-00005 Per cent, is the extreme limit of concentration
for the pea, and 0-000005 Per cent, for the maize, above which injury ensues.
All the same, sulphate of copper together with lime, under the name of 'Bordeaux
mixture ' has been used with success in combating diseases due to Fungi, and it
has a further and unexpected effect, for plants sprinkled with the mixture grow
more luxuriantly than control plants not so treated, provided that the latter
are free from infection by the fungus. Vines and potatoes treated with copper
show a greater development of chlorophyll and a more vigorous production of
organic substance. Why syringing with this solution should have this effect
cannot be explained at present, and the most varied views are held on the subject.
The favourable effect of this mixture is claimed to be due to the lime ; ADER-
HOLD (1899) recently advanced the view that it was due to the adulteration of
this substance with iron, but it is also possible that copper was the active cause.
At all events we must not draw the contrary conclusion that copper is not to be
identified chemically in the leaves, since if copper be useful it must obviously
enter in only in the very minutest traces. A full discussion of the copper
problem is impossible here ; a summary of our knowledge, along with new
experiments, will be found in BAIN, 1902.

In conclusion, we may mention that organic substances of various kinds
may act in a similar way as stimulants, just as do the inorganic salts above
mentioned. Thus in RICHARDS'S cultures cocaine and morphine acted as weak

ASH. I 89

stimulants, while amygdalin had a greater effect. These substances, however,
certainly do not belong to the ash constituents of the plant, and for that reason
we may stop at this point, returning to the question of stimuli later on, where
we will be in a better position to make the subject intelligible.

Bibliography to Lecture VII.

ADERHOLD. 1899. Centrbl. Bakt. ; II. Abt. 5, 217.

BAIN. 1902. Bullet. Agric. Exp. Station Univ. Tennessee, 15, Nr. 2.

BENECKE. 1894. Ber. d. bot. Gesell. (105).

BENECKE. 1895. Jahrb. f. wiss. Bot. 28, 487.

BENECKE. 1896. Bot. Ztg. 54, 97.

BENECKE. 1898. Ibid. 56, 83.

BEYER. 1869. Versuchsstat. II, 263.

BIRNER and LUCANUS. 1866. Versuchsstat. 8, 128.

BOUSSINGAULT. 1860. Chimie agricole (2nd ed.), i.

GRANDEAU and BOUTON. 1877. Compt. Rend. 84, 129 ; Just's Jahresbericht,
1877.

GRIS. 1843. Cited by MOLISCH, 1892.

GRUBLER. 1881. Journ. f. pr. Chem. 131, 97.

HASELHOFF. 1893. Landw. Jahrbucher, 22, 851.

HATTORI. 1899. Bot. Centrbl. 80, 171.

HELLRIEGEL. 1883. Beitr. z. d. naturw. Grundlagen d. Ackerbaues. Braun-
schweig.
>^ JODIN. 1883. Annales d. China, et d. Phys. v, 30, 485.

KNOP. 1860. Versuchsstat. 2, 65 and 270.

KNOP. 1868. Kreislauf des Stoffes. Leipzig.

KNOP. 1884. Versuchsstat. 30, 293.

LIEBIG, J. 1840. Die Chemie in der Anwendung auf Agrikultur, 7th ed., 1862.

MALAGUTI and DUROCHER. 1858. Annales sc. nat. iv, 9, 222.

MAYER, AD. 1902. Agrikulturchemie, 5th ed., Heidelberg.

MOLISCH. 1895 and 1896. Sitzungsber. Wiener Akad. 104, I, 783 ; 105, I, 633.

MOLISCH. 1892. Die Pflanze in ihrer Bez. zum Eisen. Jena.

NAGELI. 1893. Die ologodynamischen Erscheinungen. Basel.

NOBBE. 1862. Versuchsstat. 4, 318.

NOBBE. 1867. Ibid. 9, 478.

NOBBE. 1868. Ibid. 10, i.

ONO. 1900. Journ. Coll. Sci. Imp. Univ. Tokyo, 13, 141.
^ PFEFFER. 1895. Jahrb. f. wiss. Bot. 28, 238.

PORTHEIM. 1901. Sitzungsber. Wiener Akad. 1 10, I, 113.

RAULIN. 1869. Annales sc. nat. v, 11, 93.

RICHARDS. 1897. Jahrb. f. wiss. Bot. 30, 665.

RICHTER. 1901. Centrbl. f. Bakt. II, 7, 417.

SACHS, J. 1860. Versuchsstat. 2, 22 and 224.

SACHS, J. 1862. Flora, 45, 52.

SACHS, J. 1882. Vorlesungen uber Pflanzenphysiol. Leipzig.

SALM HORSTMAR. 1856. Vers. und Resultate ttb. d. Nahrung d. Pflanze. Braun-
schweig.

SAUSSURE TH. 1804. Recherches sur la vegetation; Ostwald's Klassiker, 15
and 1 6.

SCHIMPER. 1890. Flora, 73, 207.

SCHMIEDEBERG. 1877. Zeitschr. f. physiol. Chem. i, 205.

SCHRADER. 1800. Preisschrift uber die eigentl. Erzeugung der erdigen Bestand-
teile in den Getreidearten. Berlin.

SENEBIER. 1800. Physiol. vegetal., Vol. 3.

SWIECICKI. 1900. Ber. aus d. landw. Instit. Halle, 14.

TSCHERMAK. 1899. Just's Jahresbericht, 27, 2, 188.

WIEGMANN and POLSTORFF. 1842. tj. d. anorg. Bestandteile d. Pflanze. Braun-
schweig.

WOLFF. 1868. Versuchsstat. 10, 351.

WOLFF. 1871-1880. Aschenanalysen v. landw. Produkten. Berlin.

, WORTMANN. 1892. Bot. Ztg. 50, 643.

90 METABOLISM

LECTURE VIII
ASH. II

' THE mineral matters which are present in every plant, so far from being
impurities, are quite as important constructive bodies as carbon and nitrogen.'
This statement summarizes the chief results obtained in our last lecture. What
was demonstrated there by experimental methods may also be established by
observation of plants occurring wild and in cultivation. It may be proved
without difficulty that the soil whence the materials of the ash are obtained has,
quite apart from the water it contains, a most important influence on the
development of the plant. Plants do not grow nearly so well in river sand,
which is deficient in these mineral constituents, as in garden soil, nor are they
so vigorous in their growth when the supply of garden soil is limited in quantity,
as it is, for example, in pot cultivation (SACHS, 1892). These examples illustrate
sufficiently clearly the vital importance of food-stuffs being supplied in suffi-
cient quantity and of the right quality. Further inquiry into the nature of the
contents of the soil, regarded as food-stuffs, and the manner of their absorption
by the plant will serve only to emphasize this view of the problem.

Let us consider, first, soil as it occurs in nature formed by disintegration
of the rocks, not such a soil as has supported many generations of plants and
which has in turn received many of its constituents from them. The characters
of the soil have been treated of more fully than we can do here by A. MAYER
(1895) and by RAMANN (1893). [A new edition of RAMANN'S work, improved and
enlarged, has been recently (1905) published under the title of 'Bodenkunde'.]
Since the sedimentary strata have originated from the weathering and aqueous
deposition of primitive rocks, all soils must in the long run have been derived
from crystalline rock masses. Owing to the composition of these primitive
rocks, the soil produced from them must be of varied chemical composition.
An examination of granite as a source of soil gives us the following percentage
composition (according to GIRARD ; compare MAYER, 1895, II, 12) : —

Silicic acid. Alumina. Ferrous oxide. Lime. Magnesia. Potash. Soda. Water.
I. 72-6 15-6 1.5 1-3 0-3 5-0 2-3 o«8

II. 68-6 14.4 5-0 3.9 0-4 2-8 3.4 t-i

Similar results are obtained when gneiss, mica-slate, and other rocks are
analysed ; the differences between them lie merely in the relative amounts of
the individual components, the same elements reappearing, but always in
varying quantities. When we reduce such a rock to powder we obtain a soil
containing the metals potassium,' calcium, magnesium, iron, all important
nutritive elements to the plant ; sulphuric and phosphoric acids, however,
are not entirely wanting although they may be overlooked, owing to their
occurrence in relatively small quantities (the sulphuric acid as gypsum and the
phosphoric acid as apatite) (MAYER, 1895, II, i); they are present as a matter of
fact in not less quantity than in ordinary soil of cultivation.

If we now add to such a sample of powdered granite the one element which
may be wanting, or present only in very small amount, viz. nitrogen, in the
form of nitric acid, attempts to carry out culture experiments in it will lead to
very poor results, because the bases are not united with hydrochloric, sulphuric,
phosphoric, and nitric acids, as in our water-culture experiments, but chiefly
with silicic acid forming for the most part insoluble compounds, more
especially since the salts are present usually as double silicates. In consequence of
the low temperatures which have prevailed in recent geological time a compe-
tition has taken place between the carbonic and silicic acids, which has resulted
in the carbonic acid annexing the majority of the bases. These are carried
away in the form of soluble compounds, and the rock in consequence is said to

ASH. 11

weather ; but wind and weather, oxygen and water, have much less to do with
the process than carbonic acid, although it certainly requires the co-operation of
water. The carbonic acid acts with varying rapidity on silicates of sodium,
calcium, magnesium, and potassium, but is incapable of ousting the silicic acid
from its union with aluminium. The primitive rocks consist of a mixture of
different minerals whose capacity for weathering varies greatly. Looking at
granite, for instance, we find that quartz and mica are remarkably stable, whilst
the felspar (a double silicate of aluminium with potassium or sodium) weathers
more easily. Through the action of carbonic acid, carbonates of sodium or
potassium are produced from it, which are soluble in water, while on the other
hand, aluminium silicate (caolin, clay), which, while retaining water, is entirely
insoluble, is carried away by water in a state of suspension in very fine particles
and redeposited as a clay soil. When the originally compact rock is in this
way deprived of one of its ingredients, pits and cavities appear in it rendering
it liable to fresh attack from the carbonic acid, as well as to certain physical
effects of water which we need not refer to here. The net result of the whole
process is the decomposition of the granite into a mass of felspar, quartz, and
mica particles which, if they be still held together by the clay, are carried away
by water and redeposited to form an alluvial soil. Such a soil is better for the
plant than the original granite in two respects ; in the first place, it is not com-
pact, and hence plant roots can penetrate into it, and in the second place, it
contains soluble constituents, which may be continually formed until the last
particle of felspar has disappeared. The water, which such a soil holds, in
virtue of its capillarity, as well as that which actually percolates through it,
always contains substances in solution, although, it is true, only in limited
amount. Analyses of streams and rivers which have their sources in primitive
rocks teach us much on this subject (KNOP, 1868, 124) : —

Glacial rivers from cry-

Mountain rivers from

stalline Schist.

Granite and Gneiss.

One litre contains

Moll near

Of»t7

One litre contains

Regen near

Iltz near

HeiligenBlut.

Zwiesel.

Passau.

Calcium carbonate

0-0084

0.00450

Sodium chloride

0-0025

0-0059

Magnesium car-

Soda . . .

0-0056

0-0043

bonate . . .

0-0035

0-00005

Potash . .

0-0096

0-0058

Silicic acid . .

0-0072

0-00868

Lime

00154

0-0092

Ferric oxide .

O-OOIO

Magnesia .

0-0026

0-0029

Manganese oxide

0-0032

Ferric oxide

00009

0-0027

Alumina . . .

trace

Sulphuric acid

0-0020

—

Magnesium sulphate

—

O.OI30I

Silicic acid . .

OO072

0-0095

Sodium sulphate

0-0009

Insoluble matter

o 0018

0-0052

Sodium chloride

—

0-00043

Carbon-dioxide and

Suspended materials

0-0019

0.00853

organic matter .

0-0335

0-0450

0-0261

0-03520

0-0811

0-0905

Without carbon-dioxide and organic matter .

0-0476

00455

When we compare the contents of these natural soil waters with our culture
solution, as regards the minerals they respectively contain, we find that the
former solutions contain nearly 100 times less solid in solution than the latter,
and further, that much of that solid is useless to the plant. It is also obvious
that oats or maize will be able to thrive only with great difficulty in such a
medium, seeing that phosphoric and nitric acids are present in such small
quantities that they do not appear in the analysis. Soil water ought to exhibit,
however, a composition similar to, and also show a concentration equal to,
that of spring and river water, and we are thus at a loss to understand how it
is possible for a plant to live in such a soil. Observation of natural conditions,

92 METABOLISM

however, teaches us that plants of high organization which make considerable
demands on the soil, such as oats and maize, never occur on such soils, but only
plants which are unassuming in their requirements. In mountainous regions
lichens are always the first plants to appear, and these organisms, although we
must admit that qualitatively they require the same materials as Algae and
Fungi (p. 82), are content with much smaller quantities, simply because they
grow so very slowly. A higher plant, actively metabolic, would under such circum-
stances grow itself to death. As soon as lichens have effected the first settle-
ment on such a primitive soil, mosses, ferns, and finally flowering plants quickly
follow. In consequence, the substratum becomes less and less a natural soil of
disintegration and more and more of the nature of an arable soil, for each gene-
ration of plants makes the soil fitter for its successors, notwithstanding the fact
that it withdraws food materials from it. This follows from the fact that, in
the first place, every plant gives off carbon-dioxide, which, as we have seen, has
a disintegrating effect on the rock, and, secondly, that the dead parts of the
plant, not only the roots in the soil, but also the leaves, twigs, branches, and stems
formed above ground, ultimately reach the soil once more and decompose there.
In consequence, their organic constituents will be either entirely destroyed and,
amongst other things, carbon-dioxide will be produced, or such compounds as are
more resistent to decomposition will be transformed into humus, to which the
brown-black colour of the soil is due. Owing to the formation of carbon-dioxide
in the process of decomposition of plant debris the air in the soil is always very
rich in this gas ; WOLLNY (1897, 145) states that a minimum of 0-7 per cent, is
present in the soil in winter time, and a maximum of 4-8 per cent, in summer, so
that, in this respect also, vegetation has a marked effect on the rock constituents
of the soil. The humus has in addition the power of altering the characters
of the soil to a remarkable degree, both physically and chemically. From
a physical point of view the humus particles, deposited between the mineral
constituents, effect a loosening of the soil and, at the same time, increase its
capacity for retaining moisture (p. 26). Chemically ', certain special humin con-
stituents are added. Since most of these are not considered as nutrients to the
majority of plants, we need only say that they consist of little-known compounds
of hydrogen, oxygen, nitrogen, and carbon, in part neutral in chemical reaction,
in part showing the characteristics of acids. We shall have something to say
afterwards as to the latter. In addition to the humin compounds humus contains,
as well, the ash constituents of plants from which the humus has been derived,
but in a form difficult to extract with water, without diminishing the absorption
by the plant.

We have now to consider a phenomenon of the greatest importance to
plant life, known as soil absorption, viz. the capacity of the soil to extract
substances from their aqueous solutions.

In order to demonstrate this absorptive process we permit the filtration
of a solution of indigo -carmine or yellow liquid manure through a layer of earth
of a certain thickness ; we shall find that the fluids which escape are quite
colourless. Were this power of absorption limited to colouring matters, the
importance of the process, so far as plants are concerned, would be insignificant;
but inorganic salts are also firmly retained by the soil, so that solutions of such
bodies, after they have passed through the soil, have lost in concentration, and
have also lost entirely certain constituents which they previously contained.
Suppose we add to the soil one of the nutritive solutions mentioned on p. 81,
we shall find that the potassium, calcium, magnesium, and phosphoric acid are
retained, as well as ammonia and the less important plant nutrient sodium, but
that the acids, other than phosphoric, i. e. nitric, sulphuric, and hydrochloric,
are not. Absorption of the metals above mentioned belonging to the series of
alkalis and alkaline earths takes place in different ways according as they occur

ASH. II 93

as salts or as oxides. In the following summary we will follow in general the
statements of AD. MAYER (1895, II, i) :—

1. The nitrates, sulphates, and chlorides of the metals above mentioned
undergo transposition in the soil, so that the metals in these salts are exchanged
for others occurring there. These latter go into solution whilst the former are
absorbed. Thus potassium is more readily absorbed than ammonium, and that
in turn more readily than magnesium, &c., so that we can affirm that in the
series potassium, ammonium, magnesium, sodium, calcium, the succeeding
element can be ousted by its predecessor from its combination ; more especially
is it the case that frequently on the addition of salts of alkalis to the soil calcium-
salts pass into solution. No rule is, however, without its exception, for, on
the contrary, potassium can be turned out of combination by ammonium and
sodium. This replacement of bases takes place most commonly if double sili-
cates of aluminium are present in the soil which in addition to aluminium contains
another base as well. The accessory base suffers replacement while the aluminium
remains permanently united with silicic acid. Humates act like double silicates
but less actively.

2. Oxides of alkalis and alkaline earths, as also their hydrated oxides, are
absorbed in the first instance through the agency of humic acids, although no
replacement takes place as in the first case. Double silicates and even pure
caolin can act like humic acids.

3. As already noted phosphoric acid is apparently the only acid firmly
retained, and that, too, whether it occurs as a salt or a free acid ; its retention is
effected by carbonate of lime or even more firmly by hydrated ferric oxide.

The question of the causes of absorption, so often discussed by agricultural
chemists, more especially as to whether absorption is to be regarded as a chemi-
cal or as a physical question, need not concern us ; what we know is that ab-
sorption takes place only in clay, chalk, or humus-containing soils, but not in
quartz sand ; we are more concerned with the effects of absorption than with its
causes. Soils formed by disintegration of rocks which are originally poor in
salts suitable to act as plant nutrients, are thus gradually rendered more fertile,
more especially since the substances absorbed by the soil are largely protected
from being washed away by rain, although they are still capable of being taken
up by the plant. Protection from removal by water is, of course, not absolute,
but, according to PETERS (1860), it requires in most cases a very considerable
amount of water to remove an absorbed substance from the soil ; for example,
28,000-36,600 parts of water are required to remove one part of potassium.
Finally, it is of the highest importance that the individual substances so
absorbed, if they be firmly combined, should be distributed in the soil in a state
of minute subdivision. It will be seen later that in such a condition they are
much more easily accessible to the plant.

Plants in the soil encounter an extremely dilute solution of nutritive salts,
and, in proportion as they take up from it the more important of these salts
for their own use, other salts previously absorbed will pass into solution. At the
same time we must not assume that the plant's supplies are obtained only from
the substances in solution-, the plant is also able to make use of solids which it
brings into solution. Although such solution phenomena are well seen in the lower
plants, especially in lichens, still to avoid prolixity, we will confine our attention
to the special characters of the root-system in the higher plants so far as they
concern the absorption of minerals. In our earlier consideration of the root as the
organ of absorption of water, opportunity was taken to speak of its mode of
distribution in the soil. As a general rule the root meets with water and nutritive
salts in the same place, and there are few adaptations known which are especially
concerned in the absorption of salts only. Among these, the intimate fusion of
the roothair and the soil particles may be especially noted, and that subject we

94

METABOLISM

Fig. 20. Apex of a roothair in intimate union
with soil particles, x 240. (From the Bonn Text-
book.)

have not as yet considered in detail. Roothairs when grown in soil cannot
maintain the symmetrical form they have in water, e. g. in Hydrocharis ; in the
course of their growth they encounter obstacles which they cannot avoid but
round which they grow. In this way the roothair becomes irregular in
form (Fig. 20), and, further, becomes so intimately connected with these obstacles,
the minute particles of the soil, by means of its mucilaginous wall that we may,
with every justice, speak of a growth fusion between them. In fact, the soil par-
ticles adhere to those regions of the roots that are covered with hairs so firmly
that when the plant is pulled out of the soil these regions stand out prominently
in contrast with the white apices which have not as yet developed roothairs, and

also with the older parts of the root which
have lost them. The soil, in a word, sur-
rounds the root like a sleeve (Fig. 21). This
intimate union of soil particles and root-
hairs renders easy the absorption of those
substances which pass into solution on the
breaking up of the soil, and this breaking
up is in turn facilitated by certain excre-
tions formed by the roothairs.

It has been known for long that roots in
nature had a corroding effect on limestone. SACHS (1865, 189) demonstrated this
fact by allowing plants to grow in a flower-pot in which he had placed a slab of
polished marble covered with a layer of clean sand. The roots of the plant were thus
able to grow over the marble, and, after several days or weeks, corrosion figures
appeared on the plate corresponding to the distribution of
the roots. SACHS found, as a matter of fact, that the course
of thechief andsecondaryroots (e. g. of the bean) was mapped
out, wherever they were in intimate contact with the marble,
by a shallow rut about \ mm. broad, bordered by a hazy and
indistinct roughness in certain places indicative of the
presence there of roothairs. He obtained similar results
by employing dolomite, magnesite and osteolite, so that
magnesium carbonate and calcium phosphate are as
capable of suffering dissolution by roots as calcium car-
bonate. For a long time it was held that the roots gave
off free organic acids, and that these acids were the cause of
the formation of corrosion figures ; more recent research
has shown, however, that corrosion figures are due
primarily to the action of carbonic acid,. CZAPEK (1896)
employed in his researches plates of plaster of Paris arti-
ficially compounded with the mineral whose solubility he
desired to investigate, using them in the same way as
SACHS did plates of marble. The two substances were
pounded into a paste with distilled water and then spread
over a glass plate ; in this way a very fine flat surface
was obtained for experiment, quite as effective as a
polished plate of natural rock. By this means, CZAPEK
washed. (fter SACHS, established that plates of carbonate and phosphate of
Plant Pbysi- j'me were corroded by roots while aluminium phosphate
plates were not affected, and it may be concluded that quite
a number of organic acids, those, in fact, in which aluminium phosphate is soluble,
take no part in the formation of corrosion figures ; excluding these acids we
have only to consider carbonic, acetic, propionic, and butyric acids. The brown
coloration given to congo-red demonstrates that carbonic acid at least must be
considered as an agent in the formation of corrosion figures, since the other acids

Fig. 21. Seedling of white
mustard. /, taken direct

Lectures on
ology.)

ASH. II 95

mentioned give a blue reaction with congo-red. [PRIANISCHNIKOW has shown
that CZAPEK'S conclusions are by no means above criticism (Ber. d. bot. Gesell.
1904, 22, 184). At the present moment we have no certain knowledge as to
what acids are given off by the root. More recently, PRIANISCHNIKOW (Ber. d.
bot. Gesell. 1905, 23, 8) has proved that phosphates which are soluble with
difficulty in presence of salts of ammonia are available for the nutrition of plants,
in proportions quite different from those available when potassium nitrate only
is present.]

It will be shown later that all plant cells produce abundant carbonic acid,
and its presence in sufficient quantity in this relation cannot be doubted.
Again, from the sharpness of the outlines of the corrosion figures it has been
concluded that they must have been produced by a non- volatile acid ;
1 carbonic acid/ as SACHS says, ' penetrates freely into the interspaces in the
soil, and hence one might expect to find corrosion in regions at some distance
from the root.' But SACHS'S conclusions cannot invalidate CZAPEK'S view, viz.
that the carbonic acid alone produced corrosion in the substances referred to ;
for one must remember, in the first place, that carbon-dioxide does not occur
as a gas, but in solution in water, and in the second, that the water with carbon-
dioxide in solution is markedly present in the cell-walls of the root epidermis, and
does not readily exude therefrom. If the solution be in intimate contact with
the minerals, and if the dissolved particles be at once absorbed into the interior
of the cell, in course of time a quite observable effect will be produced.

Although we may look on the carbonic acid as the factor concerned in the
formation of the corrosion figures on certain kinds of rock, and although, further,
we have already ascribed to this substance, apart from the plant, an important role
in the disintegration of rock, we are not entitled to affirm that the root is un-
able to excrete other substances which may be instrumental in opening the soil
up. As we have already said, it has been generally accepted that the root
gives off organic acids, and this view was held to be supported by the fact
that organic acids could be demonstrated in the cells of the root and by the red
colour often given to litmus paper when roots were pressed against it. To CZAPEK
we owe a reinvestigation of this problem. He has confirmed the observation
previously made of the excretion of minute drops from roots grown in spaces
saturated with moisture, and has further shown that, as in the case of subaerial
;hydathodes, this phenomenon takes place only when the plant as a whole is
turgid. CZAPEK also found that these drops gave a neutral reaction. Again,
CZAPEK has grown roots in a minimum amount of water or on small pieces of filter
paper, and after a time has submitted the water or filter paper to microchemical
5 analysis. He finds potassium and phosphoric acid not infrequently present in
j quantity, smaller amounts of magnesium and chloride, and traces of calcium.
; The reaction of these fluids is on the whole acid, owing to the presence of acid-
• potassium phosphate, in other cases of acid salts of formic acid and (only in one
case) of oxalic acid. [According to PRIANISCHNIKOW (1904, Ber. d. bot. Gesell. 22,
184), the excretion of phosphates is quite possible in seedlings, since these bodies
are plentifully produced in the process of decomposition of proteids ; this, how-
ever, is not the case in mature plants.] These bodies must have been excreted
from the uninjured root-cells. Lookedat scientifically these investigations are not
above criticism, for it may be assumed that cell-sap from dead roothairs and
from the dead cells of the root-caps was present in the fluid examined, and that in
the experiments with filter paper injury was done to the roothairs in the process of
cutting off of the rootlets, so that, in both cases, the occurrence of the substances
referred to above might be explained without assuming that they were excreted
, from living cells. At any rate it is important to note that the reaction of this
sap is acid as opposed to the result of CZAPEK'S investigations on the drops ex-
creted from roothairs.

It may be further noted that the acids arising from the dead roothair

96 METABOLISM

may very well play an important part in the dissolution of soil if these cells
be sufficiently numerous. As to the amount of root-cap production we have
certainly no accurate information, but our knowledge as to the roothairs is
more complete. We know that they live only a few days and are replaced by
new hairs formed behind the apex.

The fact that substances escaping from the plant play some part in the
dissolution of the soil must be always borne in mind, since it is certain that the
plant can extract from the soil much more in the way of nutrient material than
we can with the aid of water holding carbon-dioxide in solution. Thus, e. g.,
LIEBIG (1862, 2, 108) says : —

' A young plant of rye, grown in a fertile soil, is often capable of developing
into a tuft of thirty to forty shoots, each with an inflorescence bearing a thou-
sand or more grains, and yet it obtains its nutriment from a volume of soil which,
after prolonged washing with pure water, or water containing carbon-dioxide in
solution, yields up not one hundredth part of the phosphoric acid and nitrogen,
and not one fiftieth part of the potassium and silicic acid which the plant has
absorbed from the soil. Under such circumstances how is it possible for the
plant to obtain all the materials found in it by merely dissolving them in water ? '
In order to obtain some idea how much nutrient material is available in the
soil, agricultural chemists employ a dilute (i per cent.) solution of citric acid
instead of a solution of carbonic acid ; since it has been shown that the plant is
able to absorb as much material as it would do if it excreted weak citric acid.
What the materials are which act as solvents, and which are given off by the
plant, and whether these spring from living cells or from dead, are matters still to
be made out. The corrosion figures are formed on natural or artificial plates only
when the substances of which these plates are made possess relatively great solu-
bility ; moreover, a root, without forming obvious corrosion figures, can dissolve
a considerable quantity of material if it be allowed to operate on a sufficiently large
surface ; these are undoubtedly the conditions which occur in nature. Further we
must always remember that acids may be excreted only in the presence of
definite substances, amongst which, perhaps, aluminium phosphate must not be
/ classed. In this relation, CZAPEK has shown that acid-potassium phosphate may
I be of service in consequence of the decompositions which it excites outside the
I plant, e. g. when it reacts with neutral salts of the strong acids, and so gives
/ rise to small quantities of mineral acids.

Attention has been drawn to the rapid death and constant renewal of root-
hairs and, in addition to the close union of roothairs and soil particles,this further
point is of special importance in relation to the absorption of nutritive salts from
the soil. Numberless roothairs develop from day to day on a large plant, which
penetrate into new masses of soil, grow round the particles, taking nutritive salts
from them, and rendering naturally insoluble particles soluble. Thus an ever-
increasing area of soil becomes available. Since after each absorption of soil-water
at any definite spot a movement of water takes place, tending to produce once
more an equilibrium, it might be held that this continual acquisition of new soil
particles was of little moment in the absorption of water and substances dissolved
in it; but migration of solid bodies is out of the question, and hence the intimate
union and constant renewal of roothairs is of the greatest importance. Although
the roothairs are the ordinary organs of absorption of nutritive materials, there
are many plants which normally possess none. Plants which develop roothairs
in ordinary soil, as a rule, do not do so as a rule in water-cultures. In these
cases the nutrients are absorbed either by means of the general epidermal cells
or — an(j this is a very frequent method — by the aid of Fungi which live both
on and in the root (compare Lecture XIX). In normal roots, also, the young
epidermal cells, which have not as yet developed roothairs, are capable of
absorbing nutrient substances (KNY, 1898).

Finally, a third point may be drawn attention to. Many investigators

ASH. II 97

have shown that roots branch much more freely in soils which contain abundant
food materials than in those which are poor in them. NOBBE (1862 and 1868)
has shown this by cultivating clover and maize in a soil which consisted through-
out of the same basal material, but with the alternate layers saturated with
a nutritive solution. Experiments on this subject were also made by THIEL
(quoted by SACHS, 1865, p. 178), and more recently by HOVELER (1892), who
employed soils which consisted of alternate layers of sand and humus.

Having now determined in what condition the plant finds its food-stuffs
in the soil, and how it absorbs these with the aid of its roots, there remains for
us to study how variations in the chemical composition of the soil influence its
occupation by plants. We have already considered how the primal coloniza-
tion of a rock takes place, and how it is transformed into a soil under the influ-
ence of plants which are easily satisfied. It has further been shown how the
humus compounds which arise from the decomposition of not only plant but
also animal remains, add to the fertility of the soil. Soils occur in nature,
however, which are quite free from vegetation and that for very varied
reasons. The great resistance presented by some minerals to decomposition
must be considered first, and an example of such is seen in lava, which becomes
covered with vegetation only extremely slowly (compare TREUB, 1888, on
Krakatao, and SCHIMPER, 1898, 200, on Gunung Guntur). On the other hand,
although it rarely happens, a rock may weather, but does not contain all the
elements required by the plant, or, again, it may contain too great a proportion
of mineral salts (e. g. sodium chloride, &c.) which interfere with the introduction of
plants, or it may be deficient in water. Such regions of the earth which are desti-
tute of vegetation we term deserts. The greater part of the earth's surface is
capable of supplying the plant with all the chemical compounds necessary and in
quantities sufficient for its growth, and for that reason it is covered with vege-
tation, but this vegetative covering takes on very varied characters in different
regions. In so far as we are able to understand the causes of this variation, we
may refer to climate and soil as the most important determining factors in plant
distribution. Here we are naturally concerned with soil only, and we may draw
attention to a fact with which we have been long acquainted, that a seashore
whose soil contains salt in large quantity has quite as characteristic a flora as
inland regions with, say, a lime soil, which exhibit plant societies other than those
shown by a sandy soil poor in lime, or by primitive rock. We distinguish in
geographical botany between plants which are confined strictly to soils with
definite characters, and such as are able to thrive in various kinds of soil ; the
former are * local' in distribution, the latter 'indifferent'.

Thus there is quite a number of halophytes which in nature occur by prefer-
ence or exclusively on a soil which contains a large quantity of sodium chloride,
such, for example, as a seashore, where one usually finds as much as 3 per cent,
of common salt. So far as we know common salt, however, performs no
special function in their metabolism other than it performs in the rest of the
vegetable kingdom, for these halophytes can exist in soils containing a mere
trace of this salt or none at all. The point where halophytes differ from other
plants lies in their capacity for tolerating quantities of sodium chloride which
are directly injurious to non-halophytes. In virtue of this power they are able to
exist in places from which other plants are debarred, while in ordinary soils they,
for the most part, give place to non-halophytic types. The injuries inflicted
on ordinary plants owing to the presence of an excessive amount of salt in the
soil depend, in the first place, on the osmotic action of the concentrated solutions
in the soil, and in the second place, on the difficulty of bringing about absorption
of water; further, common salt, when absorbed, has certain, to all appearance,
after-effects which are not as yet perfectly understood. Halophytes over-
come the greater difficulty of absorption by being extremely economical in

JOST H

98 METABOLISM

their use of water, and by reducing /transpiration by special methods which are
naturally adapted to the special conditions under which they live. It is of
interest to note that some of them have discovered a means, i. e. by hyda-
thodes, of getting rid of excess of salt, and thus of preventing a superabundant
accumulation of sodium chloride in their tissues (compare p. 58).

Our acquaintance with the causes which determine why many plants prefer
soils rich or poor in lime is at present much less complete. It is obvious
that the occurrence of plants in either situation is not directly connected with
the need of the plant for calcium as a food-stuff, for no soil is so poor in calcium
that it cannot obtain all that it requires from it. The so-called ' calciphobous '
plants require calcium just as much as the ' calciphilous ' plants, and they do,
as a matter of fact, absorb considerable quantities of it from the slate or primi-
tive rock on which they live. The solution of the question is rendered all the
more difficult inasmuch as one and the same species cannot live equally well
in all places. Only a few plants appear constantly to avoid lime soils, e. g.
Sphagnum and certain other aquatic mosses, the majority of Desmidiaceae, and,
among Phanerogams, Sarothamnus scoparius, Castanea vesca, and Pinus pinaster.
In regard to the last-mentioned plant, VALLOT (1883) has made many investi-
gations which show how exclusive it is in its choice of a soil. Generally speak
ing, soils in which it is said to thrive, and which contain more than about
3 per cent, of lime, are found on closer investigation to exhibit local conditions
(e. g. oases of rock poor in lime) which render its existence possible. The experi-
ments of BONNET (VALLOT, 1883, p. 202), made at Dijon, are of great interest,
confirmed, as they have been, by MANGIN (according to Roux, 1900, p. 131)
at Besanc.on. The chestnut flatly refused to grow there, and yet it was possible
to cultivate it easily when it had been grafted on the calciphilous oak. The
reason is probably that the root is injured by the excessive amount of lime in
the soil, and in support of the correctness of this view we may draw attention
to the frequently cited behaviour of Sphagnum, as well as of plants which occur
in its company, e. g. Drosera. After being watered with a solution containing lime
these plants come to grief very quickly ; solutions of lime salts act as poisons
to them. According to OHLMANN (1898), watering Sphagnum with calcium
sulphate and calcium nitrate is less harmful than treating it with calcium carbo-
nate. OHLMANN also says that a 0-05 per cent, solution of calcium carbonate
kills it in from twenty-two to thirty-two days, and that a 0-15 per cent, solu-
tion is fatal in fourteen to twenty-four days. More recently, GRABNER (1901,
p. 112) states that WEBER cultivated Sphagnum on chalk successfully. A final
decision on this question must be postponed until further investigations have
been made. [SoLMS LAUBACH (1905, Die leitenden Gesichtspunkte einer
allg. Pflanzengeographie, p. 122) has offered a more accurate analysis of WEBER'S
results, whence it would appear that Sphagnum is certainly injured by lime, since
it can scarcely live in company with other plants which require that mineral.
GRABNER' s results obtained by cultivating Sphagnum on chalk apply only to pure
cultures.] CORRENS'S (1896) observations on Drosera (compare Lecture
XXXVIII) also point to a directly injurious effect of lime. How limited our
knowledge is as to the function of lime in plant life, is evident from the investi-
gations which A. ENGLER has recently (1901) made on the distribution of
Castanea in Switzerland. Although we may consider this tree in general as
markedly * calciphobous', it occurs, according to ENGLER, on sandstones and
marls rich in lime, and which possess a large proportion of potassium.
ENGLER believes that a great need for potassium, and not the absence of
calcium, determines in general the occurrence of Castanea on siliceous soils . 1 1 must
certainly not be overlooked that in the case of plants of lower grade such as
Sphagnum,\i maybe possible that calcium, as in the case of Algae, is not an essential
food-material, while in the case of the higher plants such a supposition is less
probable.

ASH. 11 99

Although further investigations may not confirm the view that carbonate of
lime is directly poisonous to higher plants, still they may establish that this sub-
stance has an indirect effect such as may be deduced from the experiments of FLICHE
and GRANDE AU (1873). These investigators analysed the ash of trees which had
been grown in normal soil and compared it with that of other specimens which
had passed a miserable existence in a soil which was rich in lime. The result of this
comparison was to show that the ash of those grown in a siliceous soil contained
40-45 per cent, of lime, while in those from a lime soil, the percentage rose to 56-
75 ; at the same time the absorption of potassium was much reduced (from 16-22
per cent, down to 4-6 per cent.). It is conceivable that the diminution in the
absorption of potassium is due to the fact that the calcium carbonate rapidly
neutralizes the acid excretions of the root, and so interferes with its disintegrating
action on rock material naturally difficult of solution. In addition to the diminu-
tion in potassium there is a scarcity also of magnesium and iron in plants grown
in lime soils. SCHIMPER (1898, no) attributes the feeble development of
siliciferous plants on lime soils to the deficiency in iron. In support of this
view one may cite the fact that calciphobous plants become chlorotic on
lime soils (Roux, 1900), and that this chlorosis (according to a verbal state-
ment of Professor STAHL) can be set right by spraying with a solution containing
iron. [BENECKE has obtained marked chlorosis in presence of iron when the
water-culture gave an alkaline reaction (Bot. Ztg. 1904, 62, II, p. 124).]

A complete explanation of the aversion of many plants to lime is not at
present available ; we must be content with the suppositions advanced above.
Nor are we better off in regard to our knowledge of the reasons for the fondness
for lime exhibited by other plants. The view once held that these plants avoid
contact with silicic acid need not be considered. That they can endure the
presence of more lime than others can is obvious, but it is not so apparent what
use they make of it. THURMANN has performed a useful service in demonstrating
the physical differences between lime and sandy soils, especially the deficiency
in water in the former and its abundance in the latter, and has suggested this
as determining questions of plant distribution. According to THURMANN,
' calciphilous ' plants are xerophytes, ' calciphobous ' forms are hygrophytes,
and it is quite true that we do meet with plants in the most varied regions
which usually occur on chalk or, exceptionally, on primitive rock under very
dry conditions. THURMANN, for example, has emphasized the fact that quite a
series of calciphobous plants in Southern France also occur on primitive rock.
This, however, must depend not only on the amount of water present, but on the
sum- total of the physical characters of the soil, and amongst these the conditions
of temperature at all events must be especially taken into account. These con-
ditions have been specially investigated in recent times by WOLLNY (1898);
It has been shown that a quartz soil changes most rapidly with alterations
in the temperature of the air, then more slowly clay, lime, and magnesia soils
respectively, and, finally, humus soils change most slowly of all. Chalk soils
moderate the extreme temperatures of the air, being cooler in summer and
warmer in winter than sandy soils.

NAGELI (1865) has shown that it is as yet impossible to explain completely
the distribution of plants by reference only to the chemical and physical characters
of the soil ; this he has proved in a classical treatise in which he draws attention
to two new factors, hitherto entirely disregarded, which take part in determining
the distribution of plants on the earth's surface. He starts from the fact, already
referred to above, that a plant may be local in distribution in one district and
indifferent in another, or that one and the same species may be calciphobous ia
one region and calciphilous in another. NAGELI' s well-known researches were per-
formed on Achillea atrata and Achillea moschata, and we will take these also as our
illustrations. NAGELI found both species extremely restricted in distribution in

H 2

ioo METABOLISM

the Heu-Thal ; atrata on chalk and moschata on slate. ' Wherever the slate passes
into limestone, moschata at once stops and atrata begins.' In other places,
however, where only one species occurs, it appears quite indifferent as to whether
the substratum be slate or limestone. So long as only one is present both
plants are indifferent as to the soil, but when both occur in company they at
once become confined to special areas ; they adapt themselves markedly to
the chemical nature of the soil and are indifferent to the physical conditions,
for they thrive equally well in wet places and dry places, on humus, on sand, or on
rock. The mutual exclusiveness of these species of Achillea can be explained only
by assuming the influence of concurrence or of the struggle for existence between
nearly related species as the effective cause. Each species lives only on a soil
where the conditions are better fitted for its existence. NAGELI'S experiments
do not demonstrate wherein lies the reason for Achillea atrata' s preference for
limestone and moschata' s preference for slate. It may be concluded, however,
that this preference is quite limited, and yet it may be sufficient to determine,
in a state of nature, the existence or non-existence of the species. We are also
familiar in our own country with plenty of examples of rapidly spreading
American weeds which are capable of killing our native plants, and even entire
floras, in a very short time ; but we are quite ignorant as to wherein lies their
power of intruding and ousting out the native flora. A glance at our culti-
vated plants is sufficient to prove to us that organisms, when withdrawn from the
company of others, are able to exist under conditions which they cannot
tolerate in the wild state. Although we are unacquainted with the reasons
why Achillea atrata gains the ascendancy in one case and Achillea moschata in
another, still we must accept the fact that association is a factor of the highest
importance in determining the distribution of plants on the earth's surface.

NAGELI has drawn attention to another important factor to account for
\he irregular occurrence of plants which we have space only to glance at in
passing. A plant may be absent from an area, notwithstanding the fact that
the chemical and physical conditions of the soil, the plant society, and the
general climatic features are favourable, simply because none of its seeds have
as yet been distributed to that district (Historical Plant Geography).

We must content ourselves here with this brief outline ; further details
will be found in the works of SCHIMPER (1898) and ENGLER (1879). One point
at least is clear, viz. that these problems of plant geography are exceedingly
complicated and cannot be settled off-hand. In fact, the unfortunate craze
for looking for one cause instead of several only tends to obscure the real issue.
When, in the future, accurate researches on this question come to be pieced
together, other factors than those we have alluded to will, doubtless, be dis-
covered. One result, however, these researches certainly will have, they will
tend to discourage the forming of summary conclusions and draw more pointed
attention to the details in individual cases both as regards the plant and also
the soil, and so, doubtless, lead to the discovery of far more individual and
specific differences than could be expected from the older researches on the
subject.

In addition to salt, lime and siliceous soils, reference must in conclusion
be made to humus soils, which are characterized by supporting special plant
communities. We shall return to this subject later (Lecture XIX).
, Before we finally leave the consideration of the mineral constituents, we will
glance at cultivated plants as they are found in our fields and woods, and at the
same time gain a yet greater appreciation of the fundamental importance of
the minerals present in the soil. In nature, the covering of an area of soil by
vegetation leads, as we have seen, to its enrichment in nutritive substances,
since each plant on its death gives back to the soil what it took from it, and in
a form also which other plants can easily appropriate. It is true each material

ASH. 11 101

particle does not return to the identical clod from which it was derived, for wind
and water displace many a leaf and twig, a storm may remove an entire tree,
and even an entire wood with its wealth of mineral matter may be transported
far away by an avalanche. Though such removals are the exception in nature,
in husbandry they are the rule. When the ripe plants are harvested, fruits,
leaves, and often stems as well are removed from the field ; the roots alone
remain where the plant grew. Although each plant contains only a small
quantity of ash, the total amount is very considerable when an entire field is
taken into account, and, according to EBERMAYER (1882) 200-300 kilg. per
hectare of mineral substances are withdrawn when the harvest is reaped, about
half of which is nitrogen, the remainder consisting of the other constituents
of the ash. When we remember that this goes on year after year, we can
easily understand how in a short time all the nutrients in the soil that are
capable of absorption must disappear, and so in the end plant-life is pos-
sible only if the soil still contains constituents capable of decomposition. But
decomposition of soil never takes place rapidly enough for cultivated plants
to obtain the amount of inorganic nutrients they require. The soil, in other
words, becomes exhausted by continuous cultivation, but is not on that account
lost to the agriculturalist for all time, for it can be rendered fertile once more, and
even improved by artificial means and by employing appropriate applications.
Since all cultivated plants do not impoverish the soil in the same way, one type de-
manding, for example, more potassium, another lime, one may economize the
soil by rotation of crops. During the period, for example, a calciphilous plant
grows in a field the soil has time to enrich itself in potash by weathering, so that
in the following year the conditions are favourable for the development of
a plant which makes a demand on potash. Since, however, many substances,
e. g. phosphates, always occur in the soil only in small quantities, and since all
plants require a good deal of them, one cannot employ this method of rotation ex-
clusively. A second method consists in leaving land to lie fallow. In this
method fields are not made to support a crop continuously, but the land is
allowed to be idle, weeds are permitted to grow on it, and these are subse-
quently ploughed in, thus adding to the humus constituents of the soil. By
far the most important method is, however, the addition of nutritive salts to
the soil, directly replacing what has been lost. This method is termed manuring,
and was practised in husbandry long before people had learned its inward
significance. The addition to the soil of the dung of animals mixed with the
straw, as well as the general sewage of a town, naturally replaces in the soil
a part at least of the mineral substances taken from it. LIEBIG showed 'that
the chief value of these bodies lay in their inorganic constituents, so that we
can understand how it is possible to improve or supplement such natural
manures by artificial combinations.

Artificial manuring plays a great part in modern agriculture, and consists
in the addition to the soil of potash, lime, and phosphoric acid, apart from nitric
acid, of which we shall speak later. A sentence on this subject must suffice
at this point. Lime occurs so abundantly in nature that one need never fear
a deficiency of that mineral in agriculture. It is otherwise, however, with
potash and phosphoric acid. The former is manufactured in great quantities
at the potash works of Stassfurt and Leopoldshall in the form of karnallite,
cainite, and sylvinite, and of these cainite (KG . MgSo4 . 3H2O) is specially
important, as it contains not only potassium but also magnesium in the form
of sulphate, also a plant nutrient. As a source of phosphoric acid, ' Thomas-
slag,' a by-product in the smelting of ores containing phosphorus, holds a fore-
most place. It is true that the phosphoric acid occurs in the form of a tricalcic
salt [Cas (P04)2], and that this is insoluble in water. Since, however, the manure
is laid on the field in an exceedingly fine state of subdivision in the form of the

102 METABOLISM

so-called ' Thomas-slag-powder ', the plant is easily in a position to take up
the necessary phosphoric acid. This mention of cainite and Thomas-slag-
powder by no means exhausts the artificial manures used by agriculturalists
but to give further detail would take us too far ; reference must be made for
such information to agricultural literature, e. g. AD. MAYER, Agricultural
Chemistry, 1895.

Bibliography to Lecture VIII.

CORRENS. 1896. Bot. Ztg. 54, 21.

CZAPEK. 1896. Jahrb. f. wiss. Bot. 29, 321.

EBERMAYER. 1882. Physiol. Chemie d. Pflanzen. Berlin.

ENGLER. 1879-82. Versuch einer Entwickelungsgesch. d. Pflanzenwelt. Leipzig.

ENGLER, ARNOLD. 1901. Ber. Schweiz. bot. Gesell. n, 23 (Bot. Centrbl. 89, 269).

FLICHE and GRANDEAU. 1873. Annales d. chim. et d. phys. IV, 2.

GRABNER. 1901. Die Heide Norddeutschlands. Leipzig.

HOVELER. 1892. Jahrb. f. wiss. Bot. 24, 294.

KNOP. 1868. Der Kreislauf des Stoffes. Leipzig.

KNY. 1898. Ber. d. bot. Gesell. 16, 216.

LIEBIG. 1840. Die Chemie in ihrer Anwendung auf Agrikultur, 7th ed., 1862.

MAYER, AD. 1895. Lehrbuch der Agrikulturchemie, 4th ed., II, i. Bodenkunde.

Heidelberg.

NAGELI. 1865. Sitzungsber. Munch. Akad. (Bot. Mitteilungen, 2, i).
NOBBE. 1862. Versuchsstationen, 4, 217 ; 1868, ibid. 10, 94.
OHLMANN. 1898. Veg. Fortpfl. d. Sphagnaceen nebst ihrem Verb, gegen Kalk.

Diss. Freiburg (Schweiz). Braunschweig.
PETERS. 1860. Versuchsstationen, 2, 135.

RAMANN. 1893. Forstliche Bodenkunde u. Standortslehre. Berlin.
Roux. 1900. Traite des rapports des plantes avec le sol. Montpellier.
SACHS. 1865. Handbuch d. Experimental-Physiologic. Leipzig.
SACHS. 1892. Flora, 75, 171.

SCHIMPER. 1898. Pflanzengeographie auf physiolog. Grundlage. Jena.
THURMANN. 1849. Essai de phytostatique appl. a la chaine du Jura.
TREUB. 1888. Aunales Jard. bot. Buitenzorg.

VALLOT. 1883. Rech. physico-chimiques s. la terre vegetale. Paris.
WOLLNY. 1897. Zersetzung d. organischen Stofife u. d. Humusbildungen.

Heidelberg.
WOLLNY. 1898. Forschungen a. d. Geb. d. Agrikulturphysik, 20, 133.

LECTURE IX
THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. I

GREEN plants grown in nutritive solutions show a marked increase in dry
weight (p. 82), but analysis of the dry substance of such plants shows that the
predominant constituent is not the mineral matter absorbed from the solution,
but carbon, of which, as a matter of fact, one half of the dry weight consists.
Carbon occurs as a component of nearly every compound found in the plant,
and the number of these compounds depends chiefly on the fact that carbon
is able to unite with other elements in endlessly variable proportions. In one
sense we may look on carbon as the most important material in plant nutrition,
although it must be remembered that the minerals are every whit as essential
as the carbon. Under these circumstances it may at first sight appear extra-
ordinary that we have not introduced carbon into our culture solutions, or
at least have not done so intentionally.

When we inquire whence plants obtain their carbon, we discover that the
uniformity of method which prevails among them in regard to the mode of
absorption of the constituents of the ash does not hold good here ; there are
fundamental differences between different types of plant life in this respect,
some absorbing carbon in the inorganic form and transforming it into organic
compounds, others making use of carbon solely in the organic form. The

THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. I 103

former obtain all their nutriment directly from the inorganic world, and, so far
as their nutrition is concerned, are thus entirely independent of other organisms;
such forms we may speak of as autotrophic ; the other we designate hetero-
trophic, and without the aid of autotrophic organisms their existence is im-
possible. From this it will be at once apparent how important a part is played
in nature by autotrophic plants, and for that reason they claim the first place
in our attention.

Organic compounds of carbon are not available for the support of plants
grown in typical water-culture solutions, because the air contains none, or, at all
events, quantities so minute that their presence may be entirely neglected, and
because the nutritive solution in which the roots are immersed consists of inorganic
salts only. Since maize, buckwheat, and many other green plants thrive just as
well in such water-cultures as in the soil, it follows that the organic materials
occurring in ordinary soils are either not absorbed at all or that they are at least
unessential. We are thus driven, by the exclusion of other possibilities, to believe
that the source of carbon to the green plant must be the carbon-dioxide of the air.
This gas always occurs, although only in relatively small quantities, in the
atmosphere, and is never absent from the water of lakes, rivers, &c. Carbon-
dioxide also occurs dissolved in the water of a nutritive solution, unless special
precautions are taken for its exclusion, while in the soil it is generally present
abundantly. It is impossible to affirm right off whether a land plant obtains its
carbon-dioxide from the air by means of the leaves or from the soil by means of
the root. Experiment alone can settle this question ; and experiment clearly
teaches us that land plants cannot grow at all when they are prevented from
obtaining carbon-dioxide from the air ; at all events the carbon-dioxide absorbed
by means of their roots is insufficient. Again, it may be easily proved that
submerged plants are able to absorb all the carbon-dioxide they require by
means of the surface of their leaves from that naturally dissolved in the water.

The fundamental thesis, therefore, we have to prove with regard to the
assimilation of carbon by autotrophic plants is this : the carbon-dioxide is
decomposed by the energy of sunlight acting on the chlorophyll bodies of the living
cells ; the carbon is united with the elements of water to form carbohydrates while
the oxygen is given off and escapes from the plant.

[Following PFEFFER (Phys. 1st and 2nd eds.) and WIESNER (Elemente
der Botanik), the production of organic material as well as its further altera-
tion into the complex constituents of the living cell, are described in this book
under the term assimilation. Objection has been taken to this nomenclature
in a review of this work in the Botanical Gazette (1904, 37, 390) (see also Bot.
Centrbl. 76, 257). The reviewer points out, with justice, that beginners are
liable to get into difficulties if the formation of sugar out of carbon-dioxide in
autotrophic plants, and the further transformation of sugar in heterotrophic
plants, are both described by the same name. Nevertheless, there appears to
us no good reason for accepting the alternative suggestion, viz. to describe
as synthesis or syntax, the assimilation of carbon and so distinguish it from
assimilation proper, for how should we then designate nitrogen assimilation
or assimilation of minerals ? In these cases we have no distinguishable simple
bodies, corresponding to the sugar, which are assimilated, but rather potassium
nitrate, or even the free nitrogen of the air. No good reason can be adduced
for treating carbon differently from nitrogen, and therefore we have retained
the older terminology, though with full cognizance of the difficulties involved
in so doing.]

In order to render intelligible this extremely important thesis in plant
physiology it will be necessary for us to investigate closely (i) the decomposition
of the carbon-dioxide ; (2) the significance of chlorophyll ; (3) the importance
of sunlight in the process ; (4) the nature of the resulting products.

IO4

METABOLISM

The decomposition of carbon-dioxide shows itself most obviously in the
giving off of oxygen. The peculiar characteristics of water plants furnish us
with a method of demonstration of this phenomenon equally serviceable for
lecture purposes and for laboratory work. Fix a branch of Elodea Canadensis,
or a submerged leaf of Potamogeton, to a glass rod in the manner shown in
Fig. 22, and arrange that the branch or leaf while under water is illuminated
by sunlight or artificial light. Presently we shall see issuing in regular succes-
sion from the cut end a series of small air-bubbles. This stream of bubbles
may be accounted for quite simply. If we assume that the plant has been in
the dark for some time it will Have accumulated in its intercellular spaces, in
addition to oxygen and nitrogen, a certain amount of carbon-dioxide. If this
last be decomposed, the oxygen formed, in accordance with the known characters
of gases, will occupy the same space as the carbon-dioxide did ; and hence there
would appear no reason why air should escape from the cut
surface. This will occur when the disappearing carbon- dioxide
is replaced by diffusion from without, thus producing an
excess pressure in the intercellular spaces of the plant. This
excess pressure remains constant so long as carbon-dioxide is
present outside the plant and undergoes decomposition within it.
If we arrange an apparatus with the object of collecting the air
which streams from several submerged branches exposed simul-
taneously to light, e. g. by covering them with a test-tube
filled with water, we shall be able to study the gas much more
conveniently and accurately. That the gas consists largely of
oxygen is proved by the fact that it causes a glowing splinter
of wood to burst into flame, and more exact analysis establishes
the fact that it never consists of pure oxygen, but always has
a demonstrable admixture of nitrogen. This is only to be ex-
pected, since, if the intercellular spaces of the plants have been,
owing to the decomposition of carbon-dioxide, rendered richer
in oxygen than the surrounding water, nitrogen must pass into
the intercellular spaces from the water. Further, each individual
air-bubble in its passage through the water, and finally the
whole amount of gas present, must receive additions of nitrogen
by diffusion so long as the experiment continues. No doubt
we could arrange an experiment in such a way that pure oxygen
would be obtained, that is to say by removing the nitrogen
originally held in solution by the water, taking care to replace
the carbon-dioxide lost in the process, and by preventing the entrance of
fresh supplies of nitrogen. The form of the first experiment suggests
several criticisms ; for instance, the stream of air-bubbles, at least at first,
might be due to expansion of the air in the intercellular spaces by
heating, and later on, to some extent, by thermo-diffusion. Evidence, how-
ever, that the stream of air-bubbles is entirely dependent on the presence of
carbon-dioxide in the water disposes of all these objections. The addition of a
small quantity of lime-water and the consequent precipitation of the carbonic
acid (F. SCHWARZ, 1881), or the use of freshly boiled water, is sufficient to bring
the stream of air-bubbles at once to a standstill, without doing any damage to
the plant.

The method just described is not only qualitative but may be made quanti-
tative as well, since the number of bubbles which come off in a unit of time
from a given specimen forms a means of measuring the amount of carbon-
dioxide decomposed. It is true we must not compare several specimens with
each other off-hand, for a vigorously active branch may give off few but large
air-bubbles, while a weakly assimilating one may furnish many small ones. The

Fig. 23. From
DBTMER'S Prac-
tical Plant Physi-
ology.

THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. I 105

size of the bubbles, however, depends chiefly on the extent of the cut surface
at the base of the plant experimented on, although even then the size is not
constant in one and the same specimen.

Although the bubble method is often of service in quantitative experiments
owing to its great simplicity, still it is a method confined in practice to aquatic
plants. Further, it is impossible with this method to obtain fundamental data
of a quantitative kind when absolute, rather than relative values are required.
For this purpose eudiometric experiments are necessary, the principle of which
is to expose portions of plants, especially foliage leaves, to sunlight in an en-
closed chamber filled with an atmosphere rich in carbon-dioxide, and to investi-
gate what alterations take place in the composition of the air. Methods of
analyses have been perfected in many ways since the time of BUNSEN, and
have become extremely exact (unnecessarily so for our present purpose) though
somewhat complicated ; more recently, however, BONNIER and MANGIN have
introduced a much improved, and, in its latest form, exceedingly convenient
apparatus by means of which gas analysis may be carried out rapidly and
without involving laborious reductions (AUBERT, 1891). [If POLLACCI'S criti-
cism be correct (Atti Istit. Pavia, 1905), BONNIER and MANGIN'S apparatus is
perfectly worthless !] The results of eudiometric experiments may be briefly
summarized as follows : — The volume of gas remains practically constant while
assimilation is going on, because for every volume of carbon-dioxide that
disappears an approximately equal amount of oxygen is produced.

Since the giving off of oxygen is intimately bound up with the assimilation
of carbon-dioxide, it follows that there are many other methods of demonstrating
the assimilation of carbon over and above the two that have been referred to.
Oxygen possesses many properties, some purely chemical, some physiological,
which may be employed in determining its presence. The literature on the
subject contains many references to its purely chemical properties. Thus
BEIJERINCK (1890) has shown that reduced indigo-carmine becomes blue
again owing to the activity of assimilating plants ; HOPPE (1879) placed a
plant of Elodea in a sealed glass tube containing a dilute solution of
venous blood ; owing to the using up of all the oxygen, the solution then gave
the characteristic reaction of haemoglobin ; but the substance at once changed
into oxyhaemoglobin (easily recognizable by its spectrum) whenever the tube
was brought into sunlight. These methods are useful ones for demonstration
purposes, but they have been as little used as methods of investigation as the
physiological reaction described by BEIJERINCK (1901), who showed that if
luminous bacteria are brought in contact with green Algae they are phosphor-
escent, but only when the green cells are assimilating, and that in the absence of
oxygen the luminosity ceases. Another physiological method applicable to re-
search has been more often employed, viz. that described by W. ENGELMANN in
numerous memoirs, and summarized finally, under the title of the ' Bacterium
method ', in 1894. It depends on the fact that many bacteria, e. g. Bacterium
termo, are capable of exhibiting movements in the presence of minute
traces of oxygen. If one inoculates a drop of water on a slide with a pure
culture of this bacterium, and surrounds the edge of the cover glass with a ring of
vaseline to keep out atmospheric oxygen, the bacteria at first exhibit active move-
ments in the fluid ; but gradually their power of movement becomes less and
less, until, when all the oxygen dissolved in the water has been used up, they
at length come to rest. If bubbles of air be enclosed under the cover glass,
however, these become centres of attraction to the bacteria in consequence of
the presence of oxygen there ; the bacteria move towards the bubbles, collect
in their neighbourhood, and continue to exhibit movement for some time,
although motionless elsewhere. The phenomena described — and this is a point
of importance — are entirely independent of light, for they behave in the same

io6 METABOLISM

way whether the slide be placed in the dark or in the light. If in such a pre-
paration we replace the air-bubbles by specimens of a unicellular alga we find that
if kept in the dark all the bacteria quickly come to rest ; as soon as the preparation
is illuminated, however, the cells of the alga give off oxygen, and the oxygen
at once exerts an influence on all the bacteria that happen to be near the cells, caus-
ing them to rush towards them and to move actively in their immediate vicinity
(Fig. 23, /, //). When the light is removed the bacteria again distribute themselves
over the field and come to rest, while each successive illumination once more
induces movement and crowding round the alga. This method owes its
frequent use more especially to its very great sensitiveness, for the presence of
the minutest traces of oxygen may be demonstrated by its means. One may
vary the sensitiveness by employing other bacteria, e. g. Spirillum, as well as
organisms or cells belonging to other groups (Infusoria, Flagellata, or sperma-
tozooids of sea urchins) since many of these react, some only to larger, some to
even smaller quantities of oxygen than Bacterium termo. On the other hand, the
method has its drawbacks ; it must in any case be used with caution, as we
shall have occasion to see by and by.

By means, then, of the methods which have been described it is possible
to prove that green parts of plants can assimilate carbon-dioxide in light, and
this is one of the best established facts in plant physiology. Nor is it difficult
to show that this power is confined to green organs. Every experiment with
a fungus or with a root demonstrates at once absence of any decomposition of
carbon-dioxide ; while, on the other hand, the parts of the plant with the
darkest green colour, the foliage leaves, have long been known to be the most
active members in carbon-dioxide assimilation. It is true that carbon-dioxide
assimilation has also been often observed in parts of the plant otherwise coloured,
but more careful study has always shown that these parts contain green colouring
matter, which is simply masked by other pigments. The carriers of the green
colour are the chloroplasts, which are special organs of the cell, capable of
increasing in number by division, and developing the green colour under certain
conditions. Thus it is possible to prevent its formation by omitting iron from
the nutritive supply (compare p. 85), and also, in the higher plants at least,
by keeping the plant in the dark. In both these cases the chloroplast itself —
the protoplasmic basis of the chromatophore — is formed, but the chlorophyll
does not develop ; the chloroplast remains either colourless or yellow. Further,
PjEFFER (1881) has shown by the eudiometric method, and ZIMMERMANN
(1893) has confirmed his results by the bacterium method, that parts of plants
which have become chlorotic owing to the absence of iron are quite unable to
decompose carbon-dioxide, and hence we must look upon the green pigment
as a factor of the highest importance in assimilation, and all the more so because
we find that etiolated plants grown in the dark cause no decomposition of
carbon-dioxide when first placed in the light. Decomposition will, of course, take
place after a certain length of time, since the green colour becomes rapidly
developed in light. Recently, EWART (1897) has made certain observations
on etiolated cells, and has shown that if they be not too old or too young, an
evolution of oxygen from them may be demonstrated by the bacterium method,
even before the slightest trace of green colour has been developed. This would
appear to confirm an older research of ENGELMANN'S, but further investigations
are required to determine whether the interpretation EWART puts on his observa-
tions is correct or not. Further, it must be noted that an attraction of
bacteria does not take place on every occasion. Bacteria respond by mobility,
for instance, to other substances besides oxygen (Lecture XLIII), and it is im-
possible to affirm that etiolated chloroplasts may not contain substances which
have an attractive influence on such organisms. At present we may be per-
mitted to ignore E WART'S statements, and say that only cells containing

THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. I 107

chlorophyll are capable of assimilating carbon-dioxide. [MoLiscn (I.e.) finds
that etiolated chloroplasts are quite inactive.]

That it is not the entire chlorophyllaceous cell, but only the chloroplast
in particular that is the agen* in the decomposition of carbon-dioxide, may be
proved by the following observations : — If one allows two small circles of light
to fall on a Spirogyra cell (with an open chlorophyll band) surrounded by
bacteria, so that one plays on the chloroplast and the other on the colourless
cytoplasm, it will be seen that an active assembling of bacteria occurs only in
the first field of light (Fig. 23, ///). Again, it has often been observed that
solitary chloroplasts isolated from cells are able for a long time to assimilate
whilst colourless cytoplasm is quite incapable of doing so. [MoLiscn's (1904,
Bot. Ztg. 62, I, i) results should be compared in this relation. He was able to
show, with the aid of luminous bacteria, that assimilation of carbon-dioxide could
be carried on by individual chloroplasts taken from dried dead cells.] The
disputed question (compare KNY, 1897, 1898; EWART, 1898) as to whether such
functional chloroplasts must be still surrounded by a layer of protoplasm or
not, is of less interest in this relation ; it is, however, clear that in the long

• «•*.•*' . .*- -. '*•"'.;'. '••

..•V&.-y- " ••; £-• . .,/.;'* V/.:';/" ;\;»x

../•.<•.':•.. -'. • *. / •.". ' .- ••»'.•.>

••,?• .> :'?>•. ;.?.•:* £••:.../ - •/.•

. ;::^;C*'l vr-' T 1^5 .-;;;.:; • -..^'. '•„

•^}*\-^;V '':*>« ..^"'•'^ '5. i*. •,.";»^ • . '.«:

1V»* «."'•... °'° •.";>*' i^Vi''-" '.;•?•'•«• ' • '«..*'••

'?.'*'•;••*•?/•' •"-'. *5"-.':.. . . • .^r.'-'"

Fig. 23. /, a spherical, green alga centrally placed, and uniformly surrounded by bacteria (in darkness).
X 150; 77, the same preparation after a brief illumination ; 777, cell of Spirogyra illuminated at two places; the
bacteria collect only where the beam of light touches the chloroplast. x 350. (After ENGELMANN, 1894.)

run it is only a chloroplast enclosed in protoplasm that can assimilate, and there-
fore it is not of so much importance to know how quickly it loses its capacity
when isolated from the plasma. The dependence of the chloroplast on the
protoplasm need not be directly connected with the process of assimilation.

Now that we have shown that it is only the chloroplast that assimilates
in the cell, and that too only when it is green, we have next to attempt to settle
the question as to whether it is the pigment itself, the chlorophyll, that is the
agent in the decomposition of carbon-dioxide, or whether it can carry out its
function in the absence of the protoplasmic basis.

This problem compels us to examine more closely into the physical and
chemical characters of the chlorophyll. [The recent literature on the subject
of chlorophyll is to be found in CZAPEK, I.]

In certain chloroplasts green granules or drops are visible, distinct from
the colourless ground substance, and the green colour can be extracted by
means of alcohol. This crude alcoholic solution of chlorophyll is characterized
from a physical point of view by its fluorescence and by its absorption spectrum.
By transmitted light the solution is a beautiful green, but deep red by reflected
light, but this fluorescence appears only in the solution and never in the chloro-
plasts, and we should naturally conclude from this that the dye occurs in the
chloroplast in a state of combination (REINKE, 1883). The spectrum of crude
chlorophyll is shown at the top of Fig. 24.

io8

METABOLISM

It is characterized by the presence of six absorption bands, three of which
are in the more refrangible (beyond F), the other three in the less refrangible
part. The bands occurring on the other side of F appear distinct only when
weak solutions are used ; in strong solutions they merge into one. Of the
three others, the first band (A. = 670-635 //,//,) is by far the most intense ; the
second (A = 622-597 /A/A) and third (A. = 587-565 ft//,) are much feebler. The figure
further shows, just in front of line £, a band which belongs, not to chlorophyll
itself, but to a decomposition product of it. A solitary chloroplast or a leaf
presents an absorption-spectrum in all respects similar to that given by the
solution, save that the bands as a whole are displaced somewhat nearer to the
red end, whence we may conclude that the colouring matter in the chloroplasts
is dissolved in a dense medium or occurs in a combined form.

The crude chlorophyll we have hitherto been studying is, however, by no
means a simple body. As KRAUS (1872) has shown, it is possible, by shaking

too

100

Fig. 24. Absorption-spectrum of chlorophyll 'after KRAUS, 1872). 7, alcoholic extract of the green leaf (crude
chlorophyll) ; 77, sp"ectrum of the blue-green benzene extract ; 777, spectrum of the yellow pigment. (From
DETMEITS Smaller Practical Physiology.)

up an alcoholic solution with benzene, to obtain a blue-green dye more soluble
in the benzene, and a yellow dye which remains dissolved in the alcohol. The
spectra of these dyes is shown in Fig. 24, II and ///. Since the yellow dye has
nothing to do with carbon-dioxide assimilation, we may dismiss it from our
consideration in a sentence. It consists not of a simple compound but of a mix-
ture of at least three bodies :

1. Chrysophyll, an undetermined hydrocarbon with formula C26H38, which
can be crystallized and is closely related to, if not identical with, carotin, a sub-
stance widely distributed in the plant world. Its spectrum shows three absorp-
tion bands beyond F.

2. A substance which exhibits a spectrum with four absorption bands in
the more refrangible part, which are, however, not identical with those of
chrysophyll (xanthophyll, according to SCHUNK).

3. Another substance, which gives no absorption bands but produces entire
obliteration of the violet and ultra-violet rays.

The colouring matter dissolved in the benzene is also by no means a simple
body ; it consists, in addition to true ' chlorophyll ', of an admixture of a sub-
stance known as ' allochlorophyll ', though in very small quantity. Great

THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. I 109

progress has been made in the chemical investigation of true chlorophyll during
the past few years, and research has shown that it is an exceedingly complicated
body, in the composition of which lecithins and, perhaps, also proteid com-
pounds take part. A detailed exposition of the chemistry of chlorophyll, from
the pen of MARCHLEWSKI, will be found in ROSCOE and SCHORLEMMER'S Lehr-
buch d. Chemie, vol. VIII, 1901. CZAPEK (1902) also gives a summary of the
chemistry of chlorophyll ; in both works, and especially in that of MARCH-
LEWSKI, a full list of the literature is given.

There is, further, a series of by-products of chlorophyll known to us, of
which we may mention one only, viz. phylloporphyrin. This substance, occur-
ring in the form of dark red-violet crystals, has a great likeness to a by-
product of the colouring matter of the blood, haematoporphyrin. This likeness
is demonstrated by comparison of the absorption-spectra of the two bodies.
These spectra are identical save in one respect, viz. that the absorption bands
of haematoporphyrin are displaced somewhat towards the red end (Fig. 25).
A likeness is also apparent in the chemical composition of the substances, for
haematoporphyrin has a composition represented by the formula C16H18N2OS,
while that of phylloporphyrin is represented by C16H18N2O. Both substances,

Fig. 25 Absorption spectra, /, of phylloporphyrin (in ether) ; II, of haematoporphyrin (in ether). After
MARCHLEWSKI, in ROSCOE and SCHORLEMMER'S Lehrbuch der Chemie, vol. VIII. 1901.

on dry distillation, yield pyrrol. In spite of this agreement in chemical and
physical characters, chlorophyll and the colouring matter of the blood have
quite distinct physiological duties to perform.

After this digression on the characteristics of chlorophyll we may return
to the consideration of the problem as to whether the dye alone, or only when
in combination with the living protoplasmic basis of the chloroplast, is con-
cerned in carbon-dioxide assimilation.

As a matter of fact the assertion has frequently been made that a solution
of chlorophyll may, apart from the living substratum, be capable of abstracting
oxygen from carbon-dioxide. KNY (1897) has, however, given most convincing
proof that this is not so ; for when he investigated oil-drops containing chloro-
phyll by the bacterium method, not a trace of oxygen was to be found.
CZAPEK (1902) has carried this investigation a step further. He introduced
chlorophylliferous oil-drops into colourless protoplasm, but was unable to find
any evidence of the excretion of oxygen in the case of cells thus artificially
provided with the dye. It would appear, therefore, that the plasmatic
basis is quite as essential for the performance of the function carried out by the
chlorophyll-apparatus as the dye, and that other selected parts of the cyto-
plasm cannot assume the duties of the plasma of the chloroplast.

Many facts, ascertained recently, of which we shall speak later on (Lecture
XVII), render it not improbable that certain chemical compounds, perhaps quite
independently of the living chloroplast, assist in the carrying out of the ' chloro-
phyll function*. The experiments of FRIEDEL (1901) andMACCHiATi (1903),
aiming at the isolation of these bodies, have not, however, been successful

no METABOLISM

(HERTZOG, 1902). [BERNARD (1904. Beih. z. bot. Centrbl. 6) was unable to
confirm FRIEDEL'S results, which must be considered as still requiring proof.
Compare also MOLISCH, Bot. Ztg. 62, I, i.]

In the third place, the part played by sunlight in the process of assimilation
must be emphasized. Each of the methods mentioned above shows in the
clearest possible manner that the evolution of oxygen takes place only in the
presence of light and only in regions which are illuminated. The bubble method
demonstrates in the most self-evident way how assimilation ceases when a plant
is gradually removed from the window to the back of a room, and how the
process comes to a standstill even in light of relatively high intensity, which to
our eyes would appear still sufficiently intense for the purpose. At the present
moment we can only emphasize the fact that light is essential to the assimilation
of carbon-dioxide, and postpone till later a consideration of the question of
the quality and intensity of the light. We must first determine what is
the exact significance of light in carbon-dioxide assimilation, and what sub-
stances are produced from the carbon-dioxide, or in other words, we must
study the nature of the first products of assimilation.

The result of the gas-analysis method of investigation, viz. that the volumes
of the carbon-dioxide operated upon and of the oxygen evolved are equal, pro-
vides us with a certain basis on which to work. The equality of the volumes
suggests, for example, that carbon-dioxide is decomposed into carbon and
oxygen. All experience, however, is contrary to the supposition that free
carbon is produced ; carbon never occurs as such in the plant, and it cannot
be built up into organic material when it is artificially supplied in that form
to the plant. All organic bodies contain in addition to carbon, at least hydro-
gen, and this can have been obtained only from the water which is present
everywhere in the plant. If we now assume that hydrocarbons, which are the
simplest organic substances, are those which originate in carbon-dioxide assimi-
lation, then the oxygen must have been derived not from the carbon-dioxide alone
but from the water also. If that be so, however, much more oxygen should be given
off free than is, as a matter of fact, found to be the case. The experiments
of BOUSSINGAULT (1868) moreover showed that hydrocarbons cannot be further
worked up. On the other hand, the proportion observed to exist between the
carbon absorbed and the oxygen given off agrees entirely with the manufacture
of carbohydrate. Thus if we express the formation of starch or glucose schema-
tically we obtain the following equations : —

6CO2 + 6H3O = CfiH12O6 + 6O3

Carbon dioxide. Water. Glucose. Oxygen.

or, 6 CO, -r sH.O = CBH10O5 + 6O3

Starch.

In both cases the relation is-T- = I, as analysis itself also shows. The

variations from unity are in many plants quite inconsiderable ; thus BONNIER and
MANGIN (1886) found it to be in the ivy, 1-08, and in the horse-chestnut and
in Syringa, 1-06. These numbers tell us that a little more oxygen is formed
than can be accounted for by the above formulae. This excess of oxygen
reaches, however, a quite noticeable amount in other cases, for the same authors

give the value of the fraction .—- in Ilex as 1-24. We shall take another oppor-

tunity of returning (Lecture XVI) to these cases, and consider, in our pre-
liminary investigations into the origin of carbohydrates, the first-mentioned
plants only. The development of carbohydrates as a result of carbon assimi-
lation in green plants has, as a matter of fact, been established beyond all
doubt.

Among the carbohydrates which result from the assimilation process,

THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. I in

starch has been known by far the longest, because it is the most striking and
obvious constituent in the chloroplast. With the aid of a microscope we can often
demonstrate its presence in the chloroplasts after a very brief exposure to
light, if we take tL3 precaution of seeing that the plastids were free from starch
at the beginning of the experiment. We further find that when the plant has
been kept for some time in the dark, not only is a further formation of starch
prevented, but dissolution of what was present takes place. When the leaf,
freed from starch and still attached to the stem, is placed in the sun early on
a bright summer morning, we can, by using iodine solution, demonstrate the
hourly increase in amount of starch. In the evening the chloroplasts are
found to be so packed with starch grains that iodine solution turns the leaf
perfectly black. In order to employ the iodine-proof to the best advantage it
is advisable to extract the chlorophyll with hot alcohol, so that that pigment
does not interfere with the colour reaction ; for the proper demonstration of
small quantities of starch it is preferable to treat the preparation either with
chloralhydrate or with boiling water. When leaves of different plants are
subjected to similar treatment with iodine, the resulting coloration gives us
an approximate measure of the amount of starch formed, and it is easily seen
that not all plants are able to produce starch to the same extent. A. MEYER
(1885) has published a list from which one can see not only that variations
occur in the capacity for producing starch, but also that their variations are
characteristic of certain families. MEYER found : —

1. Very large quantities of starch in the Solanaceae and Papilionaceae.

2. Large quantities in the Papaveraceae, Crassulaceae, Geraniaceae, Oxa-
lidaceae, Boraginaceae, Labiatae, Dioscoreaceae, and many others.

3. Moderate amounts in the Caryophyllaceae, Ranunculaceae, Coniferae, &c.

4. Small amounts in many Lobeliaceae.

5. Very little in many Gentianaceae and Iridaceae.

6. None at all in Asclepias cornuti ; Allium, S cilia and many other
Liliaceae, and in many Amaryllidaceae and Orchidaceae.

It is easily demonstrable, however, that even where no starch occurs there
is still an active evolution of oxygen in sunlight, and that here also the volume
of oxygen given off is equal to the amount of carbon-dioxide decomposed. It
follows at once that other carbohydrates are also produced. More exact in-
vestigation further shows us that, even in the plants which are richest in starch,
the starch is not the first product of assimilation. In the first place, it is highly
improbable that an insoluble complex body like starch should arise at once,
and in the second place, one finds even in favourable instances that the starch
appears only at an appreciable interval after the commencement of the de-
composition of carbon-dioxide. Thus KRAUS (1869) found the first visible
traces of starch in Spirogyra five minutes after the illumination of the cells, and
in other cases after a longer interval ; but, by means of the bacterium method,
it may be shown that the decomposition of the carbon-dioxide takes place
simultaneously with illumination. The first products of assimilation must
manifestly be soluble, and from these starch arises as a secondary result. The
proof of this fact we owe to A. MEYER (1885), who has shown, by chemical
analysis, that during assimilation large quantites of soluble carbohydrates are
formed in plants without starch appearing, some reducing, some non-reducing,
and further, that the same substances occur in plants which possess abundant
starch. These results have been confirmed by A. F. W. SCHIMPER (1885), who
draws the following conclusion from them, viz. that the difference between
plants in the matter of starch formation does not consist in a difference in the
activity of the assimilative process, but in the fact that while one series
stores the soluble carbohydrates as such the other transforms them into
starch. [According to A. MULLER (Jahrb. f. wiss. Bot. 1904, 40, 443), leaves con-

H2 METABOLISM

taining sugar construct considerably less solid than leaves containing starch.]
In fact, it can be shown that many plants, normally without starch, can form
it provided the sugar formed during assimilation be present in sufficient con-
centration. Such a concentration may be reached in many ways ; by separat-
ing the leaves from the stem and so preventing the translocation of the
manufactured carbohydrate, by providing an atmosphere rich in carbon-dioxide,
and thereby occasioning an increase in assimilative activity, e. g. Musa and
Strelitzia (GODLEWSKI, 1877), Iris (ScniMPER, 1885), or by adding certain
carbohydrates from extraneous sources. SCHIMPER (1885) for example, showed
that Iris germanica, which is normally free from starch, can be made to form it if
provided with a 20 per cent, solution of sugar. This method, varied in detail, has
been used with great success by BOHM (1883), and also by A. MEYER (1886),
LAURENT (1887), and KLEBS (1888), to show that plants which have been
freed from starch can form it from a sugar solution in the dark. Starch
formation has thus nothing to do directly with the assimilation of carbon-dioxide ;
it occurs in all chromatophores, whether they contain chlorophyll or not,
whether they are in light or darkness, and always occurs when soluble carbo-
hydrate accumulates in large quantities in the cells. The degree of concen-
tration of sugar from which the starch is formed varies in different plants.
The authorities above named (and others) have further shown that, in addition
to dextrose and levulose, which form the source of starch in so many plants, other
carbohydrates (compare CZAPEK, 1902), such as mannose, galactose, saccharose,
as well as alcohols such as glycerine, mannite, erythrite (at least in some plants),
operate in the same way. The fact that glycerine may, in very many plants, be con-
verted into starch, proves to us in the clearest possible way that we cannot
argue backwards and say that all the substances employed in the formation
of starch must arise during the decomposition of carbon-dioxide, for glycerine
is apparently never formed as a product of assimilation.

We may, therefore, conclude that soluble carbohydrates are the first
demonstrable products after the carbon of the carbon-dioxide had become
united with water. This conclusion is, however, not satisfactory, especially from
the chemist's point of view. The complicated nature of this substance is quite
rightly emphasized, and we must, in consequence, search for a simpler body
as the first product of constructive metabolism. There are many hypotheses forth-
coming to aid us in our search for such a ' first product of assimilation ' ; of
these we may refer here to one only which has proved of special significance
in physiology, because it has led to a number of investigations. BAYER (1870)
started from the similarity found to exist between the colouring matter of the
blood and chlorophyll. Since carbon-monoxide was able to form a compound
with haemoglobin he supposed that chlorophyll might show a similar capacity.
Under the influence of sunlight he fancied the carbon-dioxide was broken
down into carbon-monoxide and oxygen ; the carbon-monoxide was united
with water, forming, after evolution of more oxygen, formaldehyde. The
following formulae express these chemical changes : —

C02=CO + 0; CO + HaO = CHaO + 0.

An optically inactive reducing sugar might then be simply produced from
formaldehyde, i. e. formose, with the formula C6H12O6 (BuTTLEROW, 1861 ; LOEW,
1886 ; compare also E. FISCHER, 1894). The fact that this sugar has not as
yet been found in the plant does not appear to us to militate against BAYER'S
hypothesis ; but the criticism, that although carbon-monoxide, as JUST (1879)
has shown, is not injurious to the plant as it is to the higher animals, it cannot
undergo higher constructive metabolism, is more to the point. [Recently it has
(CZAPEK, I, 428) been stated that carbon-monoxide may be assimilated even
though it be a poison.] One may get over this difficulty easily by assuming

THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. I 113

that formaldehyde arises from carbon-dioxide directly, without going through
the intermediate product, carbon-monoxide. The direct origin is all the more
plausible if we start, not from carbon-dioxide (CO2), as we have been doing
hitherto, but from carbonic acid (H2 CO3). We are certainly entitled to do this,
since chemists tell us that carbon-dioxide exists in aqueous solution only in the
form of H2 CO3 ; we know, further, that carbon-dioxide never occurs in the
plant cells as a free gas, but always in the dissolved condition. The con-
struction of formaldehyde would thus be represented by the equation : —

H2C03 = CH20 + 02;
Carbonic acid = Aldehyde -»- Oxygen ;

so that the hypothesis of ERLENMEYER (1877), which assumes formic acid as
an intermediate product would be in accord with this view (compare CZAPEK,

1902) :—

H2CO3 - CH2O2+O; CH2O2 = CH2O + O.

Carbonic acid = Formic acid + Oxygen ; Formic acid - Aldehyde + Oxygen.

Many arguments have been advanced from the physiological standpoint
against the origin of formic acid and formaldehyde as intermediate products
in carbon-dioxide assimilation ; for example, the regular occurrence of these
substances in the assimilating leaf has been doubted, and further it has been
pointed out that formaldehyde is very poisonous to plants. More recently,
POLLACCI (1900, 1902) has demonstrated with certainty the constant occurrence
of formaldehyde in green leaves, while CURTIUS and REINKE (1897) observed
the appearance of aldehyde in the green leaf in light and its disappearance in the
dark, and especially noted (REINKE, 1899) that it occurred not as formaldehyde,
but as a substance with the formula C8H2O2. [POLLACCI also (Atti Istit. d.
Pavia, 1904) has very strongly upheld the aldehyde hypothesis in a paper not
long published, but a critical re examination of his work is still wanted (compare
CZAPEK, 1, 502-6).] In whatever way the question may be finally settled it would
appear to us that the deficiency in a certain substance is a much better proof
of its being an intermediate product in assimilation than its plentiful occurrence,
since such intermediate stages would in all probability be passed through rapidly.
It is important to remember in the discussion of this question that although
formaldehyde, even in very small quantities (i : 20,000), is intensely poisonous
to plants, its poisonous effects could be avoided by its rapid construction into
higher compounds. Further, it is extremely significant that, as yet, all attempts
to make the chloroplasts use formaldehyde in the manufacture of starch have
failed, and yet it must certainly take place if the hypothesis be correct. Accord-
ing to BOKORNY (1897), Spirogyra certainly can (although only in sunlight)
manufacture starch out of sodium formaldehyde sulphite, and also from methylal.
Since both these substances yield formaldehyde readily, BOKORNY concluded
that the experiment furnishes evidence of the manufacture of starch from
formaldehyde. Even if his opinion were well grounded, still BAYER'S hypo-
thesis would not be directly proved thereby, for one could hold with equal
justice that glycerine was the first product of assimilation (PFEFFER, Phys. I,
p. 341), since starch is manufactured from glycerine, and that, too, in the dark.
Experiments carried out by TREBOUX (1903) on Elodea show that this plant
can live perfectly well in a 0-0005 per cent, solution of formaldehyde, but that
it was quite unable to manufacture starch from it either in the dark or in the
light.

The settlement of the question as to what is the first product of carbon-
assimilation is not of great importance in physiology, and we will there-
fore confine our attention in the following pages to the consideration of
the carbohydrates, which have been shown to be undoubtedly products of
assimilation ; and our knowledge of the phenomena taking place will be con-

JOST I

H4 METABOLISM

siderably deepened if we proceed to investigate the subject quantitatively as
well as qualitatively. Quantitative researches were first carried out by SACHS
(1884), who was able, by using the iodine method, to obtain an approximate
estimate of the amount of the assimilation products. His procedure was as
follows : — He selected large, well-developed foliage leaves on plants growing
in the open (Helianthus, Cucurbita, Rheum), and early in the morning cut out
of one longitudinal half of the leaf (avoiding the larger veins) accurately measured
portions from 50 to 100 sq. cm. area, in all about 500 sq. cm., and determined
their dry weight. In the evening corresponding areas were taken from the
other longitudinal half, which meanwhile remained attached to the plant and
had been assimilating, and these were treated in the same way. In every case
a marked increase in dry weight was observable, equivalent to the following
amounts in grams per square metre per hour : —

Helianthus, 0-914 g ; Cucurbita, 0-680 g. ; Rheum, 0-652 g.

These numbers, however, must, for two reasons, be considered as representing
only a fraction of the products actually manufactured. In the first place we
know that every part of the plant suffers considerable loss of organic substance
during carbon-dioxide assimilation, owing to respiration acting in the reverse
direction (Lecture XVI), and, further, that there is a continual withdrawal of
soluble carbohydrate taking place (Lecture XIV) into the stem. SACHS did not cal-
culate exactly the loss from respiration, and disregarded it, beyond estimating
it at about i g. per sq. metre during fifteen hours of assimilative activity. On
the other hand, he instituted special experiments to estimate the amount of the
translocated materials. With this object he investigated the loss in weight taking
place during ten hours of continuous darkness, employing the same * half-leaf '
method above described. The temperature is, however, higher by day, so that
more material is removed by day than by night. Since SACHS did not assume
a greater loss in weight during assimilation than at night, his numbers may be
taken, at all events, as not too great. He attempted to determine in another
way the loss due to translocation, viz. by using isolated leaves ; in this case,
also, the numbers obtained were certainly too small because storage of the
products of assimilation must inhibit carbon-dioxide decomposition (Lec-
ture X). The following numbers may be taken as giving only approximate
minimum values under very favourable external conditions : —

Helianthus per hour and sq. m. (Method i) 1-882 g. of dry weight formed

5> » J> k J» 2) I*7 » »

Cucurbita „ „ ( „ i) 1-502 „ „

Taking these numbers as a basis, SACHS reckoned that a vigorously active
sunflower can manufacture 36 g., and a cucumber, 185 g. of dry weight
during the course of a warm, bright summer day.

In SACHS'S memoirs it is not theory weight that is spoken of, but always starch,
since at that time all products of assimilation were assumed to be deposited
in that form. Ten years later, BROWN and MORRIS (1893) repeated SACHS'S
experiments, and confirmed them in all essentials, although they obtained
smaller values than he did ; but they also showed that only a fraction of the
products of assimilation occurred in the form of starch. In one case, for example,
Helianthus showed an increase in weight of 8-566 g. per sq. metre in twelve
hours (in addition to 4g. translocated) but of this amount only 1-4 g. was starch,
all the rest was sugar. Similar results are recorded in the works of A. MEYER
and in several other memoirs which were published in the interval. MEYER'S
studies on the nature of the sugars occurring in the leaf are therefore especially
valuable, and for such investigations he found the leaves of Tropaeolum to be
particularly adapted. Without discussing the methods employed we may give

THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. I 115

the following results of his experiments in detail,
per cent, of the dried leaf substance.

The numbers indicate grams

I.

I]

[.

II

I.

r

7.

Starch .

a.

T.OO

b.

q.QT

c.

a.

3-24

b.

4*22

a.
4.60

b.
2.08

a.

c.4.q

b.
O.QT

4.6^

8-8=;

3.86

4.04

8- 02

0.08

0.40

7.00

o.qc

Dextrose

O.Q7

o-oo

o>8i

O-OO

o»oo

o.c8

O-OO

1.04

Levulose . • .

2-QQ

6-44

O.QQ

4,-lQ

J.cn

I-4I

3-46

2. 1 1

^•16

Maltose

1.18

0-60

sj.qo

I>21

0.62

2»25

1.86

2« TI

1-28

Total amount of sugar

9-79

17-18

9-55

11-74

13.21

13.64

9-39

12.15

9-73

Expt. I, a. Leaves analysed at 5 a.m.

I,b. Leaves cut off at 5 a.m. ; examined after 12 hours of assimilation.

I, c. Leaves left on the plant and examined after 12 hours of assimilation.
Expt. II. Leaves left on the plant : (a) analysed at 9 a.m. ; (b) at 4 p.m.
Expts. Ill and IV. Leaves cut off : (a) analysed in the forenoon ; (b) 24 hours later — after
they had been kept meanwhile in darkness.

These experiments are of the greatest interest, because they form the most
detailed quantitative estimate as yet made and because they form a basis for
further inquiry; their significance is not, however, so obvious. The carbo-
hydrates, which occur in the leaf, group themselves obviously into two series
possessing quite distinct characters : —

1. Those which increase in a marked degree during assimilation in light
(I and II), viz. cane sugar, maltose, starch (di- and polysaccharides).

2. Those which increase in darkness (III and IV), viz. dextrose, levulose
(monosaccharides).

This result, at first sight, appears somewhat surprising, since one would
expect that the simpler sugars (dextrose and levulose) would appear first in
the course of assimilation. Doubtless that is the case, but they become at
once further altered and in consequence do not accumulate at all. The con-
version takes place in at least three ways ; in the first place these hexoses
migrate out in very large quantities (compare I, c. with I, b.), next they become
respired, and, lastly, from them arise cane sugar and starch. In addition to
the construction ot complicated carbohydrates a counter-formation of hexoses
also takes place, and this preponderates in the dark (III and IV). If we assume
that the dextrose and levulose are the first products of assimilation, then we
must consider cane sugar and starch as reserve substances, which are temporarily
stored by the assimilating cells, because the translocation of the assimilatory
products does not take place so rapidly as their manufacture. Many of the
features indicated in the tables are as yet inexplicable, and into the details of these
we need not go ; possibly, further research may afford an explanation of them.
As to the relations subsisting between the di- and polysaccharides, BROWN
and MORRIS have advanced the view that the starch arises from the cane sugar
and the maltose from the starch. This view can scarcely be held as correct,
and, moreover, it leaves unexplained many observed facts.

The assumption made by these authors that the whole of the carbon passes
over, in the first instance, at once into cane sugar is, at least, not proved ; doubt-
less this sugar is rapidly removed elsewhere. There is also the possibility
that the carbon, during assimilation, is at once united with nitrogen and that
the carbohydrate is formed by subsequent dissociation of this compound.
A. MEYER (1885) held that the construction of proteids during the process of
assimilation was not unlikely, but he was unable to support his view by any
conclusive evidence. SAPOSCHNIKOFF (1895) has dealt with this question in
greater detail. He determined the proportion of the dry substance formed during

i 2

n6 METABOLISM

assimilation which was carbohydrate in its nature, and found (1890) it to be,
in round numbers (on an average of various experiments), 68 per cent., 87 per
cent., and 64 per cent. He imagined that the remainder in each case, i. e. 32 per
cent., 13 per cent., and 36 per cent., was proteid, and later on (1895) endeavoured
to prove it. He also investigated the increase in carbohydrates and proteids
during light in isolated leaves. An active construction of proteid could be deter-
mined especially when the leaves were immersed in nutritive solutions con-
taining nitrogen. Further, a diminution in the light retarded the rate of
formation of carbohydrates but relatively accelerated that of proteid. SAPOSCH-
NIKOFF, notwithstanding his anxiety to reach such a conclusion, was, how-
ever, unable, from these and other experiments, to establish the view that
proteid was the ' first formed reserve of carbon-assimilation'; on the contrary,
everything was in harmony with the conclusion that proteid was a secondary
product constructed from carbohydrates.

The results which have been quoted as to the quantity of carbon assimi-
lated are in accordance with those arrived at by the use of SACHS'S half -leaf
method or approximate closely to them. Although the errors in determining the
area of the leaf surface be reduced to a minimum, still the inequalities in thick-
ness of otherwise similar portions of the leaf would account for the inaccuracies
in the amounts ; for this reason one is limited to the use of a single leaf, because
a comparison of surface areas taken from various leaves is bound to lead to errors.
It is of interest therefore to draw attention to another method of determining
the amount of the products of assimilation which we owe to KREUSSLER
(1885-90), and which is certainly much the most accurate. He estimated
how much carbon-dioxide was withdrawn by the leaf from the air, an amount
quite independent of contingencies in the leaf structure. KREUSSLER investi-
gated isolated twigs in appropriate bell- jars through which was made to pass
a known quantity of air containing a known percentage of carbon-dioxide. He
next determined how much of this carbon-dioxide passed out of the apparatus,
and at the same time worked out how much carbon-dioxide was produced in
the same period as a result of respiration. In this way the amount of carbon-
dioxide actually absorbed by the plant could be accurately determined. Since
KREUSSLER worked with air containing relatively high percentages of carbon-
dioxide, and since the material experimented on was illuminated with electric
light, his results furnish us with no information with reference to the amount
of carbon-dioxide decomposed under normal conditions of assimilation. We
need not enlarge on these results at the present moment, since they will have
to be considered in another relation presently. BROWN (1899) has employed
this method in a modified form. He worked with ordinary atmospheric air,
whose percentage of carbon-dioxide was accurately determined, and further,
employed ordinary daylight. BROWN also used in his bell- jars only single leaves,
which, however, remained attached to the plant, so more easily guarding
against withering than was possible with severed branches. Certain valuable
data may be extracted from the preliminary account he has given of his re-
markable experiments. A sunflower leaf, with a surface area of 617-5 sq. cm.
absorbed in diffuse light 139-95 ccm. of CO2 in five and a half hours, or 412 ccm.
of carbon-dioxide per sq. m. per hour. Since one may reckon that 784 ccm. of
carbon-dioxide on an average are required to construct one gram of carbohy-
drate it will be seen that, in the case cited, 0-55 gr. of carbohydrate was manu-
factured per sq. m., and this number corresponds pretty nearly with what
BROWN found by weighing; the fact that it is considerably less than the
.amount found by SACHS is explained by the feebler intensity of light employed.

[In the preliminary account of their experiments, BROWN and ESCOMBE
(1905, Proc. Roy. Soc. B. 76, 29) give only 0-4-0-5 gr. of carbohydrate per sq. m.
per hour for Helianthus annuus. They draw attention to the difference between

THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. I 117

their results and those of SACHS, and have found that the increase in weight in 1
a leaf is much greater than one would expect from the amount of carbohydrate 's
formed. They draw no conclusion from this (as one might well expect) as to the
formation of compounds containing little carbon, but hold that SACHS'S half-
leaf method was a very inexact one.]

Since in SACHS'S experiments a square metre of leaf surface of the sunflower
produced 1-8 gr. of carbohydrate, it must have absorbed, roughly speaking,
1-5 litres of carbon-dioxide per hour. This would appear at first sight unlikely,
taking into account the small amount of carbon-dioxide in the air as well as
the capacity for absorption of the plant. We shall deal in our next lecture in
detail with these relations.

Bibliography to Lecture IX.

AUBERT. 1891. Revue gen. de Bot. 3, 97.

BAYER. 1870. Ber. d. chem. Gesell. 1870, 68.

BEIJERINCK. 1890. Bot. Ztg. 48, 741.

BEIJERINCK. 1901. Kon. Akad. v. Wetensch. Amsterdam, 45.

BOHM. 1883. Bot. Ztg. 41, 33.

BOKORNY. 1897. Biol. Cbl. 17, i. (The older literature as to starch formation
from various organic substances is given in this treatise.)

BONNIER and MANGIN. 1886. Annal. sc. nat., Bot. vn, 3, i.

BOUSSINGAULT. 1 868. Agronomic, 4, 300.

BROWN and MORRIS. 1893. Journal of the Chem. Soc. 63 (Transact.), 604.

BROWN. 1899. British Assoc. Address to the Chem. Section. Dover.

BUTLEROW. 1 86 1. Compt. rend. 53, 145.

CURTIUS and REINKE. 1897. Ber. d. bot. Gesell. 15, 201.

CZAPEK. 1902. Ber. d. bot. Gesell. 20, (44).

ENGELMANN, W. 1894. Pfliiger's Archiv, 57, 375.

ERLENMEYER. 1877. Ber. d. chem. Gesell. 10, 634.

EWART. 1897. Jour. Linn. Soc., Bot. 31, 554.

EWART. 1898. Bot. Centrbl. 75, 33.

FISCHER, E. 1894. Ber. d. chem. Gesell. 27, 3231.

FRIEDEL. 1901. Compt. rend. 132, 1131.

GODLEWSKI. 1877. Flora, 60, 218.

HERTZOG. 1902. Zeitschr. f. phys. Chem. 35, 459.

HOPPE. 1879. Zeitschr. f. physiol. Chem. 2, 425.

JUST. 1879. Wollny's Forschungen, 5, 79.

KLEBS. 1888. Unters. bot. Instit. Tubingen, 2, 489.

KNY. 1897. Ber. d. bot. Gesell. 15, 388.

KNY. 1898. Bot. Centrbl. 73, 426.

KRAUS. 1869. Jahrb. f. wiss. Bot. 12, 288.

KRAUS. 1872. Zur Kenntnis der Chlorophyllfarbstoffe. Stuttgart.

KREUSSLER. 1885-90. Landw. Jahrb, 14, 913; 16, 161 ; 16, 711; 19, 649.

LAURENT. 1887. Bull. Soc. bot. Belg. 26, 243.

LOEW. 1886. Journal f. prakt. Chemie, 33, 321 ; 34, 51 (comp. Bot. Ztg. 1886,
44, 849).

MACCHIATI. 1903. Revue gen. de Bot. 15, 20.

MEYER. 1885. Bot. Ztg. 43, 417.

MEYER. 1886. Ibid. 44, 81.

PFEFFER. 1881. Pflanzenphysiologie, ist ed., i, 185.

POLLACCI. 1900. Atti d. Istit. Bot. Pavia, N.S. 6, 45.

POLLACCI. 1902. Atti d. Istit. Bot. Pavia, 8.

REINKE. 1883. Ber. d. bot. Gesell. i, 406.

REINKE and BRAUNMULLER. 1899. Ber. d. bot. Gesell. 17, 7.

SACHS. 1884. Arb. bot. Inst. Wiirzburg, 3, i.

SAPOSCHNIKOFF. 1890. Ber. d. bot. Gesell. 8, 233.

SAPOSCHNIKOFF. 1895. B°t. Centrbl. 63, 246 (Review of the Russian paper pub-
lished in 1894).

SCHIMPER, A. F. W. 1885. Bot. Ztg. 43, 737.

SCHWARZ, F. 1 88 1. Unters. bot. Inst. Tubingen, i, 97.

TREBOUX. 1903. Flora, 92, 49.

ZIMMERMANN. 1893. Beitr. z. Morph. u. Phys. i, 29.

n8 METABOLISM

LECTURE X
THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. II

LET us summarize briefly what we have learned up to the present on the
subject of the assimilation of carbon : — the green plant decomposes the carbon-
dioxide present in the air or water, evolves oxygen, and constructs carbo-
hydrate. This process can take place only -with the assistance of sunlight and
in the chlorophyll corpuscles. We will endeavour in the present lecture to
extend our knowledge of this phenomenon, more especially fixing our attention
on the manner in which external conditions affect carbon assimilation in the
plant, directly and indirectly. We shall take up the thread of our story where
we left off in the preceding lecture and study first the amount of carbon-dioxide
present in the air.

The amount of this gas present in the air has been very frequently and
very accurately measured, and it has been found that it varies within very
narrow limits, indeed it might be said to be a constant quantity so long as we
avoid selecting for analysis the layers of the atmosphere immediately overlying
the soil. BROWN'S (1899 [and 1905]) latest researches may be quoted in this
relation, and he found on an average 2-8 parts of carbon-dioxide in 10,000 of air.
This number agrees with the previous results recorded, e. g. by REISET, who
obtained 2 -9, and those of the Montsouris Laboratory, where a number of indepen-
dent observations gave, on an average, a proportion of 3 parts per 10,000. We
will not enter into a discussion of the variations in the amount of carbon-dioxide
in the air, for these are of no importance so far as the vitality of the plant is
concerned. We have already referred (p. 103) to the question as to whether the
carbon-dioxide of the air is necessary for the life of the green plant, and whether
it cannot also draw upon the stores of carbon-dioxide present in the soil. That
question was answered in the negative, still this is the proper place to refer
again to the experiments which have been adduced in proof of this fact. It
has been asserted by various investigators, most emphatically by UNGER (1855),
that the carbon-dioxide of the air was insufficient alone, and that that present in
the soil must also form part of the supply. In opposition to this view, MOLL
(1877) was able to show that a plant which was prevented from obtaining
carbon-dioxide save by its root, never succeeded in forming starch in its leaves,
and manifestly suffered from want of the gas. It is also obvious that since
the path from the root to the leaf is a long one, the transference must be slow,
and that most of the carbon-dioxide will be abstracted on the way up by the
chlorophyll bodies in the cortex of the stem.

If, however, the carbon-dioxide of the air be the source of carbon to the
green plant, the question comes to be how is it possible that the amount in the
air remains approximately constant, notwithstanding the fact that plant
activity tends continually to diminish it. As a matter of fact, the amount of
carbon-dioxide which the plant world abstracts from the air is very considerable,
as the following statement will show. According to SACHS (1884), a sunflower
with a leaf surface of about 1-5 sq. m. increases in dry weight to the extent of
about 36 g. per diem (p. 114), and since 1-5 g. of carbon-dioxide in round numbers
is produced from i g. of dry weight, it must extract about 50 g. from the air
daily, or about 1-5 kg. in a month. If we assume the amount to be only i kg.,
taking into account possibilities of unfavourable weather, and imagine the
whole surface of the earth covered with sunflowers, one to each square metre
or one million to the square kilometre, then the sunflowers existing on the
135 millions of square kilometres of land surface would consume 135 billion kg.
of carbon-dioxide in one month ; and since, according to the usual estimate,
there are 2,500 billion kg. of carbon-dioxide in the atmosphere, the supply

THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. II n9

would last about twenty months. This is, perhaps, an exaggeration, since
according to EBERMEYER'S (1885) reckoning, one hectare of forest uses up
11,000 kg. of CO2, or one square kilometre uses up i-i million kg. in a year,
whilst we have been making our estimates on the basis of sunflowers using
i million kg. per month. This discrepancy, so far as our calculation is concerned,
is of little consequence since all we want to demonstrate is that the supply of
carbon-dioxide in the air is relatively small and must, in the course of a few
months or years, be entirely used up by the activity of green plants. Since,
as a matter of fact, no such decrease has, according to the most exact analyses,
been observed, there must therefore be some processes taking place on the earth
whereby carbon-dioxide is produced in quantity sufficient to replace what is
used up.

As to the nature of these processes, on which the existence of organisms
on the earth depends, a word or two of explanation must for the moment
suffice. We are acquainted with several sources of carbon-dioxide. In in-
organic nature volcanoes and springs bring up quantities of carbon-dioxide
from day to day from the deeper layers of the earth's crust, and in the
organic world the production of carbon-dioxide in animal respiration is a
fact sufficiently well known. No estimate can, however, be made as to the
total quantity so produced, for it is only in the case of the human organism
that the necessary data are available. These data (PFEFFER, Physiol. I, 279),
however, tell us that mankind produces daily 1,200 million kg. of carbon-
dioxide, or 0-438 billions yearly, i. e. about B£00 of the total amount in the air.
Man, again, adds largely to this amount by the combustion of coal and wood,
&c. ; according to NOLL (1894, 166), Krupp's works alone give off daily into
the atmosphere 2j million kg. of carbon in the form of CO2. Then to this
we must add the results of the respiratory activity of the plant world. Although
we are unable to calculate the total amount of carbon-dioxide produced and
used up over the earth, we may still conceive the possibility of a balance
existing between them. Further, we can easily see how, owing to the con-
tinual movement of the air, a uniform distribution of carbon-dioxide in the
atmosphere must be effected, so that one finds everywhere a percentage in
round numbers of 0-03 per cent, present.

This all-important carbon-dioxide is not so uniformly distributed in water.
It is well known that the absorption of a gas by water depends on its partial
pressure and on temperature. Very different quantities will dissolve in water
according as it absorbs the carbon-dioxide from the atmosphere or from the
air in the soil. Further, the effect of temperature is so great that twice as much
carbon-dioxide is absorbed at o° C. as at 20° C. In addition to the amount of
carbon-dioxide dissolved, chemically combined carbonic acid in the water is
also available. Currents of water bring about rapid readjustment of the
differences in the amount of carbon-dioxide present in different places.

Experience teaches us that the extremely small quantity of carbon-dioxide
occurring in nature is no impediment to active assimilation in, and healthy life of,
plants. Experiment, however, has also shown that an increase of carbon-dioxide
in the air is accompanied by an increase in the products of assimilation. It has
already been pointed out that there are plants which, although unable to manu-
facture starch in an ordinary atmosphere, can do so in one richer in carbon-
dioxide, and that the occurrence of starch in them is the result of an increase in
the amount of sugar constructed. We owe our knowledge of these facts to GOD-
LEWSKI'S (1873) and KREUSSLER'S elaborate experiments. If we represent the
normal amount of carbon-dioxide in the air by i, and the amount of assimilation
carried out under these circumstances by 100, then according to KREUSSLER : —

Proportion of carbon-dioxide . i 23-57 17 35 220 440

Amount of assimilation . . 100 127 185 196 209 237 230 266 (?;.

120 METABOLISM

It will be observed that the rate of assimilation increases markedly from the
very commencement, and that when the percentage of carbon-dioxide is thirty-
five times the normal (i. e. about I per cent.) an optimum condition is reached.
GODLEWSKI'S (1873) figures, frequently at all events, show more strikingly than
those of KREUSSLER (whose last figure is doubtful) an obvious decrease after
the optimum is reached. The optimum, however, is much higher in GOD-
LEWSKI'S results than in those of KREUSSLER.

Glyceria spectabilis (Expt. 15).

Carbon-dioxide Carbon-dioxide

in air. decomposed.

3-i % 2-10

7-o % 4-73

10 4 % 5-75

13-9 % 2.27

Nerium (Expt 24).

Carbon-dioxide Carbon-dioxide

in air. decomposed.

3-6 % 4-31

13-2 % 3-62

l8-5 % 3-23

28.2 % 2.42

[A research by PANTANELLI (1904) explains to some extent why different
investigators have arrived at such varied optima for the proportion of carbon-
dioxide present in the air. PANTANELLI shows that it really varies with the
intensity of light ; it is not apparent, however, why o-i per cent, of carbon-
dioxide in the air should be so injurious to plants, as had been affirmed to be
the case by BROWN and ESCOMBE (1902).]

It is not surprising that greater concentrations of carbon-dioxide should
have an unfavourable influence on assimilation, since all vital processes are
inhibited by that gas (LOPRIORE, 1895).

Let us turn now to the other question suggested at the end of the previous
lecture, viz. how does the carbon-dioxide reach the assimilating cells of the
leaf ? Owing to the different external conditions under which aquatic plants
live, the method adopted in their case cannot be the same as that existing
in land plants. In the former case the carbon-dioxide dissolved in water
must pass through the epidermal cells and may reach the other tissues either
while still in the dissolved state, or it may escape as a free gas into the inter-
cellular spaces, and by their means be distributed through the plant. The
cuticle of aquatic plants presents no more impediment to the through passage
of dissolved carbon-dioxide than it does to water, the medium of solution.
The conditions are quite otherwise in the case of the cuticle of land plants,
more especially that of the leaves. Carbon-dioxide gas is capable of penetrating
such cuticles, even although they be quite impermeable to water, just as carbon-
dioxide can diffuse through a layer of oil though water cannot. All the same,
the partial pressure of carbon-dioxide in the atmosphere is so small that the
amount which is able to pass through the cuticle is extraordinarily minute.
Were there no other means of entrance available than by diffusion through the
cuticle, practically no assimilation could take place under ordinary conditions.
On the other hand, BOUSSINGAULT (1868) and BLACKMAN (1895) established the
fact that formation of starch took place in such leaves as had obtained their
necessary supplies only by way of the cuticle from air rich in carbon-dioxide.

It follows from this that in nature all highly organized leaves must be
provided with carbon-dioxide in some other manner. The stomata at once
suggest themselves as the portals for entrance of carbon-dioxide. Once the gas
has reached the intercellular spaces by their means, it can easily penetrate into
the individual cells after it has first become dissolved in the water of imbibition
present in the cell- walls. The amount of carbon-assimilation will thus depend
upon the number and distribution of the stomata and on the size of their
openings. STAHL (1894) and MEISSNER (1894) have demonstrated this conclu-
sively, for if a layer of vaseline be spread over leaves that bear stomata only on
their undersides, any formation of starch in the leaves is prevented. At low
temperatures leaves may be treated in this way without suffering any injury.

THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. II 121

If we create a number of artificial stomata on the upper side of a leaf whose
underside has been vaselined, by pricking it with a needle, or cutting it with
a knife, or by removing altogether a strip of cuticle or the whole epidermis, the
formation of starch takes place in the neighbourhood of such artificial aper-
tures. It is easily understood why such starch manufacture is entirely local,
seeing that the cells immediately underlying the gaps at once use up the small
supply of carbon-dioxide in the air. MOLL'S (1877) experiments, which have
been briefly referred to above, also show that starch formation is always limited
to those regions in the leaf which have direct access to the carbon-dioxide, and
it is on that account that the carbon-dioxide absorbed along with water from
the soil is of no importance so far as the leaves are concerned.

The intercellular spaces, whose exits the stomata may be considered as
constituting, are of the utmost importance for the transference of carbon-
dioxide to the individual chlorophylliferous cells. Every one of these cells
abuts somewhere directly on an intercellular space and is thus in continuity
with the external air. Movement of carbon-dioxide in the intercellular spaces
is effected, at least in the first instance, by diffusion. The entry into the leaf
through the stoma depends on the partial pressure of carbon-dioxide in the
air ; on reaching the assimilating cells the gas is completely absorbed and the
pressure is reduced to nil, so that the necessary conditions for continuous
diffusion are complied with. It cannot be doubted, however, that, in addition
to diffusion, movements of the gases in the intercellular spaces in mass, resulting
in a rapid supply of carbon-dioxide to the mesophyll, also take place. Varia-
tions in temperature as well as bending of the plant due to wind, resulting in
changes in the shape of the intercellular spaces, must assist in producing such
air currents.

Let us first of all consider stomata wide open and ask ourselves how it is
possible for carbon-dioxide, though present in such a small proportion in the air,
to enter the leaf through such minute pores in quantities so great that a sunflower
is able to manufacture i-8g. of carbohydrate per square metre of surface per hour.
A complete answer to this question is given in the recent researches of BROWN
and ESCOMBE (1900). These authors started from a consideration of purely
physical experiments. They allowed the carbon-dioxide of the air to diffuse
into a vessel through a narrow opening in a thin diaphragm, placing a solution
of potash at the bottom of the vessel. They found that the amount which
diffused through was proportional, not to the area of the opening, but to its
diameter. Thus, if an opening 4 mm. in diameter permitted two units of carbon-
dioxide to pass through in a unit of time, one unit of the gas passed through
an opening 2 mm. in diameter in the same time ; in other words, the amount of
carbon-dioxide stood in the proportion of 2 to I, while the surface extent of the
aperture bore the relation of 4 to I ; hence as the size of the opening decreases
the rate of diffusion must increase. When, therefore, numerous apertures are
made in the diaphragm, and when the activities of these are added together cases
must be conceivable where diffusion takes place through a partition pierced by
minute apertures as rapidly as if there were no wall there at all. BROWN discovered
that this result was reached if the individual openings lay so far distant from
each other that they were unable to influence each other's activity, and that
that was when their distance apart was at least ten times the diameter of the
opening. When we apply the results arrived at from a consideration of such
physical experiments to the inflow of carbon-dioxide into the leaf, the first thing
to notice is that the stomata possess not a circular but an elliptical form. If we
were now to consider the long diameter of the ellipse as the measurement on which
diffusion directly depends it might be said that the width of the opening plays
no part in the process, a view, as we shall see, directly opposed to observations.
According to the detailed statements of STEPHAN the facts point to an entirely

122 METABOLISM

different conclusion ; an elliptical opening behaves exactly in the same way
with regard to diffusion as a circular opening of the same area, each non-circular
opening must be first of all reduced to a surface area of circular outline and the
diameter of the latter taken as the figure on which diffusion depends. Thus
BROWN and ESCOMBE estimated the effective size of the opening between the
guard-cells of the stoma of Helianthm at 0-0000908 sq. mm., corresponding to
a circular area of 0-0107 mm- m diameter. The distance of the stomata apart is
equal to about eight times their diameter ; they may thus interfere with each
other, but only slightly. If we assume that the absorption of carbon-dioxide
by the mesophyll is complete, we may conclude from the number of the stomata
that 2-095 ccm. could be absorbed per sq. cm. every hour. As a matter of fact,
the leaf absorbs only 0-134 ccm- Per S<1- cm-> m order to construct a maximum
of 1-8 g. of carbohydrate per sq. m., that is to say, only about 6 per cent, of
the amount theoretically possible. From this it follows that the carbon-
dioxide can only be absorbed slowly — for it must first pass through the cell- walls
— so that the partial pressure a short distance below the stoma is far from
being reduced to zero, while in physical experiments this condition is rapidly
attained. ' The structure of a typical herbaceous leaf ', says BROWN, ' is an
admirable example of adaptation to natural laws, which in this particular in-
stance have only recently become known to us. It illustrates in a striking
manner all the physical properties of the multiperforate diaphragm, which,
with its minute apertures, set at from six to eight diameters apart, and repre-
senting only i per cent, to 3 per cent, of free area, yet allows a perfectly free
interchange of gases on its two sides, whilst at the same time affording every
protection to the delicate structures underlying it.'

The stomata, however, are by no means always wide open, the size of the
opening, on the contrary, varies with external conditions as we pointed out
when discussing transpiration Here, as on the previous occasion, we will
content ourselves with referring to the two most important factors in the case,
viz. light and atmospheric moisture. Since the maximum opening of stomata
occurs in bright light we must recognize that as an adaptation of funda-
mental importance in the process of carbon assimilation. Increase of assimila-
tion accompanies increase in light intensity (p. 125), provided that carbon-dioxide
be present in sufficient quantity. An increase of atmospheric moisture brings
about opening, a decrease a closing of the stomata, as has been already shown.
Often long before a visible wilting of the leaf has taken place a complete closure
of the stomatal slit is effected, as a result of the arrangement of the guard-cells
already described. This closure of the stoma is essential to the preservation of
life, otherwise withering must at once result ; it is a safeguard against excessive
transpiration, although at the same time a disadvantage so far as assimilation is
concerned. It has been demonstrated by many experiments to how great an extent
assimilation depends on the amount of water in the leaves ; thus KREUSSLER
(1885) observed that isolated branches, when brightly illuminated, showed a
rapid decrease in the power of assimilation, while, according to NAGAMATZ (1887),
withered leaves lose it altogether. In this case the closure of the slit is alone
responsible for the deficiency in carbon-dioxide, and it must not be thought that
assimilation depends on the existence of a certain degree of osmotic pressure in
the interior of the cells. Assimilation is not lowered owing to loss of water in the
case of plants such as mosses and lichens, which absorb carbon-dioxide by means
of their cell- walls, so much as it is in the case of foliage leaves (BASTIT, 1891, 522 ;
JUMELLE, 1892, 166), and in Algae assimilation has been observed to continue
even after the beginning of plasmolysis (KLEBS, 1888). It must not, on the other
hand, be affirmed that the amount of water present in the cell as such is a matter
of no moment in the process of assimilation, for JUMELLE'S researches show that
even in lichens assimilation is undoubtedly dependent on the amount of water
present.

THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. II 123

We have thus learned that stomata constitute an apparatus of the very
greatest importance for assimilative purposes, an apparatus whose functional
activity is dependent in many respects on external factors. This dependence
on the environment is still further increased by the fact that external factors
affect the development of stomata. It is sufficient to note that stomata are not
developed in darkness. The same is true of chlorophyll, whose first appearance
and whose functional activity later is in the highest degree dependent on external
conditions. Only in certain Algae (ScHiMPER, 1885) and in germinating Coni-
ferae (BURGERSTEIN, 1900) is chlorophyll known to develop in darkness, but
in all higher plants the chromatophores develop yellow colouring matter only,
which has no effect on carbon-dioxide. Moreover chlorophyll decomposes in
darkness, sometimes slowly, at other times quickly, in a word, both the origin
and continued existence of chlorophyll depend on light. Further, a short ex-
posure to light of feeble intensity is sufficient to induce a development of the
green colour in the chloroplast, although subsequently placed in darkness.
Again, greening is induced with varying rapidity, although, by all visible rays of
the spectrum, and not by light of any definite wave length (REINKE, 1893).
The temperature necessary for the formation of chlorophyll must not be too
low ; between o° and 5° C., light is able to induce an increase only in the yellow
colouring-matter already present (ELFVING, 1880), and even seedlings of Coni-
ferae become green in darkness, as a general rule, only if the temperature be
over 9° C.

Even when the chloroplasts develop normally, they cease acting as assimi-
lative agents under conditions which scarcely affect many of the other func-
tions of the cell, and which, at all events, act injuriously on life only
after prolonged application. Thus the activity of the chloroplasts may
be inhibited by extremes of temperature, by high percentages of carbon-
dioxide, by anaesthetics and antipyretics, acids and alkalis [compare PAN-
TANELLI, 1904], and also by prolonged insolation or accumulation of the pro-
ducts of assimilation (EWART, 1896). [According to ARNO MULLER (1904), the
accumulation of the products of assimilation in leaves containing sugar
retards the decomposition of carbon-dioxide.] At the same time respiration con-
tinues quietly, the chloroplasts appear unchanged and regain their powers of
assimilation some time after normal conditions are reinstated. The fact that
the chlorophyll persists unaltered during these interferences with its functions,
shows that it is the protoplasmic framework that is affected. Both the com-
ponent parts of the chloroplast must work in harmony if it is to carry out its
normal functions (compare p. 109). Temporary inactivity of the chloroplasts
may be unintentionally induced in the course of experiment — for example, it
may take place very easily in amputated leaves owing to the accumulation of
assimilation products — and in consequence, experiments with isolated leaves do
not furnish us with accurate estimates of the amount of carbohydrates arising
in the normal leaf (compare p. 114).

We have as yet spoken only of certain external factors which because they
influence the stomata and chlorophyll, either as regards their structure or func-
tional activity, are also of importance in the assimilation process. We must
now consider external factors in so far as they affect assimilation directly,
but it is obvious that no hard and fast line can be drawn between such
external conditions as influence this process directly and those which influence
it indirectly. One and the same factor may act in both ways. In fact, carbon-
dioxide can, for example, 'when in greater concentration, inhibit the action
of the chlorophyll, and thus can induce a decrease in assimilative activity.,
instead of causing the increase which we would expect on physical grounds.
Since we have already treated of the influence of carbon-dioxide on assimilation,
more especially in relation to its concentration, we have only to add here that

124 METABOLISM

it cannot be replaced by any other compound of carbon, least of all by carbon-
monoxide.

Along with carbon-dioxide we must study the oxygen. There are many facts
which prove to us that, at the commencement of assimilation, no traces of oxygen
are observable, and this is the more remarkable inasmuch as practically all the
vital processes in the green plant are dependent on the presence of this gas.
In the experiments on assimilation referred to above (p. 105) it was noted that
free oxygen was definitely absent in reduced haemoglobin, and yet that the
decomposition of carbon-dioxide could commence in such a medium ; when
decomposition does commence the experiment comes to an end, since oxygen
is at once produced. We learn from E WART'S (1897) investigations, however,
that certain plant -colouring matters are capable of combining loosely with
oxygen, and we are led to believe that this capacity is more widely distributed
than is generally assumed, and that, in experiments such as those quoted, it is
not free oxygen but oxygen in loose combination that is at the disposal of the
plant. At all events the plant's power of bringing about assimilation ceases in
the long run in absence of oxygen ; we have still, however, to decide whether
this takes place when the loosely attached oxygen is used up or when the
chloroplasts become inactive.

Heat, which has such a fundamental influence on plant life in general,
affects assimilation also to a great extent. The determination of the dependence
of assimilation on temperature from a quantitative point of view is by no
means easy, because, in addition to the construction of assimilation products,
a destruction of these same bodies, owing to respiration, is always going on at
the same time, and because this latter process is dependent on temperature in
other ways than the former. The most accurate experiments in this relation are
those of KREUSSLER (1890). His researches show first of all that a gradual
increase in assimilation is concomitant with a rise of temperature ; the maximum
assimilative activity is obtained somewhere between 25° and 40° C., followed by
a decrease as higher temperatures are reached. Since assimilation soon ceases
at temperatures above 45 °C., while respiration increases with a further rise of
temperature to over 50° C., there must be a certain degree of temperature at
which the two processes neutralize each other. The following table summarizes
KREUSSLER' s (1890) chief results on this subject : —

Ricinus.
Temperature. 25° 40° 45° 50° 60°

a. mg. of COa formed per

hour in the dark owing

to respiration . . . 8'5 15-0 14-8 16-4 0-75

b. mg. of CO? decomposed

per hour in light . . 35.9 29-0 16-1 —29-9 —
c . a + b = mg. of CO2 assi-
milated per hour in

44-4 44-o 30-9 —13-5 —

Prunus laurocerasus.
25° 40° 45° 50° 60°

7.7 19.9 21-7 21.3 2-5
20-9 4x-7 3-1 —337 —

28-6 61-6 24.8 — 12-4 —

Looking at line c it will be noticed that the energy of assimilation in Ricinus
is still as great at 40° as at 25° ; at 45° it has sensibly decreased, and must
soon after reach zero. In the case of Prunus it is greater at 40° than at 25°,
but falls rapidly with increase of temperature, soon becoming nil above 45°. At
all events, at 50° in light, in both cases, only carbon-dioxide is formed.

Line b shows the net profit or loss of the gaseous exchange occurring in light ;
Ricinus, in spite of concomitant assimilation, shows a smaller gain in carbon
at 40° than at 25°, while in Prunus the conditions are reversed ; at 45° the gain
in carbon in Prunus is reduced to a minimum, and, beyond that, naturally
entirely ceases, indeed there is a loss instead.

KREUSSLER (1888) has also investigated the lower limits of temperature.
He found feeble but still recognizable assimilation taking place in bramble

THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. II 125

leaves at —2-4° C. A similar result was obtained by JUMELLE (1892) in the case
of other plants at quite low temperatures ( — 30° to — 40° C.). According to
EWART (1896), however, such low temperatures must speedily cause the chloro-
plasts to become inactive. [The dependence of the decomposition of carbon-
dioxide on temperature has been investigated very accurately by MATTHAEI
(1904). This author finds that the minimum lies near - 6°, the optimum about
37°, and the maximum about 43° ; the rise to the optimum is very rapid, and
the fall beyond that point still more sudden.]

Although we have left the consideration of light to the last, it is not because
it is of little consequence ; on the contrary, it is of the utmost importance,
and may be best discussed after the other influential factors have been con-
sidered. The fact that assimilation is intimately dependent on the presence of
light may be proved with perfect ease, and branches of Elodea lend themselves
readily to such experimental treatment (p. no). [Very often the method is
employed of partially covering a foliage leaf with pieces of cardboard, when the
necessity for light in carbon-dioxide decomposition is shown remarkably clearly
by the non-formation of starch in the darkened regions. HAUG (Bot. Gaz.
1903) has recently advanced some objections to this mode of experiment.] The
evolution of bubbles of gas which takes place with a certain rapidity when Elodea
is placed in a bright window decreases markedly when we withdraw the plants
to the back of the room, and ceases entirely before what we should term ' dark-
ness ' ensues. The facts already referred to have not, however, been strictly
speaking completely substantiated. In each green cell, as we have already seen,
carbon-dioxide is formed during respiration as well as decomposed during assimi-
lation. Respiration, however, may be said to be quite independent of light,
indeed it goes on with equal activity in feeble light as in direct sunlight. There
must, therefore, be a certain intensity of light under which as much carbon-
dioxide is decomposed in the process of assimilation as is formed in the process
of respiration, and then, of course, no air-bubbles at all will escape from Elodea,
and even the specially sensitive bacterium method fails to show the existence
of any assimilation. It is only by quantitative chemical methods that assimi-
lation can be proved to occur, provided the amount of respiration in the
parts concerned is known. When the intensity of light is still further reduced
assimilation manifests itself only through a diminution of the amount of respira-
tion, and comes to an end entirely when respiration ceases to make itself apparent.

More exact estimates as to the minimum intensity of light which can still
bring about decomposition of carbon-dioxide are not forthcoming, and it is to
be expected that individual species will exhibit differences in this respect. It is
only too well known that most plants do not thrive in a room, and this is due
for the most part to the indifferent lighting of our houses. Since several plants,
such as Clivia and Aspidistra, do thrive in rooms, one might conclude from
that, that the minimum light intensity required is less in their case than in
that of others ; but it has been shown that in these, as in other shade-loving
plants, there is an entirely different reason for their power of endurance of weak
light, viz. their reduced respiratory activity, on account of which less organic
material is lost and there is less necessity for reconstruction.

Further we are not accurately acquainted with the precise way in which
assimilation is dependent on the intensity of light. All researches agree on
one point, viz. that assimilation of carbon is approximately proportional in
amount to the intensity of light ; it is questionable, however, whether this
is the rule with higher intensities. REINKE (1883) found that the amount
of assimilation was the same whether the plant was illuminated by direct sun-
light or whether a lens with a magnifying power of 60 was used. But, as
PFEFFER has noted (Physiol. ist ed. I, 209) it may be easily imagined that
a further increase in assimilation, following on increase of light, is impossible

126 METABOLISM

owing to the deficiency in carbon-dioxide. Carbon-dioxide may be present in
sufficient quantity under ordinary circumstances to employ all the energy of
sunlight, but when light is artificially increased the usual amount of carbon-
dioxide would constitute a sub-optimum. Further investigations may furnish
us with a graphic curve representing the relation existing between assimila-
tion and light intensity, similar to those which express graphically so many
physiological processes, and showing a descending curve with further increase
in light intensity, after a maximum point has been reached. So far such
descending curves as those obtained by REINKE have been established only
for lights of high intensity, which might inhibit the assimilative activity by
injuring the chloroplasts.

[PANTANELLI (1904) has shown that in Elodea the optimum decomposition
of carbon-dioxide takes place (when the water contains a normal percentage
of that gas) when the light intensity amounts to one-quarter of that of direct
sunlight, but as the light intensity is increased assimilation decreases. Doubt-
less plants with varying requirements so far as light is concerned (' shade-
plants ' and ' light-plants ') behave differently in this respect (compare WEISS,
Compt. rend. 1903). Attention has already been drawn (p. 120) to the fact
that PANTANELLI has shown that the optimum intensity of light is higher as
the amount of carbon-dioxide in the environment increases.]

Sunlight is known to be a mixture of rays of different refrangibility, of
different wave length and of different colour, and it was long ago a question
whether all these rays act in the same way or not. On this question much
has been written, but the results which have been arrived at are by no means
in proportion to the labour expended on them, on account of the difficulties
that had to be overcome — or more correctly, perhaps, that have yet to be
overcome, for no final answer to the problem has even yet been arrived at.
The first researches on the subject were those of DAUBENY (1836) ; this investi-
gator grew plants behind coloured glass plates, and this method is still of great
service for demonstration purposes, although to be rejected for exact research,
since the light so obtained is by no means monochromatic. We are unable to
make much further progress by using coloured solutions, since although the
light rays so filtered are virtually monochromatic (LANDOLT, 1894), still they
are necessarily very feeble and consequently of little use physiologically.
Hence in all recent research of an exact character the use of coloured media
has been quite subsidiary to that of light subdivided into its components
by means of a spectroscope. DRAPER (1843) was the first to put the solar
spectrum to this use, and PFEFFER (1872), REINKE (1884), ENGELMANN
(1884), TIMIRIASEF (1885), and others have made use of the same method.
It cannot be described, however, without qualification, as an exact method,
since the way in which the spectrum is produced involves the much wider dis-
persion of the more refrangible rays, so that equal areas in different regions
of the spectrum cannot be equally effective. On the other hand, in order to
increase the intensity of the light the slit of the apparatus must be opened so
widely that the spectrum becomes no longer a pure one. The first source of
error may be avoided by producing a normal spectrum with the aid of a dif-
fraction grating. Such experiments have not yet been carried out, though
REINKE (1884) has devised an apparatus by which comparable observations
may be made without the aid of a diffraction grating. The principle of this
' spectrophore * (Fig. 26) is as follows : —

Light from a heliostat is made to converge by means of a lens 0, and to
pass through the prism P, by which it is decomposed. The spectrum falls on
the screen SSr By means of two strips of wood (DDJ selected parts of the
spectrum may be blotted out, and the remainder of the rays are combined by
means of the lens S into a beam of light, which is made to play on the plant.

THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. II 127

This beam remains always in the same place, no matter how much and what
part of the spectrum is obliterated. If a scale be inserted at S5X, we can
measure on it the wave lengths by observing Fraunhofer's lines, and are able
to compare exactly equal parts of the spectrum with each other, by taking light
of wave length, A. = 700-640 /A/A in one experiment, \ = 640-580 /A/A in a second,
and \ = 580-520 /A/A in a third. The apparatus was later (Bot. Ztg. 1885) con-
siderably improved, but no further experimental results have been published
as a result of the use of the improved form.

In using the spectrum several modifications are adopted in individual cases.
The macroscopic spectrum may be employed with the gas-bubble method to carry
out eudiometric researches or estimate the amount of starch formed ; a micro-
scopic spectrum may also be thrown on the slide and the behaviour of an alga
may be studied by the bacterium method. Under certain circumstances, when
bacteria are introduced beneath the cover glass, aggregation of these at certain
regions of the spectrum gives an indication where the maximum activity occurs ;
more exact quantitative values may be obtained if one and the same object be
examined successively with light of dif-
ferent colours, and each time, by lessening
the slit of the spectroscope, a minimum
light intensity may be obtained, at which
movement of the bacteria may still be
observed. Naturally the most effective
light through the narrowest slit will still
suffice for assimilation, and vice versa.

A comparison of observations de-
rived from all the researches which
have been made brings out the follow-
ing points : —

1. Only light of wave length be-
tween 770 /A/A and 390 /A/A is Conducive Fig. 26. Diagrammatic sketch of a spectrophore (after
tn asQimilatincr artivitvin OTPPTI r>lar»t<s • REINKE, Bot. Ztg., 1884, pi. I, fig. 2). £7, projection

o assimilating activity in green plants , , p 'prism . ^ scale of A=|O to x=Vs ; A A,

these are approximately the Same rays diaphragm for cutting off extraneous rays; S, converg-

which are visible to us. ing lens ; A *°s*on of the plant «Pe«mented «»•

2. The assimilatory effect of different rays is unequal, but still not in such a
way that some only are active whilst others lying beyond these are quite inactive.

If we express the wave length by abscissae and the activity of assimilation
by ordinates, we obtain a curve which does not in the least correspond with
the curve expressing the energy of sunlight obtained by LANGLEY.

On the other hand, no unanimity has been arrived at as yet as to the form
of the curve, especially on the question as to whether there is one maximum or
two, nor as to the exact position of the first and generally recognizable maximum
point. This point lies in all cases somewhere in the red or yellow, the second
maximum appears to lie in the blue region of the spectrum (compare the curves
in Figs. 27 and 28). The importance of the second maximum is emphasized by
ENGELMANN, who discovered it by the bacterium method. Since PFEFFER (Phys.
I, 334, Fig. 53) could not convince himself of its existence by the use of the same
method, and since the reasons which KOHL (1897) has brought forward more
recently for its occurrence are by no means convincing, we must assume that
the curve of assimilation has only one apex, somewhat as REINKE (Fig. 27)
has described it, and that the maximum point lies without doubt in the less
refrangible part of the spectrum. This conclusion, at once unexpected and
important, was already drawn from a consideration of their own observations
by the older investigators. Since the strongly refrangible rays are chiefly con-
cerned in the decomposition of silver salts, e. g. in photography, it had been the
custom to regard these as the chemically active ones. The data which have

128

METABOLISM

been obtained with regard to carbon-dioxide assimilation, however, show that
this generalization is inadmissible, and that red and yellow rays are also able to
act energetically in a chemical manner.

These fundamental facts may be also demonstrated with the aid of light
which has passed through bichromate of potash on the one hand and ammoni-
acal copper-oxide on the other. In yellow light photographic paper is only
slowly blackened, but assimilation proceeds almost as rapidly under such
illumination as in white light ; on the other hand, light which has passed
through ammoniacal oxide of copper exerts a vigorous decomposition in silver
salts but has little or no effect in assimilation.

As to the position of the chief maximum point there is much dispute.
REINKE considers it to be between Fraunhofer's lines a and B (X — 720-685 /A/A),
ENGELMANN and TIMIRIASEF between B and C (A. = 685-655 /A/A), PFEFFER
(1871) between C and D (X = 655-590 /A/A). It ought to be easy to settle this
question, though it is really of minor importance. In addition to the difficulties

IS

7M '• xtf, '.&»
* B C D

B C

': !w»

e A

660 OZO JfO

Fig. 27. Curve of evolution of gas-bubbles,
compared with the absorption-spectrum of
a living leaf. (After REINKE, Bot. Ztg. 1884,
pi. I, fig. 6.)

Fig. 28. Assimilation (firm line) and
absorption (dotted line) in green cells be-
tween wave lengths A =420-750. (After
ENGELMANN, Bot. Ztg. 1884, pi. ll, fig. i.)

which have been already alluded to (e. g. the attainment of a sufficiently pure
spectrum with the necessary light intensity), one fact must be emphasized which
ENGELMANN has drawn attention to and which PFEFFER has more recently
(Physiol. 2nd ed. I. § 60) brought prominently forward. Chlorophyll has very
different powers of absorbing light of different colours. The deepest absorption
band of chlorophyll lies immediately in the vicinity of the assimilation maximum
(Fig. 24), i. e. near C (X = 661). Although this vigorously absorbed light has
also the greatest effect in assimilation, still it can only be fully effective on
structures of limited thickness. If we employ an ordinary leaf blade for experi-
ment, the uppermost layers containing chlorophyll will absorb all the light of
wave length about 660, and the layers lying below will be in darkness. Rays
outside this limited region, e. g. those whose A. = 630, will be much less absorbed
and will pass more deeply into the leaf, and will be able eventually to exert
greater assimilatory activity than those which owing to absorption are rapidly
altered. Theoretically the more interesting assimilating curve (primary curve)
is naturally that investigated by ENGELMANN, not that actually observed on
using thicker leaves. Even in layers of chlorophyll of limited thickness this
primary curve is masked; the following values which ENGELMANN obtained
from a comparison of the directly illuminated side and the reverse side of
a cell of Cladophora, only 0-028 mm. thick, shows this clearly : —

FRAUNHOFER'S lines.

B-C.

D.

D|b.

E-B.

F.

F|G.

Assimilation (upper side)
Assimilation (lower side)

100

36-5

48-5
94.0

37-o

1 00-0

24-0
52.0

36-5

22-0

I 0-0
12-0

THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. II 129

From these figures it is very evident that the maximum assimilation
in parts of greater thickness tends towards the regions of lesser wave length.

We come now to the second question, why it is of such special interest to
accurately determine the wave lengths of maximum efficiency. This is because
it has been frequently assumed that a connexion exists between absorption and
assimilation, that the maxima of absorption represented by the absorption
bands must at the same time represent the maxima of assimilation. We
have now to show that in the process of assimilation light must be absorbed, but
it by no means follows that this light is completely transformed locally.
Numerous colouring matters exhibit absorption spectra in the highest degree
characteristic, without necessitating in consequence the postulating of a special
function in the organism for the absorption bands these spectra exhibit. It is
well to remember that the colouring matter of the blood has a very remarkable
spectrum, but that it has no significance on that account in the animal economy.
Further, in the case of silver iodide (OsxwALD, General Chemistry, II, 1070) a
well-marked maximum light activity appears quite near line G, although at
the same place, optically, absorption is quite feeble. The chief argument
against the coincidence of absorption and assimilation is based on the physio-
logical facts themselves, and it appears to us chiefly to lie in this, that attempts
have always been made to establish a coincidence only between the position of
the maximum of assimilation in the red and the absorption band near B-C,
perhaps (ENGELMANN) also a corresponding coincidence in the blue, but it has
never been possible to show any rise of the assimilation curve near the other
absorption bands of chlorophyll. Let us examine ENGELMANN'S table (1884,
p. 91), wherein he shows the connexion between assimilation and absorption,
although it appears to us to support the opposite view.

Wave length (/*/*) . 718 680 622 589 558 522 506 486 468 431
Assimilation . . . 12-2 100-0 80.8 60-5 47.4 39.3 59.7 66«i 59.3 45.9
Absorption . . . 23-7 81-2 52-6 47.5 40-2 51-0 63-2 83.4 86-3 90.7

Notice especially the region \ = 622-522, where assimilation falls from 80
to about 40, while absorption decreases only a very little and has almost the
same value at 522 as at 622 ; compare further X = 680, where absorption =
81-2 and assimilation = 100, with A. = 431, where, in spite of more vigorous
absorption, assimilation reaches a value of only 45-9.

If relations exist between absorption and assimilation these are of the
most complex character and have still to be worked out. This is even more
so in the case of the yellow, red, and blue-green cells of many Algae than of the
green cells of the majority of plants. In regard to these Algae, ENGELMANN
(1884) has made very interesting statements which, however, require con-
firmation by more extensive experimental evidence.

It has already been made obvious that the activity of light in the assimila-
tion of carbon-dioxide must be bound up in a marked manner with light absorp-
tion. This may be assumed on theoretical grounds. The products of assimilation,
starch or sugar, possesses a noticeable heat of combustion, while carbon-dioxide,
from which they are constructed, being the end product, has no such value.
The energy which the plant gains in the process of constructing organic substance
can only originate from the sun, and hence, obviously, light, if it be altered
into the chemical energy of starch, must disappear as light and an absorption
of light must occur in the chlorophyll bodies. JULIUS ROB. MAYER, the dis-
coverer of the Law of Conservation of Energy, took up a perfectly correct
position with regard to the fundamental importance of this relation between
plants and light when he wrote (1847, pp. 37, 38) : — ' Nature has set herself the
task of seizing the light pouring on the earth's surface and of storing this, the
most mobile of all forces, in a fixed form. In order to gain this end she has
covered the earth with organisms capable, so long as they are alive, of absorb-
ing sunlight and of producing by its activity a continuous series of chemical

JOST K

I3o METABOLISM

differentiations ; these organisms are plants. The plant world forms a reservoir
in which the fugitive solar rays are imprisoned, and are made to subserve
certain uses.'

These reflections of MAYER have been completely confirmed, and we must
recognize in carbon-dioxide assimilation the source of the collective organic life of
our globe, life which ultimately draws its energy from the sun. Only green plants
are able to fix the sunlight in this way, and all non-green parts of such plants
as well as all non-green organisms are directly or indirectly dependent on this
primary and most important synthesis of organic substance (compare Lect. XV).

Looked at from this point of view it is certainly of the highest importance
to examine to what degree the green cells are able to make use of sunlight, how
much of that energy they store and how much of it passes out unused.

It is possible to arrive at an answer to this question, in the first place, by
theoretical calculation. Starting from BOUSSINGAULT'S work as a basis, PFEFFER
(1871) assumed that a sq. m. of the leaf surface of Nerium formed starch at the
rate of 0-000535 g. per second, and, estimating the heat of combustion of starch
as 4,100 calories per g., concluded that 2-2 calories of solar energy is used per
sq. m. per second, that is to say, less than I per cent, of the total solar energy
(which is, according to POUILLET, about 333 calories per sq. m. per second).
If, following SACHS, we reckon that a sq. m. of a leaf of Helianthus produces 1-8 g.
of starch per sq. m. per hour, this would correspond in round numbers to
7,000 calories ; but the amount of energy of the sun per sq. m. per hour is,
according to POUILLET, equal to 1,200,000 calories, so that only about 0-6 per
cent, of the total light energy is employed in assimilation. If we adopt AD.
MAYER'S (1897) results we get a somewhat higher percentage, viz. 2-4 per cent.,
but MAYER made his calculations on the total solar energy for the year. Since
a considerable portion of this solar energy falls on the earth's surface at a time
when no vegetation is there, more than 2-4 per cent, of the actually available
light would be effective. On the other hand, MAYER'S estimate of the total
annual amount of solar energy at a quarter of a million calories must be too low,
for LANGLEY'S results give double that number, so that after all corrections have
been made the value would be again somewhat under 2-4 per cent, (compare also
BROWN, 1899).

[According to the very careful researches of BROWN and ESCOMBE (1904),
not quite I per cent, of the solar energy which falls on the leaf is employed in
carbon-dioxide decomposition.]

Comparison of these calculations teaches us one thing at least, viz. that
only a small fraction of the total solar energy is gained by the plant in assimi-
lation ; how large that fraction is, however, we cannot as yet exactly tell.
DETLEFFSEN'S (1888) attempts to estimate this amount experimentally is there-
fore of the greatest interest. He made observations on the absorption of light
in a leaf with the aid of a thermopile, placing the leaf alternately in air con-
taining 10 per cent, of carbon-dioxide, and air free from that gas. In the
former case, when assimilation was going on, he found that more light was
absorbed than when there was no assimilation, and estimated that of the total
amount of light 0-9 per cent, in one experiment, 0-3 per cent, in a second, and
I -I per cent, in a third was absorbed. We must not place too much reliance
on these numbers — for the sources of error are numerous and obvious — but
DETLEFFSEN'S experiments appear to us to offer an interesting starting-point
for further research, if aided by appropriate methods. It is quite possible that
no more light is absorbed while assimilation is going on than when it is not, for
the light which serves to bring about assimilation may, when assimilation is
prevented, be transformed into heat, as in the case also of the not inconsiderable
amount absorbed by the dead leaf and by a chlorophyll solution.

Why it is that decomposition of carbon-dioxide is possible only in the pre-
sence of chlorophyll we do not know. It has often been suggested that chloro-

THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. II 131

phyll acts as a sensitizer. It is well known, for instance, that silver salts are
affected only by rays of a certain wave length, and that red light, as every
photographer knows, has no effect on them. By adding a colouring matter
which absorbs red light, the silver salts become sensitive to red rays. The
action of the colouring matters in this relation is by no means clearly under-
stood, since not all dyes which absorb red rays act as sensitizers. Apart from
that, there is one great distinction between these physical peculiarities and those
presented by the chlorophyll body, viz. that silver salts are naturally affected
by light, and their sensitivity is only increased by employing a sensitizer ;
the chlorophyll body, on the other hand, is unable to bring about the decom-
position of carbon-dioxide in the absence of the green colour, so that we cannot
look upon the dye merely as a sensitizer.

Seeing that the manufacture of organic material is necessarily bound up
with the entry of energy into the plant, it may be asked whether this energy is
of necessity always solar energy, and more especially that of the luminous
rays. It might well be supposed that other forms of energy also, heat, elec-
tricity, and chemical energy, might be employed for the same purpose, and, as
a matter of fact, it is very probable that some organisms can construct organic
material out of inorganic with the aid of chemical energy. This mode of con-
struction may be termed chemo synthesis in contrast with photosynthesis — the
process we have already been studying (PFEFFER, Phys. 2nd ed.). We shall
recur to chemosynthesis later on, but at the present moment, in view of the
importance of ' photosynthesis ', a few remarks as to the history of the subject
will not be inappropriate. (Compare SACHS, 1875 ; PFEFFER, Physiol. 2nd ed.
I, 289 ; BROWN, 1899 ; [WIESNER, 1905].)

The essential preliminary basis on which this theory of carbon assimilation
is built was laid down in a series of researches published in the last third
of the eighteenth century. PRIESTLEY was aware that the atmosphere was
rendered foul owing to the respiration of animals and to putrefaction and com-
bustion, and he strove systematically to discover by what means nature counter-
acted this. In 1771 he established the fact that this duty was fulfilled by the
plant world. It was he who in 1778 first made out that the air-bubbles escaping
from partly submerged plants contained more oxygen than ordinary air. In
the glass vessels which he employed in his researches he noticed, after long
standing, the development of green masses which also gave off oxygen in
sunlight, but since PRIESTLEY was unacquainted with the fact that these
green masses were Algae, he thought he was observing a purely chemical process
tending to the evolution of oxygen. PRIESTLEY does not seem to have
clearly appreciated the importance of sunlight in the 'purification of the
air', and it was INGENHOUSS who first drew attention to the fact that it
was the green parts of plants that alone had the power of inducing this
'purification'. Both INGENHOUSS and PRIESTLEY were supporters of the
phlogiston theory. SENEBIER was the first to investigate the subject from
the standpoint of modern chemistry as founded by LAVOISIER, and his
exposition of the subject appears to us to-day to be much more modern than
those of his predecessors. He showed more especially the connexion between
the carbon-dioxide and the evolution of oxygen, and was the first to make out
that it was concerned in the process of the manufacture of organic material.
It is true that, knowing how little carbon- dioxide was present in the air, he
thought that plants absorbed the gas from the soil. TH. DE SAUSSURE (1804),
however, first brought forward incontestable evidence that the air was the
source whence the plant obtained its carbon-dioxide, and it was he who, by his
exceedingly accurate experiments, placed our entire knowledge of the subject
on a sound basis. Later on, his correct interpretation of the facts was
neglected and ' humus ' was once more thought to be of importance in the
nourishment of the green plant, until, through LIEBIG'S sagacity and Bous-

K 2

132 METABOLISM

SINGAULT'S experimental work, SAUSSURE'S results obtained general recog-
nition, and became what they are to this day, one of the foundation stones of
plant physiology. The older investigators, for the most part, expressed no definite
opinion as to the first product of carbon assimilation ; later on, carbohydrates,
amongst other things, were suggested, until SACHS announced that starch was
' the first visible product of assimilation '. We have already referred to the
more recent developments in this relation, whereby it has been shown that the
chloroplasts can construct starch out of carbon-dioxide but also from soluble
carbohydrates as well ; on these, naturally, all the non-green parts of the
higher plants are dependent, as well as the great mass of Fungi, &c., which
are entirely unable, since they possess no chlorophyll, to carry out carbon
assimilation for themselves. Later on, we shall examine more closely the
mode of nutrition of these heterotrophic organs and organisms.

Bibliography to Lecture X.

BASTIT. 1891, Revue gen. d. Bot. 3, 522.

BLACKMAN. 1895. Phil. Trans. R. Soc. B. 186, 504.

BOUSSINGAULT. 1868. Agronomic, 4, 375.

BROWN. 1 899. British Assoc. Address to the Chem. Section. Dover.

BROWN AND ESCOMBE. 1900. Phil. Trans. Royal Soc. B. 193, 223.

[BROWN AND ESCOMBE. 1902. Proc. Royal Soc. 70, 397-412.

[BROWN AND WILSON. 1905. Proc. Royal Soc. 76, 29-137.]

BURGERSTEIN. 1900. Ber. d. bot. Gesell. 18, 168.

DAUBENY. 1836. Phil. Trans, p. 149.

DETLEFFSEN. 1888. Arb. bot. Inst. Wurzburg, 3, 534.

DRAPER. 1843. Phil. Mag. 23, 161.

EBERMAYER. 1885. Sitzungsber. Miinchener Akad. 15, 303.

ELFVING. 1880. Arb. bot. Inst. Wiirzburg, 2, 495.

ENGELMANN. 1882. Bot. Ztg. 40, 426.

ENGELMANN. 1884. Bot. Ztg. 42, 81.

EWART. 1896. Jour. Linn. Soc. ; Botany, 31, 364.

EWART. 1897. Ibid. 31, 554.

GODLEWSKI. 1873. Arb. bot. Inst. Wiirzburg, i, 343.

JUMELLE. 1892. Revue gen. de Bot. 4, 166.

KLEBS. 1888. Unters. aus. d. bot. Inst. Tubingen, 2, 489.

KOHL. 1897. Ber. d. bot. Gesell. 15, in and 361.

KREUSSLER. 1885-90. Landw. Jahrbucher, 1885: 14, 913; 1887: 16, 711;

1888: 17,161; 1890: 19,649.
LANDOLT. 1894. Ber. chem. Gesell. p. 2872.
LOPRIORE. 1895. Jahrb. f. wiss. Bot. 28, 531.
[MATTHAEI, G. 1904. Phil. Trans. R. Soc. B. 197, 47-105.]
MAYER, Ad. 1897. Versuchsstationen, 48, 67.
MAYER, R. 1845. Die organische Bewegung im Zusammenhang mit dem Stoff-

wechsel. Heilbronn.

MEISSNER. 1 894. Beitr. z. Kenntn. d. Assimilationstatigkeit der Blatter. Diss. Bonn.
MOLL. 1877. Landw. Jahrbucher, 6, 327.
[MULLER, ARNO. 1904. Jahrb. f. wiss. Bot. 40, 443-498.]
NAGAMATZ. 1887. Arb. bot. Instit. Wurzburg, 3, 389.
NOLL. 1894. In the Bonn textbook, ist ed.
[PANTANELLI, E. 1904. Jahrb. f. wiss. Bot. 39, 167-228],
PFEFFER. 1871. Arb. bot. Inst. Wurzburg, i, i.
PFEFFER. 1872. Bot. Ztg. 30, 425.
REINKE. 1883. Bot. Ztg. 41, 697.
REINKE. 1884. Bot. Ztg. 42, i.
REINKE. 1893. Sitzungsber. Akad. Berlin, 527.
SACHS. 1884. Arb. bot. Inst. Wurzburg, 3, i.
SACHS. 1875. Gesch. der Bot. Munich.
SCHIMPER. 1885. Jahrb. f. wiss. Bot. 16, i.
STAHL. 1894. Bot. Ztg. 52, 117.
TIMIRIASEF. 1885. Annal. Sc. nat. vii, 2, 99.

UNGER. 1885. Anatomic u. Physiol. d. Pflanze. Pest, Vienna, Leipzig.
[WIESNER, J. 1905. Jan Ingenhouss. Vienna.]

133

LECTURE XI
THE ASSIMILATION OF NITROGEN IN AUTOTROPHIC PLANTS

As the result of changes taking place in the carbohydrates which originate
in the chloroplast, there arises a large number of important vegetable sub-
stances, of which only the materials of the cell-wall, fats, and the numerous
organic acids may be alluded to here. All these substances are composed of the
elements carbon, hydrogen, and oxygen only, but over and above there are
numerous compounds in the plant which contain a fourth element, nitrogen ;
every plant, in fact, contains this element in a small but constant percentage
(Lecture I, p. 5). The form in which nitrogen can be utilized varies in the
different types, but we will confine ourselves at present to nitrogen requirements
of the green plant of whose absorption of nutriment we have obtained so limited
a conception. We certainly know far less about the mode of assimilation
of nitrogen than we do of carbon, and this is the more to be regretted as
nitrogen is an even more important food material than carbon. For protoplasm,
the actual living substance, always contains nitrogen, while on the contrary
those bodies which are composed of carbon, oxygen, and hydrogen only,
cannot be considered as endowed with vitality.

Let us return to the water and sand-culture methods by means of which
we. were enabled to arrive at such definite conclusions as to the requirements
of plants so far as the materials of the ash were concerned. When plants
were grown in nutritive solutions we found that a very considerable increase
in the plant's dry weight took place, but we also learned that such culture
fluids must contain all the materials needful to the support of plant life
(p. 8l). The nitrogen supplied was in the form of a nitrate of calcium or
potassium. We have now to settle the question as to whether such an
addition is really necessary, whether the enormous quantity of free nitrogen,
amounting to four-fifths of the atmosphere, may not be utilized by the
plant. The answer to this question is emphatically in the negative ; for
although we may know of methods by which free nitrogen is brought into
combination in inorganic nature, and although we shall, later on, find that there
are certain plants (Lecture XIX) which are able to make use of free nitrogen,
still we are compelled to deny this power to the ordinary green plant.

It is to BOUSSINGAULT (1860-61) that we owe the establishment of this view;
he was certainly unaware of the special powers possessed by Leguminosae,
although he carried out not a few researches on these plants as well. Since we
purpose dealing with the problem of the absorption of nitrogen by the Legu-
minosae separately in Lecture XIX, we will confine ourselves here to plants
not belonging to that group, and take as our example Helianthus argophyllus.
BOUSSINGAULT performed three series of experiments on this plant ; in the
first series he grew the plants in pure sand without any addition of minerals,
and especially with the omission of combined nitrogen ; in the second series
the sand had added to it the materials of the ash and potassium nitrate ; in
the third series materials of the ash and, in addition, potassium carbonate instead
of potassium nitrate. The result of the research is summarized in the following
table :—

Dry substance ; Organic sub- Gain in carbon Gain in nitro-

seed taken as = r. stance formed. in 86 days. gen in 86 days.

A. (Sand) ... 3.6 0-285 (g). 0-114 (g). 0-0023 (g).

B. (Sand, ash, nitrate) 198-3 21-111 „ 8-444 ,, 0-1666 „

C. (Sand, ash, carbonate) 4-6 0-391 „ 0.156 „ 0-0027 „

We see from these numbers that in Series A and C nitrogen was almost
entirely excluded ; the limited gain in nitrogen amounting to about 2-3 mg. in

134

METABOLISM

Series A, and to 2-7 mg. in C, being accounted for by the absorption of ammonia
in the gaseous form from the air. Concomitantly with this exclusion of combined
nitrogen there is a reduction in the amount of carbon and organic substance
formed, as well as in dry weight generally. It is worthy of note, however,
that an increase in dry weight may still occur, and that this increase is greater
when plants are manured with the materials of the ash
than if they be grown in pure sand. The amount of nitro-
gen present in the seed reaches a greater amount than
can be accounted for by the limited supply in the ash.
The unequal development of plants treated in
different ways comes out even more prominently from
a study of BOUSSINGAULT'S figures than from a con-
sideration of the data quoted above. At Fig. 29, two
of BOUSSINGAULT'S figures have been reproduced, side
by side, reduced to the same scale ; I, represents a plant
from series B ; 2, from series A, although it might
stand equally well as a representative of series C, be-
tween which latter and series A the differences are not
worth mentioning. From the figures it will be ap-
parent that the normal plant may reach a height of
64-74 cm., and develop a prominent inflorescence,
while that grown in absence of nitrogen reaches a
height of only 11-14 cm-» an(i produced a capitulum
of very limited size.

This experiment shows with perfect clearness that
Helianthus is unable to make any use of the atmo-
spheric nitrogen. It also proves that potassium nitrate
forms a very appropriate source of nitrogen for nu-
tritive purposes, since the increase in dry weight in
plants belonging to series B is nearly sixty times that
of plants in series A. This great increase in dry weight
is very surprising, when one remembers how little
potassium nitrate the plants have been able to obtain.
A pot containing ij kg. of sand received gradually
in the course of three months 1-4 g. of saltpetre, and
this amount was sufficient to enable two plants to
reach their full normal development.

Many hundred culture experiments in water and
sand have established the fact that nitric acid forms
Helianthus argo- an excellent, not to say the best possible source of
&Sr.l'SSSSf Imogen for the great majority of plants. [How the
(Proportionally reduced.) After divergent results arrived at by TREBOUX (1905) are

BoOSS,NGABLT,,86o,p..3. t() g expjained it is> M y/t> impossible VtQ ' ]

In principle it is immaterial with what base the nitric acid is united, still,
generally speaking, it is preferable to use such bases as are themselves essential,
hence potassium or calcium nitrates, although they are more expensive than
sodium nitrate, are most suitable in practice. It is impossible at present
to say whether nitrites as well as nitrates play any part in providing nitro-
genous nutriment to Phanerogams. According to MOLISCH'S (1887) researches
these nitrites are very poisonous when present in high percentage, although in
dilute solution (0-05 per cent, or less in the case of potassium nitrite) they are
absorbed with avidity and undergo alteration in the plant ; strange to say they
are not oxidized into a nitrate, but, on the contrary, suffer reduction. Whether
a green plant can or cannot pass through all stages of its development when
supplied with nitrites only, is not known.

29.

THE ASSIMILATION OF NITROGEN IN AUTOTROPHIC PLANTS 135

It is to BOUSSINGAULT that we owe our knowledge of the importance of
nitric acid as a nutrient to the green plant ; previous to his time it was believed,
mainly owing to the influence of LIEBIG (1840), that ammonia was the chief
source of nitrogen to the plant. This conclusion was readily arrived at because
experience had shown that excellent results could be obtained by manuring
with ammonia ; it was not known that the ammonia in the soil is transformed
into nitrate before it is absorbed by the plant. This nitrification (Lecture
XVIII), due to the action of organisms in the soil, complicates the scientific
explanation of the question as to the significance of ammonia in the nutrition
of the green plant. The recent comprehensive researches of PITSCH (1887-
1896) and of MAZ£ (1900) have conclusively proved that the nutritive value
of ammonia must not be entirely denied ; in the majority of green plants it is
second only to nitric acid in value, inducing a definite development and con-
siderable increase in dry weight. The fact that many plants thrive only
moderately well when supplied with ammonia is accounted for by the fact
that the ammonia salts when presented to the root in a more concentrated state
produce an injurious effect. Carbonate of ammonia, on account of its alkaline
reaction, is especially liable to cause injury to the plant, acting as a matter of
fact like a poison. In the case of some plants, particularly maize and other
Gramineae, ammonia is by no means of inferior value to nitric acid, for MAZE was
able to obtain as great an increase in dry weight in maize, using at most a J per
cent, solution of ammonium sulphate, as when he supplied it with a solution of
a nitrate. Similar results were obtained in cultures of Brassica and species of
Allium. Forest trees also must be dependent on ammonia, since nitrates are
seldom present in woodland soils. The significance of this is not so simple as
it appears and will necessitate inquiry later on (Lecture XIX). So far as we
know at present it is quite certain that in addition to plants which definitely
prefer nitric acid (e. g. buckwheat, potatoes, turnips) there are others which
get on just as well or even better with ammonia, so that it would appear to be
a matter of indifference to such plants whether the ammonia is supplied to
them in the form of a sulphate, a nitrate, or a phosphate ; it is only the
carbonate which, as already mentioned, is liable to produce injurious effects.

Let us now inquire into the sources of nitrates and ammonia in nature.
Minerals which are of purely inorganic origin, and which at the same time
contain nitrogen, occur only rarely in nature. ERDMANN (1896, Ber. Chem.
Gesell. 29, 1710) obtained only very minute quantities (0-028 per cent, or less)
of combined nitrogen in perfectly pure primitive rock. Nitrate of soda would
at first sight appear to be an exception, but there is little doubt but that this
form of nitrate occurs in nature as a product of organic activity [MuNTZ, 1889].

All evidence points to the fact that the total amount of fixed nitrogen
available for plant nourishment nowadays arises from the chemical combination
of free nitrogen gas. Processes are constantly taking place which result in the
combination of gaseous nitrogen, but the converse process also occurs where free
nitrogen arises from the decomposition of compounds. Every combination of
nitrogen which is effected means a gain of nutrient to the typical green plant
from a substance of no nutritive value, and every formation of nitrogen gas by
decomposition of a nitrogenous compound means a loss to it. Hence these two
processes as they occur in nature, and which we may briefly term nitrogenous
gain and nitrogenous loss, are of special interest in relation to the question
before us, and demand closer study on our part, though at the present moment
we need not do more than briefly indicate the more important points in regard
to the problem, reserving further details for study later on.

Nitrogenous gain takes place under various conditions. Apart altogether
from conditions which may be created in a laboratory, there are only two
methods of bringing about nitrogenous combination ; one of these, in which
organisms play a prominent part, we will discuss later (Lecture XIX) ; the

136 METABOLISM

other only need be referred to at the present juncture, viz. the oxidation
of nitrogen into nitric and nitrous acids, which takes place under the influence
of electric discharge in the atmosphere more especially in thunderstorms.
Rain, mist, and snow carry this nitric acid in solution down to the soil in
considerable quantities, as BOUSSINGAULT has clearly shown (1861, 325). The
greatest quantity which he found in rain was 6 mg. of nitric acid per litre,
but for the most part the amounts were 3, 2, or i mg., or even less. It cannot
be said that any marked relation exists between the frequency of thunderstorms
and the amount of nitric acid present in rain-water, for there is a relatively
large quantity of combined nitrogen present in rain even at times when
thunderstorms are entirely absent. It is possible that silent electric discharges,
which are always taking place in the atmosphere, may account for this fixing of
free nitrogen ; perhaps, also, some of the nitric acid present in the air and
carried down to the soil may have its origin in the dust which has arisen from
the soil itself.

Altogether only a small amount of freshly combined nitrogen is added to the
soil, not more than a kilogram per hectare per annum according to AD. MAYER
(1901, i, 205) [in tropical countries as much as 5 to 6 kg. (MARCANO and MUNTZ,
1889)], while the plants which grow on a surface of that extent, according to
BOUSSINGAULT, use up about 50 kg. of nitrogen per annum. It is therefore
essential that, in order that plants may continue to exist, the nitrogen derived
from dead organisms should be returned to the soil to be used in the construc-
tion of subsequent generations. The nitrogenous compounds arising from dead
animals and plants are decomposed by micro-organisms and, in general, changed
into ammonia (Lecture XVII). This substance is greedily absorbed by the soil,
and thus part, at least, of the ammonia arising from putrefaction will be fixed
in the earth. Further, owing to the influence of micro-organisms the ammonia
underjgoes oxidation into nitrous and nitric acids (Nitrification, Lecture XVIII).
In this way every soil contains in varying proportions nitrates, nitrites, and
ammonia.

Losses of nitrogen in nature may, in the first place, be quite local. The
ammonia arising from putrefactive decomposition — in so far as it remains un-
altered— is only in part absorbed ; a not inconsiderable amount passes off into
the air in the gaseous form where it is oxidized into nitrous or nitric acids, or
united with carbon-dioxide. This ammonia is returned to the soil in the course
of atmospheric precipitation. According to AD. MAYER (1901, i, 205), on an
average about 2 kg. of nitrogen in the form of ammonia falls on a hectare of
land annually, in addition to the single kg. of nitric and nitrous acids already
referred to as brought down by rain. Volatile ammonia is not entirely lost
to the plant; on the other hand, it may be transferred from one place to another ;
ultimately it may be removed out of reach of land plants, if it be carried by rain
into the sea. The case is the same with nitric acid ; when, owing to nitri-
fication of ammonia, this substance arises in the soil it is then of service
to the plants on the spot, only if it be at once absorbed by the roots.
Since the soil cannot retain nitric acid, all of it not at once absorbed by the root
will be washed away by rain and carried into rivers and, finally, into the sea.
The development of nitrogen in the gaseous form is of far greater importance
than the phenomena mentioned, which, on closer observation, resolve themselves
into translocations and transformations of combined nitrogen, and not into actual
loss of such. This evolution of free nitrogen takes place in the course of many
decompositions (Lecture XVII), and also in certain combustion processes. If
there were no organisms capable of making use of this free nitrogen, these losses
would be irretrievable. As a matter of fact such organisms are well known to
occur (Lecture XIX), and the power they possess is obviously of the greatest
importance in the circulation of nitrogen.

We shall return to such vital phenomena later, meanwhile we may note

THE ASSIMILATION OF NITROGEN IN AUTOTROPHIC PLANTS 137

that a complete explanation of their mode of operation is not as yet possible.
We may note only the fact that, despite the numerous researches which have
been made during recent years as to the phenomenon of nitrogen fixation, we
have no conception of the quantitative aspect of the question, so that it is quite
impossible to say whether the fixation of nitrogen gas or its evolution is the
more dominant feature in nature, or whether the one process is the exact
balance of the other. When one remembers that originally no combined nitro-
gen existed on the earth, one is inclined to hold that the amount of combined
nitrogen to-day is on the increase, and that as a consequence actually more
organisms are able to exist now than thousands of years ago. Without doubt, the
amount of living substance in nature depends on the amount of nitrogen, since
nitrogen occurs only to a minimum extent in uncultivated soil.

Owing to the mode of occurrence of combined nitrogen the green plant
can take it up in three different ways : —

1. It can absorb it in the form of ammonia or nitric acid from the soil by
means of the root.

2. It can take up ammonia in the gaseous form from the air by means of
the leaves.

3. It can absorb rain-water and nitrogenous substances dissolved in it also
by the leaves from the air.

The first of these possibilities is really the only one we need consider. The
power of leaves to absorb gaseous ammonia is undoubted (ScHLo'ssiNG, 1874) ;
but the fact that this gas occurs in the air only in quite limited traces renders this
capacity on the part of the leaves of no practical importance. On the other
hand, large masses of manure may certainly appreciably add to the quantity
of ammonia in the air, and it is quite possible that under these conditions it
may exert an important influence on the development of many plants
(KEENER, 1887). Any such favourable influence, it must be remembered, is
only limited, since in higher states of concentration ammonia is very rapidly
injurious. The absorption of combined nitrogen dissolved in rain through the
leaves is undoubted, yet this amount also is so small that the ordinary land
plant may be considered as entirely dependent on that absorbed from the soil.
There is a large amount of literature dealing with the presence of ammonia in
uncultivated soils, but into the discussion of these researches it is impossible
to enter here. It will be sufficient if we refer to some of the results obtained by
A. BAUMANN (1887) : —

One kg. of dry mg. of nitrogen as ammonia.

Loam (derived from granite) . . (Fir Mts. Bavaria) . . . 22-27

Weathered gneiss .... „ ... 11-05

„ porphyry .... (Rhine Palatinate) . . . 17-71

,, carboniferous sandstone ........ 4-43

,, basalt .... (Rhine Palatinate) . . . 23-37

Loess without humus .... (Munich) ..... 6-58

Sandy soil (Schrobenhausen) . . . 2-23

Moorland soil (MQnich) ..... 1-60

Soils which are unworked and unmanured vary greatly in the amount of
ammonia which they contain ; basalt and loam soils contain the most, sandy
and moorland soils the least. Further, the amount of ammonia decreases rapidly
as the deeper layers of the soil are reached.

On investigating the amount of nitric acid in uncultivated soils the same
author found it occurring for the most part in such minute traces that it
was impossible to estimate it quantitatively. On the whole, then, the plant
can obtain under natural conditions only a very small quantity of combined
nitrogen in the soil, and its growth is thus dependent on the characteristics
of the root-system already referred to, more especially the capacity it has

138 METABOLISM

for drawing upon a large extent of soil for the absorption of a substance
occurring but sparingly in it.

The continued growth of plants on natural soil indicates at least that they
are always able to obtain the necessary nitrogen from it. It is otherwise
with cultivated plants. Just as in the case of the materials of the ash, agri-
culture prospers according to the amount of nitrogen removed. If 50 kg. of
nitrogen be extracted from a hectare at each harvest, and in large part
removed permanently from the field, and at the same time not quite 3 kg, of
nitrogen is added annually, the soil must become rapidly impoverished, and
that condition can be remedied only by manuring. Part of the nitrogen
removed may be replaced by the excrement of cattle, and this explains the
favourable results obtained by manuring with excrement, customary even in
the most primitive forms of agriculture. Excrement is insufficient of itself to
replace the loss in nitrogen suffered by the land, for part of the nitrogen is
sold off the land directly with the harvest or indirectly with the cattle ; the
remainder, which is contained in dung, is entirely transformed into ammonia,
and as such becomes further diminished by evaporation or is washed out after
undergoing nitrification. The formation of free nitrogen in dunghills may be
reckoned as a further source of nitrogenous loss. Thus, in all logical schemes of
agriculture, artificial manuring with nitrogen is essential. Since nitrate of
potash is too expensive, by far the most valuable manure is Chili-saltpetre
(nitrate of soda), which occurs in immense beds in Peru, traceable in its origin
to vital activity. This substance came into use in England seventy-five years
ago and is still employed in very large quantities. In addition to Chili-saltpetre
may be mentioned sulphate of ammonia, a by-product in the manufacture of
coal gas and almost as valuable for the purpose as nitrate of soda. [Probably
calcium cyanamide (CaCN2) is also of great service as a nitrogenous manure.]
Finally, those plants which bring about the fixation of the free nitrogen of the
air are of the greatest importance in agriculture. These have been several
times referred to but we shall speak of them in greater detail later.

Having now become acquainted with the compounds of nitrogen which
may be made use of by the plant, and having noted that these substances are
absorbed especially by the root, we may turn to the question as to where and
how they are assimilated. As we remarked, however, at the beginning of the
lecture, our knowledge of the assimilation of nitrogen is very defective. The
final products of the assimilation of nitrogen at all events are proteids. These
bodies are rightly considered as forming a series of chemical compounds of
special importance, and hence deserve a few words at this stage in our work.
Unfortunately, from one point of view, the advances in the chemistry of proteids,
which have taken place during recent years, owing to the efforts of physio-
logical chemists, have dealt rather with animal than vegetable proteids (compare
the comprehensive expositions of HAMMARSTEN, 1895, COHNHEIM, 1900, KOSSEL,
1901, HOFMEISTER, 1902). Thus we do not even yet know whether the very
important animal proteids occur in the plant kingdom also, while we know little
or nothing as to the peculiarities of vegetable proteids. Consequently the
following notes, which we extract from COHNHEIM' s works, must of necessity
be very fragmentary. [Compare CZAPEK, Biochemie, vol. II.]

Proteids cannot be so easily represented by formulae as carbohydrates or
fats. Five elements for the most part enter into their composition, hydrogen,
nitrogen, oxygen, carbon, and sulphur, to which we may add also phosphorus.
The relative amount of these elements in the different proteids varies greatly
and little is to be deduced from the statements made as to the percentage com-
position of each. Generally speaking, however, proteids possess certain physical
characters, give certain chemical reactions, and especially give rise to similar
decomposition products, so that we may conclude that they form a natural series
of compounds and not merely a heterogenous collection of organic bodies which
cannot be catalogued under other and more fully studied groups.

THE ASSIMILATION OF NITROGEN IN AUTOTROPHIC PLANTS 139

From a physical standpoint the colloidal nature of proteids stands out pre-
eminent. It is, doubtless, largely owing to the size of their molecules that
proteids are unable to diffuse through parchment or animal membranes.
Nevertheless they may be regarded as forming genuine solutions which have
the peculiarity of being not very stable. Proteids coagulate on very slight
provocation, and very often this coagulation is accompanied by considerable
alterations in character. This coagulation is permanent and renewed solution
is impossible without fundamental chemical change. Such coagulations are
induced by alcohol, by boiling water, by strong mineral acids, as well as by certain
so-called alkaloid reagents (phosphotungstic acid, tannic acid, &c.). On the
other hand, proteids are transformed by salting out (especially by ammo-
nium sulphate), into a solid and often crystalline condition without being
chemically altered. This salted out proteid remains soluble.

The reagents mentioned above may be used as tests for proteids, but
certain colour reactions may also be employed, of which the most important
are the following : —

1. They give a blue- violet to red colour with caustic soda and a few drops
of weak copper sulphate solution (biuret test).

2. Heating with concentrated nitric acid gives a yellow colour (xantho-
proteic reaction).

3. Boiling with a solution of mercuric nitrate containing a trace of nitrous
acid gives a rose to dark red colour (Milton's reaction).

4. Treatment with an alcoholic solution of a-napthol and concentrated
sulphuric acid gives a violet colour (Molisch's reaction).

5. By boiling with caustic soda and a salt of lead a black precipitate is
produced (lead-sulphide reaction).

Apart from the biuret test, the reactions described are effects produced,
not by the proteid molecule as a whole, but by constituent groupings in
it ; Millon's reagent, for example, acts on a different group in the proteid
molecule than does the lead-sulphide test, and that again on a group not acted
on by Molisch's reagent. One is thus able to differentiate in the proteid
molecule a number of constituent groups, with which a study of the
decomposition products of proteid has made us familiar. Hydrolytic de-
composition more particularly has furnished us with especially valuable
data, because obviously that method entails no very profound changes on the
products of decomposition. Hydrolytic decomposition may be effected by
boiling mineral acids as well as by enzymes (proteases ; compare Lectures
XII and XIII) ; the products are similar in each case and we shall confine
ourselves at present chiefly to the action of enzymes. By the action of
proteolytic enzymes proteid is broken down, in the first instance, into smaller
molecules, which still retain many of the characters of proteids ; there arise
first the albumoses, which are no longer coagulable but may be precipitated
by salting out. From these arise the peptones, which cannot be salted out, but
which still respond to the biuret test. All subsequent decomposition products
fail to show any biuret reaction, and thus are no longer proteid. Albumoses
and peptones may still be considered as proteids, although many peptones
contain no sulphur. Among the products of further decomposition we have
next to recognize a sulphur-containing group. In what form this arises through
the action of enzymes is not as yet fully understood ; cystin (C6H12N2S2O4) rarely
occurs in plants ; sulphates on the other hand, are apparently produced directly.
[Intermediate substances between peptones and amino-acids have been dis-
covered, coupled amino-acids or polypeptides, many of which have been
synthetically prepared by E. FISCHER (€ZAPEK, Biochemie, II, 45).] Under
the head of sulphur-free proteid groups we may recognize the following

(HOFMEISTER, IQ02) I —

HO METABOLISM

I. Belonging to the aliphatic series :

1. Guanidin residue — CNH.NH2.

2. Amino-acids.

a. Monamino-acids : leucin, glycocoll, alanin, aspartic, and glu-

taminic acids.

b. Diamino-acids : ornithin (united with guanidin to form
arginin), lysin, histidin.

3. Carbohydrate groups.

II. Belonging to the aromatic series :

1. Tyrosin.

2. Phenylalanin.

III. Heterocyclic groups :

1. Pyrrol group.

2. Indol group.

3. Pyridin group.

From this general summary we may conclude that, among the proteid
reactions, the xanthoproteic test and Millon's test for the tyrosin group,
Molisch's test for carbohydrate, the lead-sulphide reaction for sulphur groups,
are the most characteristic; the biuret test alone applies to the complete proteid
molecule.

The classification of proteids given above is provisional ; it is based more
on solubility, coagulability, &c., than on constitution. For our purpose the
following summary will suffice : —

I. True Proteids :

1. Albumins. These bodies are soluble in pure water and can often

be crystallized.

2. Globulins. Insoluble in water, soluble in dilute solutions of neutral
salts, from which they may be precipitated unaltered by removal
of the salts.

3. Nucleo-albumins. Distinguished by containing phosphorus.

II. Proteids ; compounds of albumin with other bodies, and more com-

plicated than true proteids :

1. Nucleo-proteid. Compounds of proteid and nuclein ; occurring

especially in the nucleus.

2. Haemoglobins. Compounds of proteid and haematin ; a decom-

position product of haematin is haematoporphyrin (referred to at

P. I09)_-

rlui

III. Glutinoids. Bodies of simpler composition than typical proteid, in
which individual proteid groups are wanting.

So far as we know the true proteids occurring in plants belong especially
to the globulins and nucleo-albumins ; albumins proper occur only occasionally.
Owing to the sparing solubility of vegetable proteids, a fact which has been
drawn attention to by WINTERSTEIN (1901), it has come about that many pro-
teids in the plant have been quite overlooked, e. g. in Vaucheria (REINKE,
1883). We imagine that extraction with baryta water, 20 per cent, hydro-
chloric acid, &c., as tried by WINTERSTEIN, would result in the discovery of
proteids in such cases.

Let us now return to our main problem, where and how are nitrates of
potash and ammonia assimilated in the green plant ?

The nitric acid present in the soil obviously penetrates the proto-
plasm and is easily absorbed by the root in dilute solution. In many plants
nitric acid occurs in such quantities that its determination presents no
difficulty. Although micro-chemical methods fail to demonstrate its presence
in other plants we must not assume its instantaneous alteration in the root-
cells, since many secondary conditions may interfere with the ordinary tests
(e. g. diphenylamin) for the presence of nitric acid. Tobacco, turnips, sunflowers,

THE ASSIMILATION OF NITROGEN IN AUTOTROPHIC PLANTS 141

potatoes, and wheat may be taken as examples of cultivated plants which
contain large quantities of nitrates. In the last two the nitrate amounts to
from 1-5 to 2-8 per cent, of the dry weight. Even greater quantities (15 per
cent.) occur in Amarantus, to which may be added a whole series of weeds
such as Chenopodium, Urtica, &c. The maximum of nitrate is found in the
root, less in the stem and leaf, none at all in the seed. The nitrate increases as
the flowering period approaches, and decreases when fruiting takes place.
FRANK (1888) has shown that these plants contain nitrates only when they
are able to absorb it by the root ; if they be grown in nutritive solutions
containing no nitrogen, or only ammonia, nitrates are entirely absent from
them. Hence we may conclude that the nitrate is not formed in the plant,
as BERTHELOT and ANDRE (1884) thought, but that it is absorbed from
without and stored for future use. Such storing of nitrate, however, is by no
means universal ; many plants absorb no more than they absolutely require.
The nitric acid is finally employed for the most part in the construction of
proteid, and to this end the combination of nitrogenous and carbonaceous
substances is especially necessary. We are accustomed to regard the carbo-
hydrates as the material source of the carbon in proteid, but it can scarcely be
doubted that other organic substances also, especially benzol derivatives, may
serve this purpose. We are as yet quite ignorant as to what is the first product
of union of the nitrogenous and carbonaceous substances. TREUB (1895)
attempted to show that hydrocyanic acid was the first assimilation product
in Pangium edule, but the proofs he has given in this case do not appear to us
valid, and an extension of his hypothesis to other plants is scarcely justifiable.
[TREUB (1905), in a more recent research, endeavours to show that hydrocyanic
acid may be the first assimilation product of nitrogen-containing material.
Numerous and interesting as the experimental data are which TREUB has
brought forward, they are all in accord with the belief that the hydrocyanic
acid is a decomposition product of metabolism.]

No definite answer can as yet be given to the question as to where the
assimilation of nitrates and the construction of proteid takes place, though
one is, generally speaking, inclined to hold the view that all plant cells may
be seats of proteid synthesis. Many authorities hold that most of the proteid
originates in the foliage leaves ; SCHIMPER (1888, 1890), indeed, has ex-
pressed it as his opinion that nitrogen assimilation, like carbon assimilation,
is dependent on chlorophyll and sunlight.

We may quote the following experiments in which he aimed at determining
this point (1888). The leaves of Pelargonium zonale are known to contain
generally an unusually large quantity of nitrate, and the amount present may
be further increased by keeping the plant in the dark or in moderate light ; it
disappears, however, in strong light in a few days. Those parts of the leaf
which contain no chlorophyll, such as occur in certain cultivated species of
Pelargonium, exhibit no alteration of the nitrate they contain on exposure to
light, and the same is true of the aerial roots of Tradescantia selloi. SCHIMPER
also inquired into the origin of the large amounts of calcium oxalate to be found
in illuminated leaves which were provided with calcium nitrate, and he sup-
posed that the oxalic acid produced in the course of metabolism took the
place of the nitric acid and united with the lime. (As to this supposed
function of oxalic acid, compare BENECKE, Botan. Ztg. 1903 ; the subject
will be again referred to in Lecture XVI.) There can scarcely be any doubt
whatever that a vigorous synthesis of proteid takes place in leaves which are
strongly illuminated, but the influence of sunlight and chlorophyll can only be
indirect ; their influence depends on the fact that carbohydrates are present
during carbon assimilation in larger quantities in the region of origin than in
other regions to which they must first be transferred ; further, then, chemical
construction may be more advantageously effected in carbon-assimilating cells

142 METABOLISM

than elsewhere, and, finally, it is possible that when nitrates are present in a
chlorophylliferous cell a part of the carbon assimilated may be employed
directly in the manufacture of proteid without first going through the carbo-
hydrate stage. Still SCHIMPER has not proved that a synthesis of proteid is
impossible in the dark in those parts of the plant which have no chlorophyll ;
further, it has been often stated recently that nitrogen assimilation may take
place in darkness. Thus ZALESKI (1900) has observed a vigorous synthesis of
proteid in leaves of Helianthus which had been cultivated in KNOP'S nutritive
solution, when a large quantity (4 per cent.) of levulose was added at the
same time; when no sugar was added a reduction in quantity of proteid formed
was observed. SUZUKI (1898) conducted similar observations on barley, which
he found to be able to construct proteid out of nitrates in darkness in the
presence of glucose or cane sugar. Certainly researches are not wanting tending
to contradict these results, and experimental treatment of the problem on an
even wider basis is still desirable ; but we may point to the analogy offered
by many Fungi which assuredly form proteids in darkness out of nitrates,
a fact which certainly does not militate against the existence of this capacity
in Phanerogams. [GODLEWSKI (1903) has observed the formation of some
organic nitrogenous compounds to take place in darkness from nitric acid ;
increase in proteid-nitrogen takes place only in light, a statement which is
confirmed by LAURENT (1904).]

The absorption of ammonium salts in the undecomposed (nitrified) condition
has been clearly proved to take place. Since it nowhere accumulates in the plant
in appreciable quantity it follows that it must be rapidly used up. Moreover,
its rapid transformation is essential on account of its poisonous properties.
Ammonia is used up in the construction of proteid as wefl as in the formation
of simpler nitrogenous bodies, of which we shall have to speak later on. The
same question arises with regard to the synthesis of proteid from ammonia that
we have left unsettled when speaking of nitric acid, viz. the influence of light.
LAURENT (1896) held that light was essential [more recently (1904), LAURENT
has expressed the opinion that assimilation of ammonia is also possible in the
dark] ; HANSTEEN (1899), on the contrary, observed that construction of proteid
from ammonia took place also in the dark if the appropriate carbohydrates
were present. He found that glucose was of great service in this respect but
that cane sugar was useless ; unfortunately he based his conclusions on micro-
scopic investigations only. Here also comprehensive studies are urgently
needed.

The problem as to the influence of light on the assimilation of nitric acid
or ammonia is of the greatest importance for another and related reason. We
have seen how light supplies the energy required in carbon assimilation to form
out of carbon-dioxide chemical compounds containing greater supplies of energy.
There can be no doubt that an expenditure of energy is also needed to bring
about the synthesis of proteid from carbohydrates and nitric acid or ammonia,
since, as AD. MAYER (1901, i, 174) has shown, reduction processes are certainly
accompaniments of this synthesis. If it could be proved that the synthesis of
proteid takes place only in the presence of sunlight, then we might assume
that solar energy is the source of energy we are in search of. Since, however,
this is not the case, we must look around for another form of energy, and we
know of only one other form which we need consider, viz. chemical energy, set
free whenever carbohydrates are oxidized. We will return to this subject in
speaking of the phenomena of respiration, at present we need consider only one
aspect of the process : — Synthesis of carbohydrates in the green plant is un-
doubtedly a case of photosynthesis ; the sun provides the necessary energy
for carrying this out ; the synthesis of proteid, on the other hand, is to be
regarded, at least in certain instances, as a case of chemosynthesis. It has
already been shown that proteid synthesis is impossible in the absence of

THE ASSIMILATION OF NITROGEN IN AUTOTROPHIC PLANTS 143

carbohydrate synthesis, and that in the long run every case of proteid formation
is dependent on sunlight, though indirectly.

Since the nitrogenous decomposition products which appear as a result
of the breaking down of proteids, not only in the plant but apart from it,
as a result of boiling in acids, are always those mentioned at p. 139, it may
be assumed that the synthesis of these bodies, which we term amides, precedes
the synthesis of proteids [perhaps with the formation of polypeptides as
intermediate products]. In fact, these substances, more especially asparagin,
are known to be of widespread occurrence in plants, although it has not as
yet been shown whether they are, primarily, intermediate stages in the
formation of proteid from ammonia or nitric acid, or secondary products re-
sulting from the decomposition of already formed proteid. Although FRANK
and OTTO (1890) found that leaves generally contain more asparagin in the
evening after illumination than in the morning, we must not conclude on that
account that synthesis of asparagin has taken place ; it might just as easily
arise from an increase in the proteid contents of the leaf and a concomitantly
increased decomposition of proteid. It may be possible, however, to determine
whether the amides found are produced there by the breaking down of complex
molecules or by the synthesis out of simpler bodies. Perhaps an investigation
of leaves whose proteid is prevented from escaping from them by their re-
moval from the stem, may serve, in comparison with normal leaves, as a
starting-point. A more comprehensive view of the process would be obtained
if we could establish a vigorous assimilation of carbon and of nitrogen
in leaves while preventing a concomitant production of proteid. Sulphur is
present in proteids under all conditions, and although this element is required
only in small quantity it might be still possible perhaps to obtain a more vigor-
ous anabolism of amides in the leaf-blade by withdrawal of sulphates and
finally to induce a subsequent combination of these into proteids by adding
sulphur afterwards. Experiments in this direction are still much needed.

Under present circumstances it is of interest to know that the power of
the plant to construct proteid out of the nitrogenous organic substances named
above, and others also, has been repeatedly proved. Older experiments (for
literature see PFEFFER, Phys. I, 397) have shown how substances like urea,
glycocol, asparagin, leucin, tyrosin, guanin, creatin, hippuric acid, uric acid, &c.,
may be supplied to plants in water-cultures in place of ammonia and nitric
acid, whilst more recently LUTZ (1899) has shown that, in addition to acetamide,
methylamylamin, ethylamylamin, &c., may be employed. The plant can
recoup itself so far as nitrogen is concerned from such substances, although they
are not all equally good for the purpose. If a marked increase in dry weight
takes place, proteid must have been synthesized from such nitrogenous bodies.
It is a well-established fact, however, that the transition to proteid is never
direct, but that it is usually preceded by decomposition processes. This is well
known to be the case with hippuric acid, which breaks up into benzoic acid
and glycocol, the latter only undergoing further transformation. Further, all
these bodies are easily changed into ammonia through the action of micro-
organisms. Although it has been often expressly stated that the formation of
ammonia could not be demonstrated in certain experiments, it by no means
follows that it did not occur. It may well be that the ammonia is at once
absorbed by the plant as soon as it appears. The systematic exclusion of
micro-organisms has not been considered worth while in the majority of the
researches, and in those which have been carried out with antiseptic pre-
cautions (LuTZ, 1899), other sources of error are not absent (compare SCHULZE,
1902). Notwithstanding, we cannot doubt that a transformation of amides
into proteids takes place in the plant. In Lectures XIII and XIV we shall
learn that the plant produces such substances in the course of metabolism and
how it reforms proteid from them. In this case, as in the older water-culture

144 METABOLISM

experiments, the working up of amides takes place in light. We have still to
inquire, however, whether this is possible in darkness, and HANSTEEN'S (1898)
researches have shown that this is indeed the case.

HANSTEEN provided the plants he experimented on with carbohydrate
and nitrogen, either by adding these bodies to the culture solutions (Lemna
minor) or by injecting the solutions into the plant through a wound. It is diffi-
cult to see how it was possible to exclude micro-organisms in the first case.
Moreover, unfortunately, he maintained his experiments for only a few days,
so that it was impossible to arrive at any conclusion as to whether continued
growth could go on under these conditions. In addition he employed exclu-
sively microscopic methods for determining the presence of proteids, viz. by
iodine or Millon's reagent. It is well known that such evidence, especially if
quantitative in character, is extremely unreliable. Although HANSTEEN'S
experiments cannot be considered as free from doubt, we must quote his results
here for want of better. Perhaps EFFRONT'S (GREEN- WINDISCH, 1901, p. 166)
statements, which tend to show that asparagin accelerates the action of diastase,
are of importance in deciding as to HANSTEEN'S work (p. 151). [REINHARD and
SUSCHKOFF (1905) have still further called in question HANSTEEN'S results.]
HANSTEEN found : — that in the dark, proteid was produced from urea in the
presence of cane sugar just as well as with glucose ; that asparagin, glutamin (and
also, as previously mentioned, ammonium compounds) formed proteid only in
presence of glucose ; that proteid was formed from glycocoll only in presence of
cane sugar ; that as a rule no proteid was formed from nitrates, leucin, alanin,
creatin, together with the carbohydrates experimented on, although there are,
doubtless, other carbohydrates whose presence may make such a trans-
formation possible.

The chief result which HANSTEEN arrived at, viz. that in the dark proteid
is synthesized from amides and carbohydrates, has been confirmed by MALINIAK
(1900) by quantitative analysis. He observed, on supplying asparagin, that
synthesis of proteid took place in the dark in maize seedlings which had been
deprived of their endosperm, and also in etiolated leaves of Faba. The data ad-
duced to prove these facts are by no means very convincing, and the experiments,
as is often the case, were carried out on too miniature a scale. In opposition to
HANSTEEN, MALINIAK found synthesis of proteid from asparagin taking place
just as well in the presence of glucose as of cane sugar. It has been shown also
that synthesis of proteid takes place in the dark in resting and also sprouting
bulbs, tubers, and roots, without any absorption of nitrogen from without,
and without any increase in nitrogen ; this has been demonstrated recently
in a series of researches by ZALESKI (1901) and IWANOFF (1901 a) who employed
exact methods of chemical analysis. Whence these proteids arose is not
certain, but in every probability from amides. [In young seeds, also, ZALESKI
(1905) has proved the synthesis of proteid from albumoses, amido-acids, amides,
and organic bases in the dark.]

It may be seen from these remarks how little we really know on these
problems, and how desirable it is that some one should produce a really ' clas-
sical ' work on the subject, for it is impossible to give a complete picture of
the process of nitrogen assimilation based on such literature as we have been
hitherto considering. A complete exposure of the numerous contradictions
occurring in the literature at present available cannot be undertaken at present
and hence much research, which may afterwards turn out to be of the utmost
importance, has not been referred to at all.

Just as in the case of the carbon and nitrogen, so also the materials of the
ash are ' assimilated ' in the plant ; the majority of them, at least, are
probably built up into organic compounds. Since, however, we are for the
most part completely in the dark as to which elements of the ash are of service
in the assimilation of organic substances, a discussion of the process of assimi-

THE ASSIMILATION OF NITROGEN IN AUTOTROPHIC PLANTS 145

lation of the majority of these minerals is as yet out of the question ; we may,
therefore, content ourselves by briefly summarizing the more important data
available as to the assimilation of sulphur and of phosphorus. These elements
claim at least a word, since the former occurs in all proteids and the latter in
certain of them.

The source of the sulphur in proteid is exclusively the sulphates absorbed
by the root. The sulphates must certainly be reduced in the process of proteid
synthesis, but where, and under what conditions this reduction takes place, we
are quite ignorant. The same difficulty which we met with in discussing the
assimilation of nitrogen meets us also in an even more pronounced form when
we undertake an investigation into the mode of assimilation of sulphur ; for half
of any proteid, roughly speaking, consists of carbon, 15-19 per cent, consists
of nitrogen, but only 0-4 to about 2 per cent, consists of sulphur. If we write
the formula of serumalbumin, as HOFMEISTER does, as C450HTOON116S6O140 (com-
pare COHNHEIM, 1900), and assume that similar proteids also occur in plants,
it is obvious that 75 atoms of carbon must be assimilated for every atom of
sulphur. The consumption of sulphates in proteid synthesis must thus
obviously be very limited. SCHIMPER (1890) considered that the assimilation
of sulphuric acid also took place in presence of chlorophyll and under the
influence of sunlight, but his assumption is by no means well founded, postulat-
ing as it does in general the same conditions as were pertinent to the assimila-
tion of nitric acid.

Phosphorus also occurs in the molecules of certain proteid bodies ;
it is absorbed only in the form of phosphate, and it would appear
that the molecule of phosphoric acid becomes incorporated in the proteid
molecule without essential modification, at least without any reduction.
According to POSTERN AK (1900) the assimilation of phosphorus takes place
in the leaf by the direct union of phosphoric acid and formaldehyde.
The compound so produced, oxymethylphosphoric acid (H3PO4 — CH2O),
POSTERNAK claims he has found in the plant, but it is questionable whether
it is a first product of assimilation (compare IWANOFF, 1901 b). In addition
to the proteids which contain phosphorus, nucleo-albumins and nucleo-proteids,
phosphoric acid occurs in lecithins which contain no sulphur ; these latter bodies
are very prevalent in plants (SCHULZE, 1894), and, according to STOKLASA
(1893), may also arise in the chlorophylliferous leaf. Sulphur is, moreover, not
limited to proteid, it occurs also in other substances of limited distribution,
such as oil of mustard (C3H6NCS) in Cruciferae, allyl sulphide (C3H5S), in species
of Allium; nitrogen also is not confined to proteids and their anastates, but
appears to be used in the construction of the widely distributed alkaloids and
certain glucosides as well. As we know little or nothing as to the mode of
formation of these bodies it is useless for us to study them in further detail at
present.

Summarizing what we have learned from the last few lectures we may say :
that the carbon-dioxide of the air is the only source of carbon available to
green plants ; that they convert it into carbohydrates under the influence of
sunlight and in the presence of chlorophyll, from which carbohydrates starch
is produced as a product specially worthy of note ; that nitric acid is the chief
source of nitrogen, and that that element in co-operation with the carbo-
hydrates goes to form proteid especially. So far as we know, most cells can
carry out synthesis of proteid without requiring sunlight as an essential con-
dition of the process. It would also appear probable that a very large part of
the proteid is formed in the leaf. In this capacity for assimilating carbon-dioxide
and nitric acid the green plant stands in striking contrast to the higher
animal, which is unable to construct either carbohydrate or proteid out of such
simple compounds. It would be quite a mistake, however, to emphasize this

JOST L

146 METABOLISM

as a fundamental difference between plants and animals, since there are plants
which are quite unable to construct proteids out of nitrates and others which
require to be fed on carbohydrates previously synthesized. On the other hand
it would appear probable that more accurate study of the lower animals will
lead to the conclusion that the animal world also includes forms which more or
less closely resemble green plants in so far as their nitrogen requirements
are concerned.

Bibliography to Lecture XI.

BAUMANN. 1887. Versuchsstationen, 33, 247.

BERTHELOT and ANDRE. 1884. Compt. rend. 99, 683.

BOUSSINGAULT. 1860. Agronomic, Vol. I.

BOUSSINGAULT. 1 86 1. Agronomic, Vol. II.

COHNHEIM. 1900. Chemie d. Eiweisskorper. Braunschweig. (RoscoE-ScHOR-

LEMMER, Lehrb. d. Chemie, Vol. IX.) [2nd ed. 1904.]
EMMERLING. 1884. Versuchsstat. 30, 109.
EMMERLING. 1887. Ibid. 34, i.
EMMERLING. 1900. Ibid. 54, 215.
FRANK. 1888. Landw. Jahrb. 17, 421.
FRANK and OTTO. 1890. Ber. d. bot. Gesell. 8, 331.
[GODLEWSKI. 1903. Bull. Acad. de Cracovie, Math.-nat. Cl. p. 313.]
GREEN- WINDISCH. 1901. Die Enzyme. Berlin.

HAMMARSTEN. 1895. Lehrb. d. physiol. Chemie. Wiesbaden, 4th ed.
HANSTEEN. 1899. Jahrb. f. wiss. Bot. 33, 417.

HOFMEISTER. 1902. Ascher-Spiro, Ergebnisse d. Physiologic i, i (Biochemie), 759.
IWANOFF. 1901, a. Versuchsstationen, 55, 78.
IWANOFF. 1901, b. Jahrb. f. wiss. Bot. 36, 355.
KERNER. 1887. Pflanzenleben, i, 60.
KOSSEL. 1901. Ber. d. chem. Gesell. 34, 3214.

LAURENT, MARCHAL and CARPIAUX. 1896. Bull. Acad. Belg. Ill, 32, 12.
[LAURENT and MARCHAL. 1904. Bull. Acad. Bruxelles, Cl. d. Sc. p. 55.]
LIEBIG. 1840. D. organ. Chemie in ihrer Anwendung auf Agrikultur.
LUTZ. 1899. Annales Sc. nat. vn, 7, i.
MALINIAK. 1900. Revue de Bot. 12, 337.

[MARCANO and MUNTZ, 1889. Compt. rend. Acad. Sc. Paris, 108, 1062.]
MAYER, AD. 1901. Agrikulturchemie, 5. Ed. Heidelberg.
MAZE. 1900. Annales Instit. Pasteur, 14, 26.

MOLISCH. 1887. Sitzungsber. Wiener Akad., Math.-nat. Cl. 95, i, 221.
[MuNTZ. 1889. Gompt. rend. Acad. Sc. Paris, 108, 900.]
PITSCH. 1887-96. Landw. Jahrb. 34, 217 ; 42, i ; 46, 357.
POSTERN AK. 1900. Revue ge*n. de Bot. 12, 5.

[REINHARD and SUSCHKOFF. 1905. Beihefte bot. Centrbl. 18, i, 133.]
REINKE. 1883. Quoted from Theoret. Biologic (1901), p. 236.
SCHIMPER. 1888. Bot. Ztg. 46, 65.
SCHIMPER. 1890. Flora, 73, 207.
SCHLOSSING. 1874. Compt. rend. 78, 700.
SCHULZE. 1894. Versuchsstat. 43, 367.
SCHULZE. 1902. Ibid. 56, 97.

STOKLASA. 1898. Zeitschr. f. physiol. Chemie, 25, 398.
SUZUKI. 1898. Bot. Centrbl. 75, 289.
TREUB. 1895. Annales Jardin Buitenzorg, 13, i.
[TREUB. 1905. Ibid. 2nd ser. 4, 86.]
[TREBOUX. 1904. Ber. d. bot. Gesell. 22, 570.]
WINTERSTEIN. 1901. Ber. d. bot. Gesell. 19, 326.
ZALESKI. 1900. Bot. Centrbl. 87, 277.
ZALESKI. 1901. Ber. d. bot. Gesell. 19, 331.
[ZALESKI. 1905. Ibid. 23, 126.]

147

LECTURE XII
THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. I

THE DISSOLUTION OF STARCH IN SEEDS BY MEANS OF ENZYMES

CHLOROPHYLL is not developed in all the cells of the higher plant ; in
the petiole and most stems it is small in amount as compared with the colour-
less protoplasm, while subterranean parts, such as roots, rhizomes, &c., are
quite free from it. Cells without chlorophyll, however, as we have seen, are
entirely incapable of forming carbohydrates from carbon-dioxide, and conse-
quently they are forced to obtain it from green cells. It necessarily follows
that the products of assimilation in the leaf blade must migrate from it to
undergo conversion elsewhere. This is probably true only of the carbo-
hydrates, since, to all appearance, with their aid, cells free from chlorophyll are
also capable of forming proteids in the dark. Should proteids also arise in
the leaf to any great extent they too probably migrate. Migration of the pro-
ducts of assimilation out of the leaf may also be deduced from other evidence.
Leaves in their earliest stages of development are colourless, and consequently
must depend for their further growth on external supplies of organic material ;
at a later date, after the formation of chlorophyll, they begin to exhibit the
phenomenon of carbon assimilation, and we must conclude that they employ
the products of that assimilation in the first instance for their own construction.
After a short time, however, a stage in development is reached when the leaf
has attained its definite size, and then arises the question, what becomes of
the products if they be not translocated ? Experience teaches us that accumu-
lation of starch in the confined limits of the chloroplast limits its assimilatory
activity, but, as a matter of fact, such an injurious accumulation of starch
does not take place under ordinary circumstances, because any excess under-
goes translocation. We shall find in the next lecture that a leaf filled with
starch may often lose it all in the course of a single night, and the fact that in
isolated leaves no such disappearance of starch takes place demonstrates that this
disappearance in the normal leaf is due not to a consumption of the starch
in situ, but to its transference from the leaf to the stem by way of the petiole.

Starch is, however, solid, insoluble and incapable of migrating either actively
or passively from cell to cell. Its translocation is possible only in the form of
a soluble carbohydrate, after undergoing chemical alteration. There is no fact
more clearly established in vegetable physiology than this, that a supplyof soluble
organic plasta passes from the assimilating leaf to regions of the plant which
have themselves no power of bringing about carbon assimilation. A study of the
transformations which these organic substances undergo, reveals to us certain
functions performed by them, of which the following are the most important : —

1. The products of assimilation in the leaf blade act as constructive mate-
rials ; they are transferred to wherever the plant is using these bodies — to
the growing points of the stem and root and also to the cambium. In these
situations, the organic materials manufactured by the leaf are devoted to the
formation of new cells.

2. The products of assimilation act as reserve substances , either where they
are formed or, after translocation, are stored up more or less permanently in
other situations. Such reserves are afterwards converted into plasta and
employed as constructive materials or for other purposes.

3. The products of assimilation are oxidized, and in consequence become
once more altered into simple inorganic bodies such as are used by the leaf in
the manufacture of organic compounds. Katabolic processes, such as these,
are inseparable from all vital activities. Those substances which are sacrificed
in this way may be termed working materials.

4. Since the conversion of materials is accompanied by translocation we

L2

148 METABOLISM

may consider, finally, translocation products. It must be noted, however, that
this subdivision, based on the function of these substances in the plant, gives
no indication whatever of the chemical nature of the compounds. The four
different types of material may be chemically distinct but they need not ;
glucose, for example, may occur as a primary assimilation product, as a trans-
location compound, as a plastic substance, a reserve, or as a source of energy.

It would be obviously most natural to commence our study of the migra-
tion and translocation of the products of assimilation by observation of the
method by which such substances are removed from the leaf ; but for many
reasons it is advisable to begin with the reserves, which are usually regarded as
assimilatory products redeposited in ' secondary storehouses '. Reserves are
deposited in these storehouses in such quantities that plants are often able at
their expense, to develop to a considerable extent in darkness without needing
to have recourse to any direct products of assimilation. Leaves, on the con-
trary, contain very little in the way of reserves, are rapidly deprived of them,
and are liable to injury if kept in darkness for any length of time. Conse-
quently the most important researches have been carried out on storehouses
of reserves, and more especially on seeds. When we have mastered the con-
ditions which prevail in these structures the transformations which take place
in the foliage leaves will be easily understood.

The most important constituent of every seed is the embryo. It consists of
a small, often microscopic young plant, in which we may distinguish one or two
more or less well developed cotyledons, all other parts being still in an embryonic
condition. Between the cotyledons may be distinguished the plumule, or growing
point of the stem, surrounded by a few leaves, and, at the other end, the
growing point of the root, the radicle. The whole embryo is in general enclosed
in a special tissue, the endosperm, and that in turn by a seed-coat. When the
seed is separated from the parent it cannot at once undergo development, for it
is deficient in moisture, without which growth is impossible. In addition to
certain other external factors which are conditions of germination, viz. warmth
and oxygen, water is primarily essential ; when that is supplied the seedling
begins to grow. Generally speaking, the root bursts the seed-coat and imbeds
itself in the soil ; later on, the plumule is extended and gives rise to leaves above
ground. As soon as the leaves develop a green colour on exposure to light, the
plant becomes independent and can nourish itself by products which it itself
has manufactured, but its entire development up to this stage is possible only if it
be provided with reserves supplied to it by the parent. These reserves are as
a rule capable of supplying all that is necessary for much later stages in the
development of the seedling, so that from large seeds, such as those of the bean,
plants of considerable dimensions may be produced in the dark, entirely at the
cost of these reserves. The reserves are often deposited in the seedling itself,
and the relatively bulky seed-leaves are frequently the seat of such deposition.
The endosperm, however, a tissue external to the seedling may be the seat of
deposit of such reserves. It is unnecessary for us to enter on the discussion of
such purely morphological matters as the difference between endosperm
and perisperm, nor need we concern ourselves with the question as to why
some plants deposit their reserves wholly or in greater part in endosperm, or
only in cotyledons or in both situations. For the purposes of physiology it is
sufficient for us to know that such reserves are placed in the neighbourhood of
the growing parts of the seedling. It is important, however, that we should
become acquainted with the chemical nature of these reserves.

In seeds we meet with organic substances as well as minerals as reserves,
and the former we recognize as of two kinds, one nitrogenous and the other
non-nitrogenous. These three kinds of materials are not, however, always
stored up in seeds in the proportions in which the young plant makes use of
them. If that were the case the assimilated reserves at any given time

THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. I 149

would be used up in equal proportions, as when germination takes place in
darkness and when the seedling is provided with distilled water only. In
nature, the root, as we have said, generally forces its way at once into the soil
and draws therefrom the necessary minerals, and since its duty is to supply
the seedling with such materials from the very first, naturally we need not
expect more than a trace of minerals in the seed itself. This is the reason why
GODLEWSKI (1879) found that seedlings of Raphanus developed far better in the
dark when he provided them with nutrient salts than when he gave them
distilled water only. It was only then that the seedlings could make full use
of the organic reserves and so attain twice the weight they reached when
grown in pure water. If excess of non-nitrogenous reserves be supplied and no
nitrates be given, or if these bodies cannot be assimilated sufficiently in the
dark, growth in the long run comes to a standstill. The converse is true of
many Leguminosae ; although development is inhibited in darkness, one still
finds in the seedlings nitrogenous organic substance accumulated in quantity
out of all proportion to the amount of non-nitrogenous substances present.
The degree of development is determined here also by the nutrient present
in minimum quantity (compare Lecture VII, p. 83).

The reserves in seeds are bodies either entirely insoluble in water or charac-
terized by having very large molecules (colloids). This has two advantages ;
in the first place, substances containing no water take less room, and in the
second, the high osmotic activity of concentrated solutions of crystalloids is
avoided. We shall find that non-desiccated storehouses behave quite differ-
ently. The non-nitrogenous reserves which occur most commonly in seeds are
starch, cellulose, and fat, the nitrogenous bodies are represented by proteids. In
order to understand clearly how the dissolution and translocation of reserves
is effected it will be most convenient for us to begin with a consideration of
starch, for it is not only a very common reserve, but has also been the most
thoroughly studied.

As already remarked, starch is insoluble in water unless chemically altered.
Such alterations as give rise to soluble products may be effected, apart from the
plant, in very various ways. Water at a high temperature acts in this way,
causing starch to turn first of all into a paste and finally altering it into dextrin
and dextrose. A similar decomposition is induced by mineral acids (e.g. hydro-
chloric acid), especially if these be warm. Other products are formed, however,
when starch is dissolved in alkalis, calcium nitrate, chloralhydrate, &c. In the
plant, starch, whether it be growing or whether it be dissolved, is surrounded by
the chromatophore and cannot come in contact either with acids or alkalis ;
its dissolution is effected in the plant by means of a substance with quite peculiar
properties, viz. diastase, belonging to the physiological group of enzymes
or ferments (compare SCHLEICHERT, 1893). Diastase is a product of the activity
of the organism, but is capable of carrying out its functions apart from it. The
most convenient method of obtaining diastase for study is to take some seeds
containing abundant starch, such as barley, a short time after the commence-
ment of germination, grind them down and extract them with water at a tem-
perature of about 50° C. The diastase and other soluble bodies dissolve in the
water, and we thus obtain a barley or malt extract for purposes of investigation.

On treating starch grains with this extract, we find that they gradually
dissolve in precisely the same way as they do in the uninjured germinating
seed (Fig. 32, p. 155). We may obtain a knowledge of the resulting products
more readily by investigating the alteration effected in starch paste. Employ-
ing the iodine test we find that the original blue reaction rapidly gives place
to a wine-red coloration. Finally, this latter reaction also disappears. Even
without using iodine the fluid exhibits a marked alteration in appearance.
Originally it is semi-fluid and opalescent ; now it becomes transparent and
quite watery. The starch, as such, has disappeared, and dextrin and maltose

150 METABOLISM

take its place (LINTNER and DULL, 1893 ; A. MEYER, 1895). Maltose betrays
its presence by the fluid being capable of reducing alkaline copper sulphate
(Fehling's solution).

We do not yet know exactly how the change into sugar is effected in all
cases, but we must assume that the decomposition of starch is a gradual one,
dextrin being formed first of all, which later is changed into maltose. By treat-
ment with iodine it is often possible to distinguish a series of dextrins, but we
are quite unable to say in what relation these stand to starch. They appear
to have the same chemical composition as starch, differing from it only in the
smaller size of their molecules. The dextrin molecule is still a large one in
comparison to that of maltose, its molecular weight being about eighteen
times as great. The formation of maltose appears to be effected according to
the following equation : —

C216H3W0I80+i8H20 == i8(C12H32On).
Dextrin Maltose.

On the absorption of water a hydrolytic decomposition takes place ; but it
does not appear impossible that a similar hydrolysis occurs previously in the
formation of dextrin itself.

The transformation of starch into sugar by the agency of malt extract
can be demonstrated in a few minutes in a test-tube kept at suitable tem-
perature. As in the case of germinating barley, so other germinating seeds yield
diastase after digestion with water or glycerine ; moreover, diastase may also
be shown to occur in many other amyliferous plant tissues as well as in digestive
secretions in the animal body (saliva ; pancreatic secretion). There is no doubt,
however, that diastase is by no means the same in character in each case ;
indeed, profound differences have been discovered to exist, not merely as re-
gards the products of the reaction but also with relation to the influence of
external factors. To all appearance there are several kinds of diastase. It is
very likely, for example, that the transformation of starch into dextrin is
effected by a diastase differing from that which decomposes dextrin into maltose.
By heating the malt extract to a temperature of about 80° C. (compare DUCLAUX,
1899, 400), its capacity for forming maltose is destroyed, although the formation
of dextrin still continues. Further, the decomposition of dextrin does not al-
ways occur in the same way ; often maltose only is produced, at other times
glucose appears as well. In the latter case a hydrolytic decomposition of the
maltose molecule into two molecules of grape sugar takes place. Those dia-
stases which produce maltose only may be distinguished from each other by their
intermediate products (compare BEIJERINCK, 1895).

If we now compare the effect of diastases with that of a hydrochloric acid
solution, it would appear that the former have a more limited activity than
the latter. While one acid is sufficient to transform starch into glucose, three
different diastases are required, each having a restricted but definite part to
play in the total result. The same is true of other kinds of enzymes. The
enzymes are thus much more delicate agents than the acids, and to this is due
the importance attached to them in modern physiological chemistry.

Inquiring now into the chemical characters of diastase, we have first of all
to note that our malt extract is by no means a pure solution of diastase, for as
yet it has not been found possible to isolate it completely from the other con-
stituents of the extract. . If we add alcohol to the solution a precipitate is
obtained which gives the proteid reaction, and which, when dissolved in water,
exhibits the same power of dissolving starch that diastase has, though to a
rather less degree. If this solution be heated above 80° C., proteid, and with it
the diastase, separate out ; the latter no longer, however, has the power of dis-
solving starch. It might be thought, therefore, that diastase was a proteid which
is coagulable at high temperatures, but this view cannot be taken as proved,
for diastase might have, chemically, nothing to do with the proteid. It might

THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. I 151

be a body of quite unknown composition, not as yet isolated from the proteid,
Since very small quantities of diastase possess great hydrolytic powers, the
actual diastase itself may form merely a trifling impurity in the proteid
obtained by alcoholic precipitation from the malt extract.

What has especially attracted the attention of physiologists to diastase is,
on the one hand, its behaviour at different temperatures, and on the other, its
action on certain substances. At o° C. the dissolving action of diastase on
starch is scarcely noticeable ; an increase in temperature is followed by a rapid
increase in its activity, until at 50° C. it reaches a maximum maintained till
63° C. is reached ; if it be heated still further, the activity of diastase again de-
creases, until finally, at about 85° C., its power becomes destroyed (KJELDAHL,
1879). If we construct a graphic curve (Fig. 30) whose abscissae indicate
degrees in temperature and whose ordinates show the amount of starch dissolu-
tion effected by the diastase, it will be found that the curve bears a strong
likeness to those other curves which express the dependence of various functions
of the living plasma on temperature, such as we have still to study in growth
and movement, and which we have already seen in the case of carbon assimilation.
The maximum, minimum, and optimum points
of this curve given by different diastases
are not always coincident (LINTNER and ECK-
HARDT, 1890). The diastatic curves differ
from other physiological curves chiefly in the
fact that the optimum point always stands
very high ; indeed, it stands so high that it is
never reached in the plant, since at 5o°-6o° C.
carbon assimilation is impossible and gene-
rally the limits of life itself are reached or ex-
ceeded before the Optimum effect is Obtained, diastase on temperature. After KJELDAHL

In considering the influence on diastase (I 79

of certain substances we will begin with those which tend to retard its activity,
and which act on diastase just as poisons do on protoplasm. According to BOKOR-
NY (1901), formaldehyde is to be considered in this light, since, even in a con-
centration of o-oi per cent., it affects both protoplasm and diastase injuriously,
after it has operated for a certain time. Diastase, however, responds to the
majority of poisons in a different manner from protoplasm, viz. in being
much less sensitive to them. While the latter is destroyed by very minute
quantities of corrosive sublimate (0-00005 percent.) and silver nitrate (o.oooooi
per cent.), a concentration of o-oi per cent, in both cases is required to produce
a poisonous effect on diastase. We know also that by introducing certain
poisons in sufficient quantity it is possible to kill micro-organisms, and yet
the enzyme remains fully active. Since, however, micro-organisms, as we
shall see by and by, can greatly influence experiments with malt extract, their
exclusion becomes of very great significance. Thymol or chloroform is usually
employed for this purpose, not corrosive sublimate.

In contrast to these inhibitory poisons, other substances are known which
act in the highest degree as accelerating agents. Generally speaking, all addi-
tions of free mineral acids act in this way, if they be present in traces only ; so
also do neutral salts (e. g. sodium chloride) in somewhat larger doses, and
finally, salts of aluminium, phosphoric acid compounds, and asparagin in suffi-
ciently high concentrations. Thus EFFRONT (cited by GREEN, 1901) found
that in a certain time, the following unequal amounts of maltose were pro-
duced by a malt extract from starch paste : —

1. Without any addition 8-63 maltose

2. With addition of 0-5 % calcium phosphate 46-12 „

3. „ „ 0-25 % ammonium alum 56'3° »

4. „ ,, 0-25 % aluminium acetate 62-40 ,.
5- „ „ 0-05% asparagin . . 61.20 „

Fig. 30. Dependence of the activity of malt
Kjl

152 METABOLISM

HANSTEEN (p. 144) has drawn certain conclusions as to the formation of
proteid from the disappearance of starch on the addition of asparagin ; these
researches show, however, that he was only dealing with an acceleration of the
diastatic activity by asparagin.

Whether such an accelerating agent be present or not, a solution of diastase
is unable to transform the whole of the starch into maltose — usually a certain
amount remains in the form of dextrin. Doubtless the reason for this is not
that the diastase is used up after the dissolution of a certain quantity of starch,
but that its power of transforming starch into sugar is inhibited by the accumu-
lation of the products of the reaction. If adequate arrangements be made for the
removal of the sugar formed, all the dextrin is finally turned into maltose,
and theoretically only a small amount of diastase is necessary to transform an
unlimited quantity of starch, without its losing its diastatic power in the process.

Quite a number of peculiarities, which we have now recognized as belonging
to the diastases, are found in other substances formed by the organism, and to
these bodies has been given the name of enzymes or ferments. Those with
which we are at present concerned induce hydrolytic decompositions ; later on we
shall have to study enzymes which bring about decompositions otherwise than
by hydrolysis (Lecture XVI). These enzymes act in very small quantities and
take no part, or at least no permanent part, in the reaction. The reaction is
always incomplete and may be accelerated or retarded by certain substances ;
their activity is dependent on temperature in the same way as we have seen
that of diastase to be. The enzymes may be extracted from the organism by
water or by glycerine, from which media they may be precipitated by alcohol.

Each individual enzyme apparently attacks only one or at most a few
related bodies. We may distinguish at least five classes of enzymes, although
apparently their number is much greater and their spheres of operation much
more limited : —

1. Amylases, or diastases, which transform starch into sugar.

2. Cytases, which manufacture sugar from cellulose and the other carbo-
hydrates associated with it in forming the cell-wall.

3. Invertases, which change disaccharides into monosaccharides, e. g. cane
sugar into dextrose and levulose, maltose into two molecules of dextrose.

4. Lipases, which break up fats into glycerine and fatty acids.

5. Proteases, which act on proteids and produce from them diffusible
bodies already enumerated elsewhere (p. 140).

In addition to these specific (hydrolytic) activities, the enzymes (all?)
possess the power of splitting off oxygen from peroxide of hydrogen. In many
respects, more especially in their dependence on temperature and many chemi-
cals, the enzymes resemble organisms themselves, and it has for long been the
custom to regard them as portions of the protoplasm, or, at least, as very highly
complex substances. But it is by no means necessary to make such an assump-
tion, since enzymes resemble in many ways a series of inorganic bodies which
possess peculiar characteristics, and which are known as catalytic agents, and
it is now the custom more than ever to regard the activities of enzymes as
catalytic in their nature.

Catalytic agents are those which alter the rate of a reaction without them-
selves entering into the final product (OsrwALD, 1902). The catalytes which
specially interest us for the moment are those which accelerate reactions. As
a type of such catalysis we may take the decomposition of hydrogen-peroxide
into oxygen and water in the presence of finely divided metals. Peroxide of
hydrogen, it is true, also decomposes spontaneously, but the separation of
oxygen takes place much more rapidly in presence of the metal, a very small
quantity of the catalyte decomposing a very large amount of the peroxide
without suffering any loss of power in the process. But the catalytic value of
the metal is entirely dependent on its finely divided condition ; platinum wire

THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. I 153

is quite useless for this purpose, but platinum black or so-called colloidal
platinum is, on the other hand, exceedingly active. BREDIG (1901) has obtained
such colloidal solutions by electric spraying. Platinum and other metals may
be broken up by kathodic spraying under water into particles so fine that they
remain in suspension in the water, and cannot be seen even with the best micro-
scope ; such a fluid we term a colloidal solution. BREDIG has made a number
of experiments with such solutions, which are of extreme interest to the phy-
siologist. He was able, in the first place, to estimate more exactly than pre-
viously the amount of platinum necessary, and showed what was the minimum
amount which could act catalytically on hydrogen-peroxide, viz. 0-000003 mg-
From his further researches the catalytic effect of certain substances may be
estimated which are known to be inhibitory. Minute quantities of sulphuretted
hydrogen, bisulphide of carbon, hydrocyanic acid, &c., destroy the catalytic
power of the platinum solution, but they do not do so permanently ; after
removal of the substance the catalysis begins anew.

BREDIG finds a strong likeness to exist between enzymes and such colloidal
metal solutions, so much so, indeed, that he terms the latter ' inorganic en-
zymes '. This likeness is expressed in their colloidal form, their mode of action,
and the effect produced upon them by the substances named. We must
leave as a debatable question whether we may correctly designate the sus-
pension of platinum particles in water as a colloid corresponding to organic
substances capable of swelling and confine ourselves to the consideration of
the other two points of comparison. In addition to the catalytic action of
apparently all of them on hydrogen-peroxide * enzymes have over and above
specific effects on definite substances, but these specific effects do not appear
to us to be as yet proved to be possessed by a colloidal solution of platinum
(compare Zeitschr. f. phys. Chem. 31, 262, note). The specific hydrolytic
action of the enzyme cannot have anything to do with the decomposition of
hydrogen-peroxide ; the one action may be differentiated from the other by
heating to a certain temperature QACOBSON, 1892). Pancreatic secretion
for example after heating to a temperature of 61° C. can still transform starch
into sugar, but it can no longer abstract oxygen from peroxide of hydrogen.
It is very obvious that by this means we have separated out a substance
which, in its behaviour it is true, shows a strong resemblance to colloidal
platinum, and that the actual enzyme remains uninjured. We arrive at similar
conclusions on a closer analysis of the action of the poisons mentioned above ;
of these, hydrocyanic acid is especially a case in point, because it acts poison-
ously on enzymes in the same way as it does on colloidal platinum. In reality,
the hydrocyanic acid affects only the activity of the impure enzyme on per-
oxide of hydrogen, and leaves the specific action of the enzyme quite intact
( JACOBSON, 1892). Further differences between BREDIG' s ' inorganic enzymes '
and organic enzymes may perhaps come out on a closer study of the effect
of temperature on the reactions. At present, at all events, it is by no means
certain whether the so-called inorganic enzymes show a temperature curve
with minimum, optimum, and maximum points, but if this be considered of
secondary importance we have, at least, one other difference of greater weight,
i. e. the close of the reaction. The platinum solution remains active as long as
a trace of peroxide is present ; in other words, the reaction is complete ; in the
case of enzymes, however, the reaction (see p. 152) is incomplete unless the pro-
ducts be withdrawn. It would not be out of place here to go into the question
of the incompleteness of the enzyme reaction, but it is possible in the present
condition of the science only to draw attention to the diametrically opposite views
advanced on the question, and on which no decision has as yet been reached.

The reason for the incompleteness of the reaction generally lies in this, that
it does not consist of one reaction only, but of two processes, which induce
opposite changes and which lead to an equilibrium at a definite temperature.

154 METABOLISM

Thus, in the hydrolytic decomposition of an ester by hydrochloric acid, alcohol
and acid are formed, but the alcohol unites again with the acid, water being
given off, and an equilibrium is brought about if the formation of the ester goes
on as rapidly as its decomposition. If enzymes behave in the same way as the
hydrochloric acid in this example, then they must be able to induce not only
a hydrolysis, but, under certain conditions, a synthesis also. Something
like this has been observed by HILL (1898). He obtained an enzyme
from yeast, which changes maltose into dextrose, allowed it to act on a 40 per
cent, solution of dextrose, and found that after a long time 14-5 per cent, of
the dextrose was changed into maltose. A synthesis took place when water
was withdrawn, and an equilibrium was reached when 14-5 per cent, of the
sugar was composed of maltose and 85-5 per cent, of dextrose. The con-
dition of equilibrium depended essentially on the concentration of the solution,
as the following summary shows : —

Amount of dextrose
originally present.

40%

Amount of dextrose after the
action of the enzyme.

85-5%
90-5,,
94-5 »
98-0,,
99-0,,

Amount of maltose after the
action of the enzyme.

14-5%
9-5,,
5-5 »

2-0 „

The more dilute the solution of dextrose the less the amount of maltose
formed. HILL'S work has been confirmed in many respects by WENT (1901),
and a reversible action has been established in the case of other enzymes as well
(lipase, HANRIOT ; maltase, EMMERLING, 1901). In spite of this reversible action,
it is quite obvious that we, as a rule, see only one aspect of the enzymic activity,
i. e. hydrolysis, and that when the products of the reaction have been
effectively removed, a complete hydrolysis of the products of the process will
be found to have taken place.

The view advanced by HILL is opposed to that taken by TAMMANN (1892).
According to this investigator no reversible action occurs during enzymic activity.
He holds that when hydrolysis remains incomplete, the reason is that the enzyme
becomes altered into an inactive variety under the influence of the accumu-
lation of decomposition products. Further research is needed to determine
which view, HILL'S or TAMMANN' s, is the right one ; but if, as we have no doubt,
HILL'S theory be correct, then BREDIG would have to show that a formation
of hydrogen-peroxide took place in presence of his ' inorganic ferments ', if he
desired to establish a comparison between these bodies and organic ferments.

The similarity between these substances does not seem to us to be very
great, and to be confined to the fact that enzymes and platinum solutions are
catalytic agents. Catalytic substances, however, belong to very diverse cate-
gories (OSTWALD, 1902). We might, indeed, doubt whether enzymes are cata-
lytic agents at all, seeing that many of the decompositions in question are not
perceptible in the absence of the enzyme. Starch, for example, under ordinary
conditions, produces no maltose in water ; and it would appear doubtful
whether we are entitled to regard the action of the enzyme merely as a case of
acceleration of a previously existent process. If we find, however, that
hydrolysis certainly occurs at higher temperatures without an enzyme being
present, and that the reaction gradually ceases when the temperature is
lowered, we cannot deny spontaneous hydrolysis at ordinary temperatures,
even if the products arising from the action do not make themselves apparent
perhaps till years afterwards. At all events the classification of enzymes in
the category of catalytic agents is the best hypothesis we can put forward
at present.

After these general remarks on enzymes and on diastase in particular, let
us return to the consideration of the germination of seeds. Dissolution of
starch may be studied best in the seeds of Gramineae, which possess an especially

THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. I 155

abundant supply of this reserve (about 80 per cent, of the dry weight). Fig. 31
shows a longitudinal section through the lower end of a grain of wheat. Within
the testa, which is in this case fused with the wall of the fruit, we observe the
embryo (Em), well developed and possessing a special organ, the so-called
scutellum (Sc), lying against the abundant endosperm (End). The contents of
the cells of the endosperm are not homogeneous. A single layer of peripheral
cells (the aleurone layer, Al) containing aleurone grains, lies immediately within
the testa, while the large central mass is packed with starch. Diastase may be
demonstrated even in the resting endosperm, and it becomes very noticeable
when water is absorbed at the beginning of germination. It produces a disso-
lution of the starch which may become very extensive if the maltose which
arises in the process be removed. In normal germination this is effected by the
seedling absorbing greedily the sugar presented to it by means of the super-
ficial layer of the scutellum. If the embryo be removed, not only does the
translocation of the sugar come to an end, but its formation also ceases and the
starch grains remain intact. HANSTEEN (1894) and PURIEWITSCH (1897) were
able to show that an emptying of the endosperm cells took place in the absence
of an embryo, if the seed were placed in contact with a large quantity of water

Fig. 31. Longitudinal sec-
tion through the lower part of
a grain of wheat ; End, endo-
sperm, Al, aleurone layer, £t»,
embryo, oVr, scutellum. After
SACHS (1862), slightly magnified.

Fig. 32. Starch grains from germinat-
ing barley. 1-4, successive stages in the
dissolution of the grain. From the Bonn
Textbook.

with due antiseptic precautions, in such a way that only a small part was sub-
merged. The experiment was so arranged that in place of the embryo a plug
of gypsum was applied to the endosperm where the scutellum had been, its
lower part being in contact with water. It was found in this way that various
grass seeds exhibited after about a week many corroded starch grains in the
endosperm (Fig. 32) ; after eight to fourteen days most of the cells were com-
pletely emptied, and a sugar, which was capable of reducing Fehling's solution,
could be demonstrated in the water which removed it from the endosperm.
Whether this sugar corresponded in amount to the starch which had disap-
peared seems not to have been determined, but obviously the determination of
this point is of great importance in coming to a decision on the question.
Further, a complete evacuation of the starch in PURIEWITSCH' s experiments
took a much longer time than in the case of the normal seedling. It is not
probable that the imperfect removal of the sugar formed was the cause of
this, although no other explanation is available at present. It has been
clearly proved (LiNZ, 1896) that the scutellum contains at any time more (or
more active) diastase than the endosperm, so that it may be that in normal
germination diastase from the scutellum penetrates the endosperm and there
assists in the dissolution of the starch. Although it cannot be doubted that

156 METABOLISM

such an excretion of diastase does take place from cells, and although it has
been shown that it is a phenomenon specially characteristic of the grass
embryo, e.g. by BROWN and MORRIS (1890), it cannot be said that the evidence
for it is altogether above criticism (Lraz, 1896 ; GRUSS, 1897).

As noted above, a reducing sugar was always found in the fluid used in
the experiments where the endosperms of grasses were allowed to undergo
transformation in absence of an embryo ; in addition, however, cane sugar also
appears in not inconsiderable quantity, that is to say, another disaccharide
capable of reducing only after treatment with hot acids. This sugar, according
to our present knowledge, cannot be produced by hydrolysis of starch, and so its
appearance requires to be explained. At present we must be content to believe
that it is present as such in the seed, a view not entirely improbable, seeing that
SCHULZE (1899) has found cane sugar in seeds of the Gramineae. Again PURIE-
WITSCH has shown, from other investigations, that the emptying of the endo-
sperm is not due simply to diffusion. He agrees with HANSTEEN in believing that
the emptying takes place more rapidly if large quantities of water be supplied
with the view of dissolving the sugar. The explanation seems simple enough :
diffusion can take place only so long as the fluid outside is less concentrated than
that in the cells ; for if the sugar formed in the cells cannot diffuse out then the
hydrolysis of starch ceases. Thus PURIEWITSCH found that far less starch was
dissolved from the endosperm when he used in place of water a 1-3 per cent,
solution of cane sugar or dextrose. Although it has not been definitely proved
to be the case that dextrose is actually formed from starch, one may still regard
these two sugars as the direct cause of the inhibition of diffusion. The situa-
tion is quite altered when glycerine, potassium nitrate, and sodium chloride
are used, for PURIEWITSCH found that these substances vigorously inhibited
the dissolution of starch. It is possible to explain this result physically,
but one may doubt the purely physical action of cane sugar and dextrose, and
we must remember, further, that the cells of the endosperm are living, and as
such are affected in a variety of ways by the environment. Sufficient evidence
is not forthcoming to show how it is that organisms are able to inhibit the
action of an enzyme, but we do know of many facts which prove to us that
they have this power. PURIEWITSCH, for example, found that the removal of
starch ceased when he supplied the endosperm (in the absence of the seedling)
with air free from oxygen, or with air containing chloroform ; and yet we
know that, apart from the cell, a solution of diastase acts equally well on
starch whether oxygen or chloroform be present or not. Experiments such as
these are full of lessons for us. It is only right that great stress should be laid
on the study of enzymes in modern physiological chemistry, since these bodies
obviously have important functions to perform in the organism ; but we must
not hope for too great results from such studies. We may learn in this way
what reagents the living cell uses in dealing with these substances, and so
imitate in many cases what goes on in the cell, by reproducing in test-tubes
the chemical transformations that go on in the organism, with the aid of acids
or enzymes ; but these transformations are not those which are characteristic
of the organism ; the secret lies in controlling the enzyme so that at one
moment it is active, at another it is quiescent. Further, we can give no
general answer to the question how an accumulation of the products of the
reaction interferes with the activity of the enzyme. Probably substances
acting as inhibiting agents or poisons to the enzyme play a principal part
in retarding enzymic activity (compare CZAPEK, 1903), but as yet we are
unable to understand how the organism governs the production at the same
time of an enzyme and an anti-enzyme. It cannot be doubted, however, that
such a co-ordination or appropriate production of these bodies really takes
place. Similar cases of regulation of secretions occur everywhere in organisms,
as we shall find later on.

THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. II 157

Bibliography to Lecture XII.

BEIJERINCK. 1895. Centrbl. Bakt., Abt. II, i, 221.

BOKORNY. 1901. Bot. Centrbl. 85, 293.

BREDIG. 1901. Anorganische Fermente. Leipzig.

BREDIG. 1902. Ergebnisse der Physiologic (Spiro-Ascher), i.

BROWN and MORRIS. 1890. Jour. Chem. Soc. Trans. 57, 458.

CZAPEK. 1903. Ber. d. bot. Gesell. 21, 229.

DUCLAUX. 1899. Traite de Microbiologie, II. Diastases. Paris.

EMMERLING. 1901. Ber. d. chem. Gesell. 34, 600 and 380.

GODLEWSKI. 1879. Bot. Ztg. 37, 97.

GREEN, J. R. 1901. Die Enzyme (German edition by Windisch). Berlin.

GRUSS. 1897. Jahrb. f. wiss. Bot. 30, 645.

HANRIOT. 1901. Compt. rend. Paris, 132, 146.

HANSTEEN. 1894. Flora, 79, 419.

HILL. 1898. Journ. Chem. Soc. Trans. 73, 634.

JACOBSON. 1892. Zeit. fur physiol. Chem. 16, 340.

KJELDAHL. 1879. Meddelelser fra Carlsberg Labor, i, 121.

LINTNER and DULL. 1893. Ber. d. chem. Gesell. 26, 2533.

LINTNER and ECKHARDT. 1890. Quoted in Koch's Jahresbericht iiber Gahrungs-

organismen.

LINZ. 1896. Jahrb. f. wiss. Bot. 29, 267.
MEYER, A. 1895. Die Starkekorner. Jena.
OSTWALD. 1902. Verhandl. d. Gesell. d. Naturforscher zu Hamburg 1901,

Leipzig 1902.

PURIEWITSCH. 1897. Jahrb. f. wiss. Bot. 31, i.
SCHLEICHERT. 1893. Nova Acta Acad. Leopold. 62, i.
SCHULZE. 1899. Zeit. f. physiol. Chem. 27, 267.
TAMMANN. 1892. Zeit. f. physiol. Chem. 1 6, 271.
WENT. 1901. Jahrb. f. wiss. Bot. 36, 611.

LECTURE XIII
THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. II

DISSOLUTION OF THE REMAINING RESERVES IN SEEDS
OTHER STOREHOUSES OF RESERVES

IN the course of their researches on starch dissolution in germinating barley,
BROWN and MORRIS (1890) found that the cell-walls of the endosperm were
also dissolved during germination. The cell-walls of the endosperm of barley are
relatively thin, and hence the sugar resulting from their dissolution cannot
play any very essential part in the nutrition of the seedling. Probably the
significance of the absorption of the cell-walls in this case lies merely in the fact
that the other enzymes are thus enabled to enter the cells more readily. The
walls of the endosperm cells of other seeds, on the contrary, are remarkably
thick, as, for example, in palms and many other Monocotyledons, and here also
a dissolution of the cell-walls takes place in germination, so that it is quite legiti-
mate to consider the materials of which they are composed as reserves, par-
ticularly as other carbohydrates are absent or present only in small quantity.
The chemical composition of the cell-wall is still imperfectly known. At least
two groups of substances take part in its formation — pectins and celluloses. The
former of these we need not discuss at present since the most divergent views
are held as to their chemical characters (SCHRODER, 1901 [compare CZAPEK,
Biochemie, I, 545]). The celluloses on treatment with acids give rise by hydro-
lysis to various types of sugar, dextrose, mannose, galactose, and, following
E. SCHULZE (1890-92), we may regard them as anhydrides of these hexoses as
well as of certain pentoses (arabinose and xylose). The cell- wall only rarely
consists of a single chemical compound ; in most cases it is formed of a mixture
of several. Such celluloses are deposited as reserves in seeds especially, giving

158 METABOLISM

rise to mannose and galactose in quantity but only to a little dextrose, on
treatment with dilute acids, but only if the acids be dilute. SCHULZE terms
them hemicelluloses in contrast to the true celluloses, which are capable of
hydrolysis only by the action of concentrated acids. These latter can ob-
viously be dissolved by the plant with great difficulty and never take any
further part in metabolism.

The celluloses are perhaps better characterized by their behaviour in the
presence of enzymes than in the presence of acids. Not only is this of more
importance biologically, but it gives us also a clearer insight into their chemical
nature. It is well known that the hemicelluloses are dissolved by enzymes in
seeds, although we have no precise knowledge of these enzymes. BROWN and
MORRIS (1890), from germinating barley, and NEWCOMBE (1889), from the
cotyledons of lupins, as well as from the endosperm and the cotyledons of
Phoenix, by the same method used in the extraction of diastase, have obtained
a soluble enzyme which dissolves the cell-walls of the endosperm of barley
rapidly, and more slowly the reserve cellulose of lupins. Although this power
of dissolving the cell-wall has been attributed for several reasons to diastase,
which as a matter of fact is never absent from such extracts, NEWCOMBE was
able to prove conclusively the presence of a special enzyme, cytase. Although
it has not been as yet possible to separate this enzyme from diastase, still NEW-
COMBE'S conclusion would appear fully justified because the amylolytic and
cytolytic capabilities of the extract are not at all proportional to each other,
the extract of lupins and of Phoenix having a very vigorous action on cellulose
and a feeble action on starch, while malt extract acts conversely.

The distribution of the cytases has not as yet been fully determined, but
it may be assumed that they occur wherever cellulose requires to be dissolved.
That they frequently cannot be recognized may be accounted for by the fact
that they are present only in very small quantities ; indeed we find that the
endosperm of palms, for example, takes a much longer time to dissolve than that
of the Gramineae. [HERISSEY (1903) has shown that cytases are widely dis-
tributed, and that there are many types of them, each with its own specific
activity.]

In addition to starch and cellulose a third non-nitrogenous substance
occurs in seeds, viz. fatty oil. Fatty oils occur not in seeds alone but in all
cells ; indeed it is very doubtful whether there is such a thing as protoplasm
free from fat. It is a normal component of the chloroplasts of certain
plants, and was in such cases for long believed to replace starch as the final pro-
duct in the process of carbon assimilation. More exact investigations (HoLLE,
1877, GODLEWSKI, 1877) do not confirm this view. It seems much more likely
that the oil globules occurring in chloroplasts are not to be considered as pro-
ducts of assimilation, at least in the higher plants (although in Vaucheria
FLEISSIG (1900) holds that fat does take the place of starch), and that they
are not capable of further conversion. It is for that reason we have made no
mention of fats in the preceding pages. In seeds, fats are undoubtedly reserves,
and in many seeds they form the chief part of them, as may be seen from the
following table : —

Almond . . . 5.39 24.18 53-68 7-23 6-56 2-96

Hazel-nut . . 3.77 15-62 66-47 9>O3 3<a8 I>83

Poppy-seed . . 5.79 14-09 47-69 18-74 5-76 7-93

Coco-nut . . . 5-81 8-88 67.00 12-44 4-06 1.81

We must treat of the fats in this section, although we cannot enter into
a discussion of their mode of origin at the present moment.

The fats are really glycerine esters of various fatty acids, belonging to one
or other of three series. The first series has a composition represented by the

THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. II 159

formula CnH211O2, the second, CnH2n_2O2, and the third, CnH2n_4O2. To the first
group belong laurinic acid (C12H24O2), myristic acid (C14H28O2), palmitic acid
(C16H32O2), stearic acid (C18H96O2), and arachidic acid (C20H40O2) ; to the second
series belong hypogaeic acid (C16H30O2), oleic acid (C18H34O2), and brassidic or
erucic acid (C22H42O2) ; to the last group, linoleic acid (C16H28O2) (compare
SCHMIDT, 1891). [An enumeration of all the fats which occur in seeds will be
found in CZAPEK'S Biochemie, I.] The glycerine esters of these fatty acids are
briefly described as palmitin, stearin, olein, &c., and they are to be considered
as formed from glycerine and a fatty acid by withdrawal of water, and con-
versely they may, by absorption of water, be decomposed into glycerine and
a fatty acid : —

C3H5(CJ6H3102)3 + 3H20 = C3H5(OH)3 + 3(C16H3202).
Palmitin Glycerine Palmitic acid.

Such a decomposition, which we may obviously consider as a case of
hydrolysis, actually takes place in the course of germination. According to
R. H. SCHMIDT (1891), germinating seeds contain an appreciable quantity of
free fatty acid, amounting to from 10 per cent, to 30 per cent., and there can
be no doubt that its occurrence is due to the action of an enzyme, lipase. GREEN
(1890) extracted such an enzyme from seeds of Ricinus, and was able by its
means to obtain glycerine and a free fatty acid from castor oil apart altogether
from the seed. The presence of glycerine, however, has not hitherto been
demonstrated in the germinating seed, obviously because it rapidly undergoes
transformation and migrates easily from cell to cell. Fatty acids, until a short
time ago, appeared to be incapable of translocation from cell to cell ; the cell-
wall, saturated with water, was held to form an impassable barrier to any such
movement. According to R. H. SCHMIDT, this is perfectly true of artificial
cellulose walls, although not for the walls of living cells, which allow fats to
penetrate them in considerable quantity, more especially if the walls contain
a certain amount of free acid. It may be supposed that some substance present
in the cell-wall unites with the free fatty acid to form a soap, and that this
soap permeates the cell- wall and thus permits the fat to pass through. This
takes place all the more readily if the oil be subdivided into very minute
drops, that is to say, is emulsified, and the fatty acids are well known to have
an emulsifying power.

In addition to Ricinus, several other seeds, such as those of rape, poppy,
and hemp, have been shown by SIGMUND (1890-2) to contain a fat-splitting
enzyme, so that we have every right to assume the general distribution of
fatty oils and the corresponding frequency of lipase. More detailed investi-
gations on this subject are not as yet forthcoming, and we know still less as
to the process which microscopical research has established beyond a doubt
(SACHS, 1859), but which, from the purely chemical point of view, is incompre-
hensible, viz. the transformation of fats into sugar. The amount of sugar
present in germinating oily seeds renders it impossible that it has been
derived from glycerine only ; the fatty acids must obviously also contribute
to the formation of carbohydrates (compare Lecture XIV).

In addition to non-nitrogenous we also meet with nitrogenous reserves
deposited in seeds in the form of proteid ; the relative proportion of nitro-
genous and non-nitrogenous substances is in the highest degree variable.
Whilst in general the non-nitrogenous substances are predominant, there are
many plants, more especially the Leguminosae, which contain a very large
percentage of nitrogenous reserves. A glance at the following table (KoNiG,
1882) demonstrates very effectively this variation : —

Seed. Per cent, of nitrogenous substance in dry weight.
Rice (undecorticated) 6-49

Wheat 14.30

Kidney beans 29*94

Linseed 29.33

160 METABOLISM

Proteid, in so far as it is a reserve substance and not a constituent of
the protoplasm or nucleus, occurs in the endosperm as well as in the cotyledons
in a definite morphological form, as aleurone. The aleurone grains originate
as vacuoles in the protoplasm of the storage cells, which are always richer in
proteid and poorer in water, and finally become solid bodies by desiccation.
Owing to loss of water the various substances present in the yacuole
separate out ; certain proteid bodies appear in the crystalline form, other
more complicated substances form spherical secretions, and both are
enclosed in a coagulated ground substance ; the constituents of the aleurone
body thus enclosed are known under the names of crystalloid and globoid.
From the chemical point of view, the most important researches on these
bodies are those of WEYL (1877), SCHMIEDEBERG (1877), GRUBLER (1881), also
those of CHITTENDEN, OSBORNE, and their pupils (summarized by GRIESSMAYER,
1897) ; more recently TSCHIRCH (1900) has checked the results arrived at by
these American investigators by micro-chemical methods. From this it would
appear that the aleurone grains are composed of globulins. The percentage
composition of the crystalloids of the brazil nut is, according to WEYL : —

0,52.43; H, 7.12; N, 18.1; S, 0.55 ; O, 21-3.

while GRUBLER gives as the analysis of the crystalloid of the cucumber : —
C, 53-21; H, 7.22; N, 19.22; S, 1-07 ; O, 19-10.

OSBORNE has identified a large number of globulins in the crystalloids of
different aleurone grains, and has given them special names, and has also shown
that the individual crystalloid is composed of several globulins. The differences
existing between these are of no special interest to the physiologist, so that we
need not enter into a discussion of them here. The globoids appear also to be
composed of globulins, but these are united with calcium, magnesium, and
phosphoric acid, and hence are very varied in character ; they must be reckoned
among the ' nucleo-albumins ' or the ' proteids '. Finally, the ground substance of
the aleurone body consists of globulins, mixed in all probability with albumoses.

During germination these reserve proteids must be altered into forms
capable of undergoing diosmosis and migrating from cell to cell. Simple
solution would not achieve this end without chemical alteration taking place
at the same time ; the large molecules must be broken up, and this is effected,
as we know, by proteolytic enzymes (proteases). These enzymes have been
much more thoroughly investigated from the animal than from the vegetable
side, still the results which have been obtained may, without hesitation, be
considered as applicable to the vegetable kingdom. Two types of proteases
are known, which differ both so far as regards the conditions under which they
operate, and also in the products to which they give rise. To the first type
belong the pepsins, which act only in an acid solution, decomposing the proteids
only into albumoses and peptones and in this way rendering them diffusible.
Pepsin in the animal is produced in the stomach, while trypsin, the other type
of protease, is formed in the pancreas. Trypsin differs from pepsin in
the first place in operating best in an alkaline solution (i per cent, soda),
and also in inducing a much more thorough decomposition in the sub-
stances which it attacks. The albumoses and peptones, which are the
first products submitted to its action, are broken down still further into
the amido-compounds we have already referred to, and more especially into
arginin, histidin, lysin, and ammonia. [In addition to pepsin and trypsin,
a ferment erepsin has also recently been identified as of very widespread
occurrence. The final products are the same as those of trypsin, but it attacks
peptone only, never proteid. Its occurrence in seeds has not as yet been proved.
(ViNES, 1905).] It would appear that trypsin, or an enzyme like it, is of very

THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. II 161

general occurrence in nature ; in the plant world pepsins seem to be entirely
absent. Since, in the germination of seeds with abundant proteid, amino-acids,
or other similar substances produced by the action of the pancreatic secretion,
may be recognized, more especially if by cultivation in darkness an accumulation
of these bodies is induced (Lecture XIV), we may legitimately conclude the
presence of a trypsin-like enzyme in the germinating seed. A ferment of this
kind has been discovered by GREEN (1887) in lupins and Ricinus (1890), and by
NEUMEISTER (1894) in barley, poppy, wheat, maize and rape. NEUMEISTER,
however, searched in vain in many other seeds for a protease, although one
would have expected to find such enzymes most readily in seeds, such as those
of many Leguminosae (lupin, vetch, pea), containing large quantities of proteid,
BUTKEWITSCH (1900) has fully confirmed GREEN'S work on the lupin in
opposition to NEUMEISTER'S conclusions, and was able to show conclusively
that leucin and tyrosin were formed from proteids of Leguminosae by acting
on the latter with a glycerine extract of germinating seeds. WINDISCH
and SCHELLHORN (compare GREEN, 1901) have also discovered a trypsin-like
protease in germinating barley. There is thus nothing to prevent us assuming
the presence of proteases in all proteinaceous seedlings. [These have also been
found in unripe seeds by ZALESKI (1905).] According to the observations of
PURIEWITSCH (1897), such an assumption is certainly not necessary in all cases.
Whilst only amido-compounds can be recognized in the culture fluid when
cotyledons of lupins empty themselves of their own accord, other seedlings
treated in a similar way give off into the surrounding fluid either amides in
addition to proteid or peptone, or peptone and proteid only. It must be
assumed from this that proteid as such can pass through the protoplasm and
cell- wall, a possibility which we have not hitherto generally taken into account,
but one which, especially after our experience of the ability of fats to penetrate
such walls, cannot be regarded as unlikely.

In the decomposition of proteid, sulphates also are set free, as mentioned
on p. 139. The globoids give off phosphoric acid (compare ZALESKI, 1902), which
may arise also from lecithin. Many seeds contain not inconsiderable quantities
of this organic compound, which contains phosphorus but no sulphur, and hence
in the process of germination much phosphoric acid may arise from this source.
[According to more recent investigations only a very small part of the phosphoric
acid arises from lecithin ; phytin (oxymethylphosphoric acid), on the other
hand, appears to be the most important source of this acid. (Compare
p. 145). (POSTERNAK, 1903 ; IwANOFF, 1902).] It is doubtful whether
phosphoric acid occurs in seeds in a less complex combination, e. g. as an in-
organic salt. The same may be said of other ash constituents ; calcium and
magnesium, and apparently iron also, are present in the globoids in organic
form, but perhaps they may occur in seeds in other forms as well. [POSTERNAK
(1905) shows that all the essential materials of the ash are present in the aleurone
grain, potassium, silicon, and manganese, as well as those mentioned ; whether
they are all present in the globoid he does not say.]

Reserve substances are by no means restricted to the storage tissues of
seeds, on the contrary they are found wherever individual cells or tissues occur
independently of special assimilatory activity. Next to seeds stand those organs
which subserve the purposes of reproduction and multiplication, viz. those end-
less types of structures which are known as spores and propagative buds, and
amongst these must be included the pollen-grains of the flowering plants. The
special significance of the reserves in all these cases lies in the fact that they
serve to support the new individual in its development until it has reached
independence and can live on the products of its own assimilatory activity, or
until it has fulfilled its function (pollen). In other cases, the reserves render
possible the formation of new vegetative organs in such plants as pass through
a resting period during which they rid themselves of all superfluous structures.

JOST M

162 METABOLISM

This is the case with those perennials which lose all their aerial parts in winter,
and in trees which, at least in many cases, cast off their leaves in the winter season.
Furthermore, reserves develop in assimilatory organs themselves when the
assimilatory products are formed more rapidly than they are used up or
removed. In all cases, however, before the reserves are actually employed they
must be chemically altered and rendered mobile. This we have yet to consider,
although we may dismiss the subject in a few words since the methods
employed are fundamentally like those already referred to in seeds.

The reservoirs for reserves in perennials are in the form of aggregations of
large-celled storage parenchyma situated in the interior of the plant, and often
indicated externally by conspicuous swellings. The storage tissue may occur
in the root, in the hypocotyl, in the stem, or in the leaf, and hence we are led,
from the morphological point of view, to distinguish such swellings as root
tubers, stem tubers and bulbs. In close relation to such storage tissues we
find one or more buds which are capable of developing into shoots in the
following year. The reserves present in these bodies are on the whole the
same as those found in seeds, and consist of nitrogenous and non-nitrogenous
organic substances as well as constituents of the ash, which we need not con-
sider further. In one point only seeds differ from subterranean storage organs,
viz. that when ripe they are for the most part in a desiccated condition, and
the absorption of water is the essential preliminary to their germination. Sub-
terranean reserve organs, on the other hand, always contain a considerable
percentage of water ; if they be artificially deprived of water to the same extent
as seeds are, they would in most cases soon come to grief. It is well known, for
instance, that potato tubers are able to develop shoots if not in too dry an
atmosphere, owing to their possessing a store of water, and that some bulbs and
tubers may give rise to shoots bearing flowers, without any absorption of water
being observable. MEDICUS (1803) noted this in the case of Veltheimia
capensis ; HILDEBRAND (1884) drew attention to the same fact in Oxalis
lasiandra, and recently Sauromatum guttatum has been put on the market as
a curiosity in consequence of its power of forming flowers in the complete
absence of water, after being heated. We may indeed say that water itself
is, in many subterranean storage regions, a reserve substance.

Amongst non-nitrogenous reserves, carbohydrates are entitled to the first
place for they are even more abundant in these underground storage organs
than they are in seeds. On the other hand, fats, so common in seeds, occur but
rarely in subterranean storehouses (e. g. Cyperus esculentus). The carbohy-
drate present is very frequently starch, although we often find present in
addition, or exclusively, substances which we have not mentioned in speaking
of seeds, because they are either absent or are of secondary importance. Such
substances are mucilage and varieties of sugar. Mucilage as a reserve may be
found abundantly in the tubers of Orchidaceae and in the rhizome of Sym-
phytum (compare FRANK, 1866). [Mucilage corresponds to reserve cellulose
in these situations and gives, on hydrolysis, mannose and galactose. HERISSEY,
1903.] Among the forms of sugar glucoses occasionally occur as reserves, e. g.
in the onion. These sugars are capable of being used in the plant for other
purposes without further transformation ; since, however, their accumulation
must obviously cause an increase in osmotic pressure, we may conclude that the
plant for the most part unites several molecules of glucose into a molecule of
larger size with separation of water. Thus, for example, the transformation
of glucose into cane sugar reduces the osmotic pressure by one-half, and it will
be further reduced if substances such as inulin are formed, which have a com-
position similar to that of starch, but which remain in solution in the cell-sap.
Inulin occurs as a reserve especially in the Compositae and Campanulaceae ;
a substance at least very like it in composition is found also in certain Liliaceae.
Cane sugar is the dominant reserve in sugar-beet and also in the sugar-cane

THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. II 163

(which, however, strictly speaking, should not be referred to here). A more
detailed account of the distribution of the different varieties of sugar is as
out of place as a complete enumeration of those which have been found.
We need only note that cane sugar and inulin undergo alteration during
germination, although we might assume a direct conversion from their
solubility in water. The alteration once more consists in a hydrolytic decom-
position by the agency of enzymes. The enzyme invertase decomposes the cane
sugar into equal parts of dextrose and levulose, while inulase transforms
the inulin into levulose.

Proteid is always present among the nitrogenous reserves of perennial plants,
occurring occasionally in the form of crystals (e. g. in the potato) ; it, however,
never forms aleurone grains owing to the large amount of water present. Besides
proteid, amido-compounds, such as asparagin, leucin, and tyrosin, also occur,
and these, in the majority of cases, may be assumed not to arise from precedent
proteid but to be stored as such. In the tubers of a certain variety of potato,
SCHULZE (1882) found 56 per cent, of the total nitrogenous substance present
consisted of amido-compounds, and only 44 per cent, of proteid. In the sugar-
cane SHOREY (1897) found the simplest amino-acid, glycocoll, a substance which
has not as yet been shown to occur elsewhere in plants.

The most extensive storage tissue occurs in trees, where all the parenchyma
cells of the wood, of the cortex, and also of the pith, both in root and stem, are
filled with reserves. The central elements of the wood of many trees which
gradually pass over into duramen and, on losing their vitality, cease to store
reserves, form an exception. Starch is the most generally distributed of all the
non-nitrogenous substances in such plants, and, owing to the ease with which it
can be recognized, its behaviour can be closely followed. Storage of starch begins
in May or June, and generally first of all in the cells of the root ; later, it appears
in the stem, and finally the cells of the branches and finer twigs become filled with
it. During the winter it undergoes alteration, completely or partially, enabling
it to be translocated to other regions ; but in spring, before the buds come out,
the same conditions as in autumn are re-established, and in order to render the
starch mobile, diastase must be produced in sufficient quantity. [In addition
to starch, hemicelluloses also occur as non-nitrogenous reserves in trees ; these
bodies take the form of thickenings on the walls of the wood fibres or cortical
parenchyma, which become again dissolved in spring (LECLERC, 1904 ; SCHELLEN-
BERG, 1905).] Our knowledge of the nitrogenous reserves in trees is much less
extensive ; in general, they consist of proteids and amides. Proteid in a crys-
talline form may be demonstrated in some situations, for example in the bud-
scales of certain trees, which act a storage organs in the same way as do
bulb-scales, and which contain not only proteid but non-nitrogenous substance
as well — mostly in the form of reserve cellulose.

The last type of storage organ, the foliage leaf, brings us back again to
structures with which we are more familiar. We have already studied in detail
the synthesis of carbohydrate in these organs and have seen that it is probable
that proteid is formed there also — if not exclusively, still in very large quantity.
It has also been more than once indicated that fats of value in metabolism are
scarcely at all formed in the foliage leaf. [In leaves which last for several years,
fats acting as reserves certainly occur, at least in winter, just as in the case of trees
(p. 175).] So long as the leaf is growing the products of assimilation are at once
made use of, or are so rapidly translocated that an accumulation of them is im-
possible. In general, the products of assimilation, or more accurately, the surplus
over immediate wants, become reserves in the regions of their formation, but
they do not remain in this condition for long ; most commonly, they again
become mobile during the night succeeding their construction and migrate out of
the leaf. In the case of starch, such periodic formations and dissolutions have
been definitely demonstrated. Phenomena such as these, however, suggest

M 2

164 METABOLISM

several new problems, and more especially lead us to inquire whether the
dissolution of reserves in the leaf is also effected by means of enzymes.

From a comparison of the results of experiments conducted by VINES (1891),
JENTYS (1892), and also by BROWN and MORRIS (1893), the occurrence of dias-
tase in the foliage leaf cannot any longer be doubted. Although WORTMANN
(1890) arrived at another conclusion, still his failure to find diastase can be
readily explained ; for diastase, in the first place, is present in the foliage leaf
only in small quantity, and further, it cannot be extracted completely by
water, considerable further loss taking place during filtration. The chief reason
for the apparent absence of diastase lies in the tannin so frequently present in
the leaf, which by precipitating the diastase makes it inactive. To BROWN and
MORRIS we owe convincing proof of the fact, not only that diastase is present in
the leaf, but also that it occurs in sufficient quantity to transform into sugar all
the starch present there. These authors also showed that different leaves contain
the most varied amounts of diastase. For this purpose they estimated the
amount of maltose which was produced from so-called soluble starch in
forty-eight hours by the action of an extract of 10 g. of dried and powdered
plant substance, and found that 10 g. of malt gave 634 g. of maltose, 10 g. of
the leaf of Pisum gave 240 g., log. of Lathyrus leaf gave ioog., log. of
Tropaeolum leaf gave 4-10 g., and of Hydrocharis leaf, 0-3 g.

In comparison with malt the amount of diastase present in leaves is gene-
rally small, although many leaves have large supplies. Definite proportions
evidently exist between the starch contents of a leaf and diastase, since the
Leguminosae which have been examined and which are next after malt in dia-
static power, are at the same time very rich in starch ; on the other hand it must
be noted that not all the numbers obtained can be accepted as an exact measure-
ment of the diastatic capabilities of the leaves concerned without further con-
firmation ; more especially we must note how the very inactive leaf of Hydro-
charis owes its place at the end of the series chiefly to the large amount of
tannin which it contains, which, as we have already said, inhibits the action
of diastase. Among external factors, so far as they have not been already
mentioned, we must note light, which has an inhibitory action on diastase
(GREEN, 1897), so that more starch is altered into sugar at night than by day.
EMMERLING (1901) was unable, however, to confirm this observation. Again the
increased activity of diastase in darkness, due to carbon-dioxide (MoHR, 1902),
must tend to make the amount of sugar formed at night greater-, since
at night the leaf contains much more carbon-dioxide than by day. How far
our previous conclusions as to the amount of material formed during the
process of assimilation by day are actually affected by this fact we cannot at
present say ; it will be remembered that we assumed that quite as much carbo-
hydrate was translocated by day as by night. Perhaps also SACHS'S estimates
(p. 114) are too high.

As we have seen, diastase, generally speaking, transforms starch into mal-
tose, i. e. a reducing disaccharide, nearly related to cane sugar. The maltose
is by hydrolysis broken up into two molecules of dextrose, while cane sugar is
decomposed into a molecule of dextrose and a molecule of levulose. Since
cane sugar has been shown to be without doubt a product of carbon assimilation,
at least in certain plants, but that maltose is a product of the decomposition
of starch, it comes to be a question whether these disaccharides undergo further
conversion as such or whether they must be hydrolysed first in the manner
described. A final decision on this question has not yet been reached. Reference
may first be made to the germination of beet, where there is no doubt that
cane sugar is transformed into invert sugar. That the presence of invertase
is necessary for this decomposition has been established with sufficient certainty
in various plant organs (GREEN, 1901), and BROWN and MORRIS have proved
its presence in the leaves of Tropaeolum, KOSMANN in buds of trees, O'SuLUVAN

THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. II 165

in barley seedlings, VAN TIEGHEM and GREEN in pollen-grains ; in a word the
wide distribution of invertase can scarcely be called in question. At the same
time we do not wish to imply that cane sugar cannot be directly converted in
some other way. Still more difficult is the case of maltose. We noted (p. 150)
that, according to BEIJERINCK, dextrose could be formed from starch by means
of a special diastase, and this effect might well be produced by the action of
glucase, a member of the diastase group. Such a ferment has been shown to
occur in yeast, but not much is known as to its occurrence in the higher plants.
We must also remember in what has yet to be said that, besides dextrose and
levulose, saccharose and maltose may occur as migratory carbohydrates, not to
speak of glactose and mannose, about which as yet very little is known.

Our imperfect acquaintance with the mode of formation of nitrogenous
products of assimilation in the foliage leaf has already been frequently noted.
Proteids and amides have frequently been found in the leaf ; the amides are
capable of translocation without further alteration, but the proteids must, at
least in some cases, undergo decomposition first. That such a decomposition
of proteid is probable may be deduced from the fact that an accumulation
of amides has been observed in such organs as have been kept in the dark
(BORODIN, 1878). The formation of amides is not influenced by darkening,
but the reconstruction of proteid from them is prevented, and hence their
accumulation is explained. We have indirect evidence also of the de-
composition of proteid. The occurrence of a tryptic ferment in a number of
succulent organs has been demonstrated, and this ferment is capable of bringing
into solution the proteid formed in the course of assimilation, at all events
there is no other purpose known which it could fulfil. The most thoroughly
investigated is a trypsin found in the fruit of the pineapple, and which
CHITTENDEN (compare GREEN, 1901, 198) has termed bromelin. It acts very
energetically on fibrin and on egg albumen, and gives as products of its
action, peptone, leucin and tyrosin. Were this enzyme limited in its occur-
rence to this fruit it would be of little interest to us here, since we are investi-
gating the enzymes present in the foliage leaf, but another proteid-dissolving
enzyme, papain, known at first only in the fruit of the papaw tree, has been shown
by WURTZ (1879) to occur also in leaves, so that we may believe that bromelin
also may occur in the vegetative organs of the pineapple. Tryptic enzymes
have also been obtained by MARCANO from the expressed sap of the leaves of
many species of Agave, by BOUCHUT and HANSEN from the sap of the fig (Ficus
carica), and also by DACCOMO and TOMMASI from Anagallis arvensis (for litera-
ture, see GREEN, 1901, 212 ; compare also FERMI and BUSCAGLIONI). In
comparison with the doubtless quite general distribution of diastase, evidence
of the occurrence of proteases is as yet very scanty, and it would be rash on
our part to conclude from such evidence only that these enzymes were at all
generally dispersed.

Bibliography to Lecture XIII.

BORODIN. 1878. Bot. Ztg. 36, 801.

BROWN and MORRIS. 1890. Journal Chem. Soc. Trans. 57, 458.

BROWN and MORRIS. 1893. Ibid. 63, 604.

BUTKEWITSCH. 1900. Ber. d. bot. Gesell. 18, 185 and 358.

EMMERLING. 1901. Ber. d. chem. Gesell. 34, 3810.

FERMI and BUSCAGLIONI. 1899. Centrbl. Bakt. II, 5, 63.

FLEISSIG. 1900. Physiol. Bedeutg. d. olartigen Einschl. in Vaucheria. Diss. Basel.

FRANK. 1866. Jahrb. f. wiss. Bot. 5, 161.

GODLEWSKI. 1877. Flora, 60, 215.

GREEN. 1887. Phil. Trans. 178 B, 39.

GREEN. 1890. Proc. Roy. Soc. 48, 370.

GREEN. 1897. Phil. Trans. 188 B, 167.

166 METABOLISM

GREEN. 1901. Die Enzyme. German edition by Windisch. Berlin.

GRIESSMAYER. 1897. Die Proteide d. Getreidearten. Heidelberg.

GRUBLER. 1881. Journ. f. prakt. Chem. 131, 97.

[HERISSEY. 1903. Rev. d. Bot. 15, 345.]

HILDEBRAND. 1884. Lcbensverhaltnisse d. Oxalisarten. Jena.

HOLLE. 1877. Flora, 60, 113.

[IWANOFF. 1902. Ber. d. bot. Gesell. 20, 366.]

JENTYS. 1892. Bullet. Acad. d. Cracovie.

KONIG. 1882. Chem. Zusammensetzung d. menschl. Nahrungsmittel. Berlin.

[LECLERC DU SABLON. 1905. Rev. d. Bot. 16, 341.]

MEDICUS. 1803. Pflanzenphysiol. Abhandlungen, 2, 140.

MOHR. 1902. Centrbl. Bakt. II. Abt. 8, 601.

NEUMEISTER. 1894. Zeitschr. f. Biol. 30, 447.

NEWCOMBE. 1899. Annals of Botany, 13, 49.

[POSTERNAK, 1903. Compt. rend. 137, 202, and 1905, 140, 322.]

PURIEWITSCH. 1897. Jahrb. f. wiss. Bot. 31,1.

SACHS. 1859. Bot. Ztg. 17, 177 (Ges. Abh., i, 557).

[SCHELLENBERG. 1905. Ber. d. bot. Gesell. 23, 36.]

SCHMIDT. 1891. Flora, 74, 300.

SCHMIEDEBERG. 1877. Zeit. f. physiol. Chem. i, 205.

SCHRODER. 1901. Bot. Centrbl. Beihefte, 10, 122.

SCHULZE. 1882. Versuchsstationen, 27, 357.

SCHULZE. 1890-2. Zeit. f. physiol. Chem. 14, 227 ; 16,387.

SHORE Y. 1897. Cited in Revue g£n. de Bot. 1902 ; 14, 283.

SIGMUND. 1890-92. Cited by Green, 1901.

TSCHIRCH. 1900. Ber. d. pharm. Gesell. 10, 214.

VINES. 1891. Annals of Botany, 5, 409.

[VINES. 1905. Ibid. 19, 171.]

WEYL. 1877. Zeit. f. physiol. Chem. i, 72.

WORTMANN. I800. Bot. Ztg. 48, 581.

WURTZ. 1879. Compt. rend. 89, 425.
ZALESKI. 1902. Ber. d. bot. Gesell. 20, 426.
[ZALESKI. 1905. Ibid. 23, 133.]

LECTURE XIV
THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. Ill

THE TRANSLOCATION AND TRANSFORMATION OF DISSOLVED RESERVES

THE reserves accumulated in storage organs become at definite times
mobile, that is to say, they are transformed from an insoluble and non-diffusible
into a soluble and diffusible state. The object of this transformation is to
permit of these substances migrating from cell to cell, and such translocation
of material from the storage regions to the places where they are used up
is a phenomenon of wide occurrence in the plant. It is very easy to prove the
migration of carbohydrates out of the foliage leaf, and much research has been
carried out on this subject. One can often observe that a leaf which is full
of starch in the evening has become quite empty of starch next morning, after
a warm night, if it remains attached to the plant, but that if it be cut off, the
amount of carbohydrate suffers but little change during the night. We may,
therefore, conclude that a migration of carbohydrate takes place out of the
normal leaf in the dark. This migration does not cease, however, by day.

It is more difficult to prove the migration of nitrogenous substances from
the foliage leaf. Exact research on this subject is as yet not forthcoming.
KOSUTANY, it is true (1897), has made careful comparative researches on the
amount of nitrogenous substance present in vine leaves in the afternoon and in.
the morning before sunrise, but unfortunately he based his calculations oil

THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. Ill 167

corresponding dry weights and not on similar leaf areas. In loog. of dry
substance he found as follows : —

Total Nitrogenous matter. Proteid. Non-proteid.

In the afternoon 3-537 3-199 0-338

In the early morning 3-621 3-385 0-236

He concluded from these numbers that during the night a formation of proteid
from non-pro teid (nitric acid and amides) took place, and that the total amount
of nitrogenous material increased. This conclusion does not appear to us to
be sound. We will attempt to reduce his determinations to a calculation of
similar leaf surfaces, using as a basis the values which SACHS obtained for the
loss in weight during the night of the leaves of Helianthus and Cucurbita : —

Evening. Morning.

i sq. m. Helianthus (dry) weighs 80-44 g. 70-80 g.

I sq. m. Cucurbita (dry) weighs 59-92 g. 51-22 g.

Total 1 40- 36 g. r 22-02 g.

On an average one sq. m. of dry leaf substance 70-00 g. 61-00 g.

If 70 g. of dry leaf substance in the evening represent the same leaf area as
61 g. in the morning, then 100 g. in the evening will be represented by 87 g. in
the morning ; in other words, a definite area of leaf blade loses 13 per cent, of
its dry weight during the night by translocation of the products of assimila-
tion. If we assume for the vine in KOSUTANY'S researches a loss of only 10 per
cent., to use round numbers, then 100 g. in the afternoon would correspond
to 90 g. in the morning. We might then rewrite KOSUTANY'S tables, for similar
leaf surfaces, in the following way : —

Proteid' Non-proteid.

A definite leaf area contains in the afternoon 3-539 3-r99 0-338

The same leaf area contains in the morning 3-259 3-°47 0-212

From such a calculation it would be possible to deduce conclusions as to
the migration of nitrogenous substance during the night, and not as to its
increase in the individual leaf. An experimental confirmation of our argument,
resting as it does on a somewhat insecure basis, would be certainly of value.

We have to note as well that, in addition to this daily translocation, another
transference of nutritive material occasionally takes place from the leaf. In
the first place, the leaves of evergreens frequently act as storehouses of reserve
and empty themselves in spring, just as do the cotyledons of a seedling ;
before it dies, however, certain bodies migrate out of the leaf back into the
permanent living parts of the plant. This transference of material was for long
greatly overestimated, until WEHMER (1892) pointed out that it was in no way
substantiated by facts. More recently RAMANN (1898) has demonstrated in
forest trees, and FRUWIRTH and ZIELSTORFF (1901) in hops, that, as a matter
of fact, nitrogen, phosphoric acid, and potassium do migrate from the leaves
in autumn. This translocation can scarcely, however, be considered of much
importance ; this brief reference to the subject must therefore suffice.

We need not pause here to give in detail the evidence for the translocation
of materials from other storehouses ; we have had other opportunities of
considering the subject, at least in part, and, in the course of our further
investigation, we will return to the subject when we study the causes of trans-
location in greater detail and the path by which the trans] ocation products move.

In the first place, we may consider certain purely physical causes of trans-
location. We have already, in speaking of germination, and in the discussion
of the researches of HANSTEEN and PURIEWITSCH, drawn attention to one
fundamental principle of every translocation, viz. diffusion. It is quite im-
material whether the diffusion takes place from one cell to another or from the
cell to the exterior ; all that is necessary for the initiation of diffusion is that the

168 METABOLISM

solutions of a substance in two regions shall have different degrees of con-
centration. Thus we saw that when the endosperms of grasses were immersed
in a large quantity of water the cells were gradually emptied, while a small
amount of water rapidly came to contain so much sugar that diffusion could no
longer take place. When the migration of the sugar formed from the starch
ceases hydrolysis of the starch also comes to an end and the endosperm-
cells remain full. It was further noted that the emptying of the endosperm
was stopped more rapidly by immersing the storage region in a solution of sugar
than by employing a small quantity of water. The same method of emptying
reserve stores was also employed by PURIEWITSCH in the case of isolated cotyle-
dons, root tubers, rhizomes, bulbs, and branches. [Compare WACHTER, 1905.]
It might also, perhaps, be possible by appropriate methods to induce an empty-
ing of an isolated foliage-leaf filled with products of assimilation. In the case
of the storehouses mentioned, another highly important method of investi-
gation into the migration of reserves may be successfully carried out, for which
endosperm is obviously not suited, owing to the fact that it dies off after the
emptying of its cells. As already noted we may prevent translocation by
means of a sugar solution of appropriate concentration, but if the concentration
be increased one finds the opposite process taking place, for the sugar enters
the storage tissue and there forms starch. This phenomenon is identical with that
recorded at p. 112, where it was seen that the formation of starch took place in
the foliage leaf when sugar was supplied from without. In this experiment there
are two points of interest for us which we have not previously paid attention
to. In the first place, it shows us that the direction of the nutritive stream
is determined by the degree of concentration at two different points. Whether
an outflow or an inflow of materials takes place in any cell depends, at least
to a certain degree, on its surroundings ; both an outflow and an inflow may take
place at the same time if the contents of the neighbouring cells on one side show
a higher, on the other a lower, degree of concentration of the nutrient in
question. Just as in the case of the single cell, so also a tissue situated
between two other tissues with different sugar concentrations will permit
sugar to stream through so long as this difference is maintained. In
other words, physical conditions well known and easily understood govern
the situation in this case. The refilling of empty storage regions is in-
structive from a second point of view. It shows that a continuous removal of
the sugar formed, using a very large amount of water in the emptying experi-
ments, is by no means always necessary for the maintenance of the diffusion
current, and that the translocation may be replaced by storage and trans-
formation. In fact, the movement of materials into an empty cotyledon
would soon stop, were it not that the entering sugar is changed into starch,
and thus room is made for new supplies. It need scarcely be mentioned
that the principle of diffusion already mentioned, i. e. the maintenance of the
flow by translocation or storage, is not limited to the case of sugar and starch,
where it can be conveniently demonstrated, but that it applies to all other
migratory substances. The principle is by no means a new one; on the
contrary, it has been discussed in detail in relation to the osmotic characters
of the cell, although its importance may be again emphasized here.

The diffusion flow is, however, not the only important factor in the migra-
tion of materials ; the permeability of the protoplasm is of equal importance.
It would be unfortunate for the plant if its cells permitted all the reserves they
collected to diffuse outwards in the way they do in PURIEWITSCH' s researches
on storehouses of reserve. A rapid streaming of materials would then take
place towards the roots, and from them into the soil ; the existence of the plant
would then become impossible. If the plant is not to lose all its reserves by
diffusion, the external walls, which are in close relation to water, must not
permit of the passage of the reserves through them. In all probability this

THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. Ill 169

impermeability lies in roots and submerged plants in the external layer of the
protoplasm, whilst in aerial parts the impermeable cuticle prevents the washing
out of reserves by rain. It is very desirable that research should be undertaken
to determine more accurately than has hitherto been done, whether permeability
and impermeability are constant characteristics of the protoplasm of a definite
cell, or whether, as is more probable, the characters of the protoplasm'are capable
of variation according to the demands made upon it. [Compare NATHAN-
SOHN, 1904.] There is quite a number of phenomena which could, perhaps,
be interpreted in the former sense, but which might just as well be dependent
on some more complex influence of the protoplasm. At all events, the
facts cannot be explained merely by taking into account the principle of diffu-
sion, for we have to deal, not with a simple osmotic apparatus, but with a per-
petually changing organism. The facts to which we allude have been already
mentioned (p. 156). If the emptying of the storehouses of reserve were governed
by purely physical factors, then the artificial emptying of the endosperm in
PURIEWITSCH'S experiments could only have been arrested by such bodies
as are formed in the hydrolysis of reserves ; as a matter of fact, substances
also act in an inhibitory way, which are not subject to the laws of diffusion;
oxygen and chloroform especially have an influence on the emptying process.
Thus CZAPEK (1897), experimenting with leaf stalks, was able to show that
killing or stupefying with chloroform retarded the translocation of products of
assimilation, whilst an atmosphere of carbon-dioxide did not affect it. WORT-
MANN (Bot. Ztg. 1890), however, has arrived at an exactly opposite conclusion
in experimenting with carbon-dioxide. The possibility that phenomena of this
kind depend on variations in the quality of the plasmatic layer must be con-
ceded, but we must not forget the far-reaching influence of the whole proto-
plasmic machinery of the cell ; translocation is thus by no means so simple
a process as it has been hitherto considered.

We are driven to the same conclusion for other reasons. Diffusion works
much too slowly to accomplish by itself the transportation of the materials
in the plant. DE VRIES (1885) has shown that, from STEPHAN'S calculations,
a milligram of sodium chloride, one of the most rapidly diffusible salts, takes
319 days, almost a year, in order to migrate from a 10 per cent, solution
into water a metre distant. The same result would be obtained in the case
of cane sugar in two and a-half years, and in the case of proteid in fourteen
years. The slowness of diffusion may be demonstrated very clearly in the follow-
ing way : — Take a long glass tube, closed at one end, and place in it a solid
coloured salt such as copper sulphate, and then fill the tube with water or a not
too concentrated solution of gelatine. The rapidity of diffusion is quite as
great in the latter as in water. After a week, the copper sulphate will have
reached a height of 5 cm., after five weeks, 13 cm., after three months, 20 cm.
If the tube filled with stiff gelatine be inverted it will be seen that weight has no
influence on diffusion, a matter of importance in determining the translocation
of materials in the plant.

This experiment leaves us in no doubt that simple diffusion cannot account
for the translocation of such quantities of material as migrate from the foliage
leaf in the course of a single night. Some means of accelerating the movement
must exist. One factor is the streaming movement taking place within the cell,
thus bringing about a mechanical mixing of the materials. Such rapid move-
ments result from unequal heating of different parts of the cell, perhaps, also
in consequence of electric currents which are widely diffused in the plant, and
also in consequence of protoplasmic movement. Although in this way, in a very
short time, a uniform degree of concentration of a certain substance may be
reached in a single cell, still diffusion is needed to account for the passage from
cell to cell and for the penetration of the cell-wall and the two layers of proto-
plasm which lie against it. It must not be supposed that the cell- wall places

170 METABOLISM

greater difficulties in the way of the diffusion of substances soluble in water.
As a matter of fact, one sees that the aniline dyes mentioned previously pass
very rapidly through the outer walls of a cell of an alga, and the cell-walls lying
between the parenchyma cells may be compared in their characters with
such an algal cell. In the walls of all cells pits occur with great regularity,
that is to say, places where the wall remains thin on both sides, and these may
serve to shorten the path which the diffusing particles have to take through
the membrane. This fact led to the belief that the cell-wall was more difficult to
penetrate than the protoplasm. Since we know that the pits are pierced by
numerous fine pores, and that by their means not only is the protoplasm of one
cell in continuity with that of its neighbours, but that in this way the proto-
plasts of the whole plant form one connected system, we must view the pits, as
agents for permitting translocation of materials, from an entirely different
point of view. It might be imagined in the first place that formed particles
of plasma or entire starch grains might be squeezed through these minute canals ;
as a matter of fact, MIEHE (1901) and KORNICKE (1901) have seen even nuclei
pass through the membrane, doubtless by way of these protoplasmic bridges,
but such migrations, owing to the minuteness of the pores, can be possible only
under high unilateral pressures, such as scarcely ever occur in nature. PFEFFER
(1892), in investigations specially devised for the purpose, was unable to observe
any passage of protoplasm through the pores in the pit-closing membrane.
Although the significance of the protoplasmic bridges as agents in the transport
of materials in mass is, to say the least, doubtful, still they are obviously of great
service in diffusion movements. We may assume that the protoplasm of each
bridge consists of the outer plasmatic layer and the inner plasma, so that
although the external layer is impermeable, as it is in the cell, still the materials
may be able to diffuse through the inner plasma. It is true the canals
are very narrow, but they are, on the other hand, very numerous and very
short, and from BROWN'S work (1900) (compare p. 121) we know that, with
a suitable arrangement and size of the pores, diffusion can be quite as great
as though the entire pit-closing membrane were absorbed.

According to these determinations it appears that a transference of mate-
rials will succeed more easily in long cells in which few partition walls have to
be passed through than in short ones. This leads us to consider somewhat
more closely the tissues which subserve translocation of materials in the plant.
Each normal parenchyma cell can fulfil this function and, as a matter of fact,
we find in certain regions that these cells are the only ones carrying out this
function. In endosperm parenchyma cells alone are present and other elements
are absent from all growing points. But it must be noted that growing points
exhibit only very slow growth changes and hence a rapid transference of materials
is not required. Behind these, where active growth is taking place, tissue
differentiation is more manifest, and there we find cells, which obviously are
adapted specially to the transport of material, more accurately, of mobile organic
substances. These are the sieve- tubes, which, not only from their great length,
but also from the partial absorption of their transverse walls by sieve-pores,
are peculiarly suited to this purpose. They form long strands lying close to
the vascular cords and constitute along with these the ' vascular bundle '. Let
us consider, as an illustration, the emptying of a leaf that has been assimilating
all day, and inquire as to the part played by the sieve-tubes in the process.
SCHIMPER (1885) made an interesting investigation on Plantago, in which type
it is possible to remove the vascular bundle from the leaf stalk without causing
excessive injury, the leaf meanwhile remaining attached to the stem. SCHIMPER
found that a leaf so treated could transfer its starch into the stem in the dark,
and he imagined he had discovered in the elongated cells which surround the
vascular bundle the so-called ' bundle-sheath ', the conducting organs for sugar
translocation. On the other hand, CZAPEK (1897) pointed out that, although

THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. Ill 171

there was no reason to doubt the conducting power of these cells, still they
were not sufficiently extensive to carry out the transport of the whole of the
materials ; he held that this was the chief task of the sieve-tubes. Although
CZAPEK was unable to advance any definite proof of his view, nevertheless it
would seem highly probable that this is the case. He made incisions into
the leaf-stalk of Vitis on a summer evening, cutting through half of the vascular
bundle, and found in the morning that the part of the lamina which was thus
partially isolated was still filled with starch, but that the other portion was
empty. Since, in this experiment, not only the sieve-tubes but also the bundle
sheath was cut through, one cannot draw any conclusion against SCHIMPER'S
view ; we can only conclude that the general fundamental parenchyma of the
petiole is not sufficient for the translocation of the products of assimilation,
and further that this transference cannot be effected in a transverse direction,
and hence that elongated elements are necessary. The same conclusion
is arrived at from experiments in ringing trees. If from an unbranched
stem we remove a ring of cortex, right into the wood, the part of the
tree situated below the region of ringing is not filled with starch, whilst
the part above the ringing accumulates it abundantly. If a narrow bridge of
cortex be retained, connecting the upper with the lower part of the tree, the sugar
flows backwards but spreads laterally very slowly. If the bridge be in the form
of a step ( 4 ) no backward passage of the products of assimilation takes place,
since the horizontal part of the bridge does not conduct them. The wood region
in the tree is unable to carry the products of assimilation backwards, and,
in the cortex, conduction takes place only in the longitudinal direction. This
may depend on the special capacity of the cortical parenchyma ; it is more
likely, however, that it is the sieve-tubes that are the organs of conduction.
[HABERLANDT (1904) has, however, advanced arguments against this view.]

If we accept CZAPEK' s hypothesis as to the function of the sieve-tubes we
obtain the following picture of the translocation of carbohydrates from the
organs of assimilation. The sugar arising from the transformation of starch
eventually reaches the sieve-tubes after migrating through the assimilating
cells and the bundle sheath. G. KRAUS (compare PFEFFER, Phys. I, 592)
found 38 per cent, of the dry weight of sieve-tubes consisted of soluble carbo-
hydrates. In the sieve-tubes it is capable, by mechanical means, by streaming
of all kinds, of travelling rapidly for long distances. The sieve- tubes may,
under certain conditions, form a strand several centimetres or even decimetres
in length, which operates just like a single cell ; by diffusion it receives the
sugar in at the upper end and gives it off at the lower. Movement in the
intermediate region seems not to be effected by protoplasmic streaming, since
such streaming appears to be absent in sieve-tubes (STRASBURGER, 1891, 363) ;
still we may conceive of a movement in mass, caused by varying osmotic
pressures in the surrounding parenchyma. An exudation of contents due to
pressure of neighbouring cells may indeed be observed when sieve-tubes are cut
across. We must not, however, suppose the function of a phloem strand to be
merely to serve as a means of communication between two regions some dis-
tance apart, as though these were in connexion by means of a glass tube. On
the contrary, the sieve-tubes during their entire course are in lateral connexion
with the phloem parenchyma and give over to them all surplus carbohydrate,
and these, owing to vigorous starch formation, are always ready to absorb new
materials. The parenchyma which lies in contiguity with the sieve- tube, acts
as a storage tissue, and, indeed, as in the case of trees, in a double sense. In
the first place, certain reserves are deposited in them, as in the parenchyma
of the medullary rays, of the cortex, and of the wood, for next spring ; but
starch, not only in the stem, but in every petiole, is also deposited in the
phloem parenchyma as so-called temporary reserve, that is to say, the surplus
inflowing sugar wanders out of the sieve-tubes and from time to time may

172 METABOLISM

be transformed, when a direct supply from the leaf ceases. Such transitory
formation of starch always accompanies a translocation of sugar, whether it takes
place for long distances in sieve-tubes or for short distances in parenchyma.
According to statements already made this formation of starch is easily under-
stood, since it serves the purpose of maintaining the degree of concentration
necessary for diffusion.

If the circulation of carbohydrates in sieve-tubes be interrupted, these
elements are found to be in general the conducting organs of migratory organic
substances. The sieve-tubes have been for long claimed to subserve the carriage
of proteid, and the open passage from segment to segment has been referred to as
especially of importance, rendering possible the rapid movement of a substance
in itself diffusible with difficulty. We need not enter further into a consideration
of the translocation of proteid and its decomposition products ; what little is
known proves that conditions similar to those governing the translocation of
carbohydrates prevail here also. It must be noted, however, that materials of
the ash also, sometimes as such, at other times in an organic form, must be
transported by the same path as proteid and sugar, after they have ascended
in the transpiration current from the root and been partly altered into some
other form. It is more than possible that sieve-tubes are aided in the per-
formance of their functions by laticiferous tubes (compare HABERLANDT, 1883,
SCHIMPER, 1885, GAUCHER, 1900). [According to KNIEP (1905), this does not
as yet rest on sufficiently secure evidence.]

There remains for us to inquire into a phenomena which is especially
exhibited by trees. When in springtime starch is dissolved, the sugar, in order
to reach the seat of metabolism has often to travel from a few to more
than 100 metres. Hence it may be concluded that it travels by another path
and not, or not entirely, by the sieve-tubes, but follows the water-stream in the
vessels, just as the salts of the soil do after being absorbed by the root. This
conclusion is arrived at from experiments on ringing, as TH. HARTIG has already
shown (1858). While, as above noted, such a cortical ringing prevents the ac-
cumulation of starch in the basal part of the stem, if the operation be performed
after the storage of starch in autumn, the whole of it disappears in the following
spring out of the wood and cortex of the stem base. After A. FISCHER'S (1890)
researches there can be no doubt that the glucose is transferred to the opening
leaf -buds by the wood and especially by the vessels. And since the transpiration
current is effected for many metres while diffusion perhaps is active for only
millimetres or microns, one can comprehend the advantage which a plant obtains
by this arrangement. Again, since in the sap excreted by a tree in the process of
bleeding both amides and proteids have been found, one may well assume that
nitrogenous materials travel the same way as do carbohydrates.

TH. HARTIG and also A. FISCHER (1890) and STRASBURGER (1891) have gone
further in this respect. They affirm that in trees the upward movement of car-
bohydrates in spring takes place exclusively in the wood, and that only a down-
ward movement can take place in the cortex. The reasons advanced in support
of this view do not appear to us to be quite sound, and it may be that fresh
experiments may show that the phloem also is capable of transporting mobile
reserves. This would appear all the more probable since in herbaceous and
shrubby plants the vessels are said never to be called into service for the upward
transport of reserves, and such a fundamental difference between woody and
herbaceous parts would scarcely be intelligible.

The destinations of the migratory materials are always those regions of the
plant where materials are being actively used up. The more rapidly the altera-
tion of these materials takes place in the regions of activity the greater the
diffusion between the two termini of the movement and the more rapid the move-
ment itself. The dissolution of the reserves will also be accelerated when the
migration of the dissolved substances takes place rapidly. In nature there exist

THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. Ill 173

quite definite regions of transformation and others which might be considered
as supplying their wants. The demand for materials is most prominent in all
growing points. In these regions, it is true, no great quantity of materials iscalled
for at any one time, but, since a continual formation of new cells takes place
in these places, substances needed for the formation of cell-walls, of protoplasm,
and of osmotic substances are required almost the whole year round. In a tree,
as soon as the extension of the shoots of the present year is completed, frequently
rapidly and always at the expense of last year's materials, early in the year the
primordia of the next year's buds are laid down, the development of which
slowly progresses during the winter. Besides, the cambium is also active, and
by its constant production of wood and bast, demands a continuous supply of
nutritive materials. After the formation of the flowers comes the construction
of fruit and seed, and finally the accumulation of stores in the root and stem,
beginning at the base and gradually extending upwards. In all cases we
recognize as migratory materials the substances so often mentioned, sugar,
proteid, and amides, and further, we see that reserves are formed from them,
either temporary in their nature or destined to remain quiescent for longer
periods. Trees are distinguished from annuals inasmuch as the latter store
reserves permanently in their seeds only, while the distinction between trees and
perennial herbaceous plants lies in this, that the latter deposit their reserves
not in aerial but in subterranean storehouses.

A transference of material takes place normally from all storehouses of
reserve, but the plant is, however, able under abnormal conditions to permit
of consumption and transference also taking place in other than the normal situa-
tions. For example, if we remove the growing points and thus render their
development impossible, other appropriate organs may become centres of
consumption (compare VOCHTING'S experiments, Lecture XXVI), and if we
allow the growing points to develop with insufficient food supplies, materials are
drawn from older parts of the plant and in such quantities that these older organs
die off. In cultures carried out in darkness one often notices apical growth
proceeding at the cost of the older and moribund leaves.

We have still, in conclusion, and as an appendix to our treatment of the
subject of alteration and migration of materials, to note the changes which
these moving plasta undergo when they reach their destinations. These changes
are most varied, especially when the plasta are altered into constructive mate-
rials. We have to compare only such relatively simple migratory bodies as
soluble carbohydrates, amides, and minerals with the complicated structure
of the cells made from them. As to these metamorphoses of materials we are
still very much in the dark. The processes by which reserves are formed from
translocatory materials are better understood, since these become transformed,
on the whole, into the same bodies as those from which they were derived.
Here, also, however, we come face to face with many debatable points.
Although, for example, starch or reserve cellulose is formed from glucose
we are ignorant of the immediate conditions of the transformation ; we can
only say that the changes are not very extensive, and it is only a question
of time before we shall be acquainted with all the chemical details of the
process. The matter is not nearly so simple with proteid. This sub-
stance, as we have seen, breaks down into bodies whose constitution is very
different from its own. If germination takes place in light no noticeable
accumulation of amides takes place, because they are at once retransformed
into proteids at the regions where consumption is going on. But if we allow
the seeds to germinate in the dark these bodies accumulate in such quantities
that we can demonstrate their crystals with the greatest readiness under the
microscope, after precipitation with alcohol. Obviously the conditions for
the reformation of proteid are not fulfilled in darkness, and hence a culture in
the dark is always employed when it is desired to obtain amide bodies in large

174 METABOLISM

quantity. If we now compare the amides appearing in a darkened seedling
with those which may be obtained from the decomposition of proteid outside
the plant by the aid of acids or enzymes, we find them to exhibit several remark-
able differences. In the first place we generally find an abundance of asparagin
and glutamin in the plant, whilst the related aspartic and glutaminic acids
appear outside. Then again the proportional amounts of the several amino-
acids in the plant and outside it are by no means the same ; in the plant, one
is generally predominant, and the greatest differences present themselves in
different plants in this respect. Thus we find in the seedlings of Leguminosae
and Gramineae that asparagin is especially abundant, whilst in Cruciferae,
Ricinus, and Cucurbita, glutamin, and in Coniferae, arginin is the dominant
compound. These differences are not to be explained by assuming a different
composition in the reserve proteid of the seeds concerned ; since in the individual
species one does not find as a rule the substances under discussion always in the
same relation. E. SCHULZE (1898), who has gone most deeply into these
questions, formulates the hypothesis that the same decomposition products
arise from proteid in the plant and apart from it, but that in the plant a further
alteration takes place which affects the individual products of the hydrolytic
decomposition in varying degree. Changes in the composition of the mixture
of organic nitrogenous bodies can be determined directly by analysis. This
is apparent in a comparison, for example, of an analysis of pea-seedlings one week
old with those three weeks old : — »

Leucin. Tyrosin. Arginin. Asparagin.

i week abundant little present absent

3 weeks much less absent almost absent very abundant

Further, SCHULZE was able to demonstrate the presence of arginin and
amido-acids only in the cotyledons of the lupin, but he could find no asparagin,
while this latter substance was present in the stem of the seedling ; similarly,
the cotyledons of the cucumber contained no glutamin although that substance
collects abundantly in the stem. Finally, it would appear from quantitative
analyses that the occurrence of asparagin goes hand in hand, not with the dis-
appearance of proteid, but with that of amino-acids.

SCHULZE assumed that the amino-acids first arising from the proteid sub-
stances, in addition to which perhaps also primarily asparagin and glutamin
may arise, break down further into ammonia, and from this, in presence of
a suitable carbohydrate — perhaps glucose — asparagin and glutamin are con-
structed. These amides would be thus, not the main products of decomposition,
but rather the first stages in a higher synthesis, and their formation from this
point of view is not inconceivable. According to HANSTEEN'S experiments
the amino-acids appear, so far as they have been investigated, much less adapted
for the formation of proteid than ammonia or the two amides above mentioned ;
an accumulation of ammonia would be a disadvantage, however, because that
substance, which can be detected only in traces, readily acts as a poison in
larger quantities. This hypothesis of SCHULZE, which has been recently sup-
ported by BALICKA-IWANOWSKA (1903) [and also by PRIANISCHNIKOW, 1904,
and earlier], appears to us to explain best the facts known to us at the present
time in this difficult region of investigation ; much still remains to be done,
however, quite apart from the fact that the mode of origin of asparagin from
ammonia and glucose in a purely chemical manner cannot be discussed at all at
present.

A new difficulty presents itself when we attempt to investigate how proteid
arises from amino-acids or amides. It cannot be doubted that the process
takes place in light, and PFEFFER (1873) has further given proof that light acts
indirectly in the process. BALICKA-!WANOWSKA (1903), however, shows that
in all likelihood light has also a direct influence. Illumination in conjunction
with an atmosphere free from carbon-dioxide does not result in the disappearance

THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. Ill 175

of such amides ; certain products arising during carbon assimilation are necessary
for proteid formation. We must again take into consideration the carbo-
hydrates in this relation, all the more so since if these are supplied in sufficient
quantity proteid reformation can take place in the dark (compare p. 144). The
establishment of this fact is far, however, from solving the chemical problem
of the synthesis of proteid from glucose and asparagin. The glucose at all
events must undergo a fundamental change during this process, since it is incon-
ceivable that it can be introduced into the proteid molecule exclusively as
a carbohydrate group ; indeed, it is by no means certain that such a group
occurs in vegetable proteid. Since, however, in animals carbohydrates can be
formed from proteid, we must admit the reverse process to be possible in the
plant.

In addition to the regeneration of proteid out of the products of its de-
composition we must glance finally at the construction of fats, which we have
seen in our last lecture to be present as a reserve material in seeds. Fat also
occurs in the vegetative organs, and, according to certain authorities, it may be
supposed to travel in these organs either as fat or, after preliminary decom-
position into glycerine and a fatty acid. No one can believe, however, that
the entire mass of fat occurring in a seed could have entered it in that form.
On the other hand, the same materials may be seen travelling to oily seeds as
to those poor in oil, i. e. carbohydrate or, e. g., mannite in the olive, a substance
which takes the place of carbohydrate there. Again, in all oily seeds (PFEFFER,
1872) in the young condition large quantities of starch occur, which, when the
seeds are ripe, is replaced by a fatty oil. This does not appear to be effected by
the respiration of starch and its change into water and carbon-dioxide while fat
wanders in to take its place ; on the other hand, the fat must be derived from the
starch, for one can demonstrate its appearance in isolated unripe seeds into
which no entry of fat is possible. Just as we saw earlier that an alteration
of fat into carbohydrate took place during the germination of seeds, so now
we may note that in ripe seeds starch is changed into fat. From the chemical
point of view this change is extraordinarily difficult to appreciate, i. e. the origin
of a substance poor in oxygen from one relatively rich in that element. A
chemical reaction of this type has not as yet been observed externally to the
cell. We must therefore content ourselves for the present with the observation
of the fact without being able to enter more deeply into the meaning of the
phenomenon. It must be noted, however, that the alteration of starch into fat
is not limited to seeds. In trees also starch is, during winter, at least partly,
altered into fat, and in spring starch is again reformed from fat. [According to
NIKLEWSKI (1905), the relationship between sugar and fat cannot be explained
from the chemical point of view and physiologically also it is very doubtful.]
Both these processes depend in a variable manner on temperature (A. FISCHER,
1890) ; low temperatures tend to induce the formation of fat, high temperatures,
starch. Hence one can in the middle of winter bring about a reformation
of starch in amputated twigs by bringing them into a warm room. The
significance of this phenomenon is still a great puzzle, and its physiological
reasons are also but little understood. An increase in dissolution of starch
accompanying a decrease in temperature is known to be independent of the
formation of fat ; in the potato, for example, the ' sweetening ' depends, at
temperatures just above o° C., on the formation of sugar out of starch
(MULLER-THURGAU, 1882), which is promoted by quite low temperatures. This
disappearance of starch at low temperatures cannot be accounted for by special
peculiarities of diastase.

We have now obtained a knowledge of the changes which certain organic
compounds undergo in the green plant ; at the same time, we have glanced
at only a relatively limited number of chemical substances, namely, the proteid
bodies and the crystalline nitrogenous organic substances resulting from their de-

176 METABOLISM

composition, the fats and the carbohydrates (we will deal with the organic
acids in succeeding lectures). It requires no depth of chemical knowledge to know
that the wealth of chemical compounds in the plant is not thereby exhausted.
One need only refer to the odours which are peculiar to so many plants to
recognize at once a large series of bodies of wide distribution, such as ethereal
oils, resins, &c. Further, we must not forget the colouring matters, which
make their appearance in such variety, not only in the flower region, but in the
vegetative organs as well, taking the place of chlorophyll. Finally, we are
acquainted with bodies to which many plants owe their poisonous or curative
powers, the glucosides and alkaloids. Since such substances appear to occur
again and again under the same conditions in the same plant, they, as well as
sugar, proteid, &c., must be products of metabolism, and the same questions
must be asked about them, viz. how are they formed ? What becomes of them ?
What significance have they in the plant economy ? Although we have not
entered into a discussion of these questions in our consideration of the metabolism
of the green plant it is not because they are without interest, but because the
researches which have been hitherto carried out on them have led to no, or
only to indifferently, conclusive results. We know that many of these bodies
do not undergo any further transformation in the plant, we may consider
them as final products of no value, as excreta in short. [Many glucosides,
hitherto regarded as waste products, act as reserves according to WEEVERS
(1904.)] Such a purely chemical conception is, doubtless, one-sided. The cell-
wall in the majority of cases is not further altered in the course of metabolism,
but no one would consider it as an excretion, since it is of the very highest
importance in the life of the organism. Many such examples might be brought
forward ; and it follows that the so-called biological significance of materials
demands notice, and this is true especially for such substances as scents,
colouring matters, alkaloids, and glucosides, and their significance has been
often looked for and found with greater or less success. To enter into that
aspect of our problem would take us too far and so we must content ourselves
with this brief summary. We may at least draw attention to some of the more
important works on the chemistry, physiology and biology of these metabolic
end-products [CzAPEK, Biochemie], such as ethereal oils, resins, &c. : —
TSCHIRCH, 1900 : Die Harze und die Harzbehalter, Berlin ; H. MULLER,
1873 : Die Befruchtung der Blumen, Leipzig ; DETTO, 1903 : Flora, 92, 147.
Colouring matters : — ROSCOE : Ausf. Lehrbuch d. Chemie, vol. 8, 1901 ; H.
MULLER, 1873 : Die Befruchtung der Blumen, Leipzig ; STAHL, 1896 : Bunte
Laubblatter (Annales Buitenzorg, 13, 137) ; Alkaloids and glucosides : — ROSCOE :
Ausf. Lehrbuch d. Chemie, vol. 8, 1901 ; VAN RIJN, 1900 : Die Glykoside,
Berlin ; PICTET, 1900 : Die Pflanzenalkaloide, Berlin ; STAHL, 1888 : Pflanzen
und Schnecken (Jen. Ztschr. f. Naturw. 22).

Bibliography to Lecture XIV.

BALICKA-IWANOWSKA. 1903. Bull. Acad. Cracovie.

CZAPEK. 1897. Sitzungsber. Wien. Akad. Math.-nat. Cl. 106, I, 117.

FISCHER, ALFR. 1890. Jahrb. f. wiss. Bot. 22, 73.

FRUWIRTH and ZIELSTORFF. 1901. Versuchsstationen, 55, 9.

GAUCHER. 1900. Annal. Sc. nat. vn, 12, 241.

HABERLANDT. 1883. Sitzungsber. Wien. Akad. Math.-nat. Cl. I, 87, i.

[HABERLANDT. 1904. Physiol. Pflanzenanatomie, 3rd ed. Leipzig.]

HARTIG, TH. 1858. Bot. Ztg. 16, 332.

[KNIEP. 1905. Flora, 94, 129.] •

KORNICKE. 1901. Sitzungsber. Niederrhein. Gesell.

KOSUTANY. 1897. Versuchsstationen, 48, 13.

MIEHE. 1901. Flora, 88, 105.

MULLER-THURGAU. 1882. Landw. Jahrb. n, 751.

[NATHANSOHN. 1904. Jahrb. f. wiss. Bot. 39, 607.]

THE NUTRITION OF HETEROTROPHIC PLANTS 177

[NIKLEWSKI. 1905. Beih. z. bot. Centrbl. 19, i.]

PFEFFER. 1872. Jahrb. f. wiss. Bot. 8, 485.

PFEFFER. 1873. Monatsber. Berl. Akad.

PFEFFER. 1892. Stud, zur Energetik, Abh. Sachs. Gesell. 18, 275.

[PRIANISCHNIKOW. 1904. Ber. d. bot. Gesell. 22, 35.]

RAMANN, E. 1898. Zeitschr. f. Forst- u. Jagdwesen, Rev. Bot. Ztg. 56 231.

SCHIMPER. 1885. Bot. Ztg. 43, 756.

SCHULZE, E. 1898. Zeitschr. f. physiol. Chem. 24, 18 ; ibid. 30, 241.

STRASBURGER. 1891. Bau u. Verrichtung d. Leitungsbahnen. Jena.

DE VRIES. 1885. Bot. Ztg. 43, i.

[WACHTER. 1905. Jahrb. f. wiss. Bot. 41, 165.]

[WEEVERS. 1904. Jahrb. f, wiss. Bot. 39, 229.]

WEHMER. 1892. Landw. Jahrbucher 21, 513.

WORTMANN. 1890. Bot. Ztg. 48, 581.

LECTURE XV

THE ACQUISITION OF CARBON AND NITROGEN BY
HETEROTROPHIC PLANTS

WE shall now leave autotrophic plants and turn to the consideration of
heterotrophic forms, that is to say, forms which have no power of forming carbo-
hydrate from carbon- dioxide, nor the capacity for building up proteid out of
nitrates or ammonia. They are dependent on previously manufactured organic
substance, and also, in nature, on the nutriment they are able to take from other
and autotrophic plants. In reality the contrast is not so sharply defined as
it appears. In the first place, so far as the acquisition of carbon is concerned,
only certain definite cells in autotrophic plants are really autotrophic, i. e. those
which contain chlorophyll, all others are actually heterotrophic. We have seen
that all subterranean organs, even the aerial stem itself, all growing regions
and growing points, embryos, &c., are entirely dependent on already con-
structed organic substances. It may be further noted that the foliage leaf
even, the specific organ for autotrophic nutrition, may, under certain
conditions, be constructed exclusively out of carbohydrates, &c., brought to
it from without (Josr, 1895). Although it is impossible in general to nourish
higher plants in a purely heterotrophic manner in the absence of carbon-
dioxide, the reason lies rather in the purely experimental difficulty of
the research than in the natural and fundamental difficulties of the case.
In some cases (LAURENT, 1898) these difficulties may be overcome. [LAURENT
has shown in a recently published work (1904) that in the case of maize seed-
lings fed on sugar in the dark, the increase in weight always remains quite
small.] Typical autotrophic plants in nature certainly live on carbohydrates
manufactured by themselves, but there are also nontypical forms which,
according to external conditions, are able to exist either in an autotrophic or
heterotrophic manner (Euglena : ZUMSTEIN, 1899). [ARTARI (1899 and 1904)
showed that the growth of certain lower Algae could be furthered by adding
sugar.]

Since, then, the contrast between autotrophic and heterotrophic organisms
is not so fundamental as it at first appears, we need not expect to find any
entirely new feature in the nutrition and metabolism of heterotrophic plants.
At the same time it is the proper course for us to devote a special section to the
treatment of heterotrophic plants, since they exhibit in many respects peculiar
conditions of life, and are much better adapted for the study of many problems
in nutrition than are autotrophic plants.

Not infrequently the plant exhibits many diagnostic characters both in
form and in mode of life which serve as criteria for determining whether it is

JOST N

178 METABOLISM

nourished in an autotrophic or a heterotrophic manner. Since the decom-
position of carbon-dioxide is dependent on the presence of chlorophyll we may
conclude from the absence of that colouring matter that the organism is of
necessity obliged to fall back on organic materials containing carbon. Experi-
mental observations on a large number of Bacteria and Fungi have fully
confirmed this conclusion in the great majority of cases. On the other hand,
the constant occurrence of an organism in soil which is rich in organic material
suggests that it may live heterotrophically even though chlorophyll be present.
Although the plant be dependent on nitrogen, sulphur, phosphorus, or other
ash constituents in an organic form, it does not always follow that it also
requires the carbon to be in an organic combination. The most remarkable
condition is where another living organism, animal, or plant serves as a sub-
stratum rich in organic nutriment, where, in a word, the mode of life is
•parasitic. A large number of Fungi as well as certain higher plants, e. g.
Lathraea and Orobanche, exhibit this type of heterotrophic life ; in these,
chlorophyll is for the most part wanting. A colourless parasite must, so far
as nutrition is concerned, obviously behave just like the colourless member
of an autotrophic plant, e. g. the root. It might, therefore, be thought that it
would serve the purpose if we were to treat such parasites as an appendix to
our study of autotrophic plants. In reality, however, we know very little
indeed as to their nutrition ; we are much better acquainted with the behaviour
of certain Fungi and Bacteria which live on dead organic matter. Whilst
parasites are always present on certain definite plants, often on a single
species or variety, many saprophytes, that is to say, heterotrophic organisms
which live on dead organic materials, appear on the most varied substrata, and
thus are specially adapted for the study of the nutrients which make life
possible for them. We shall begin with these plants and especially with their
dependence on carbon in the organic form. [BENECKE has recently published
(1904) an important and comprehensive memoir on the nutrition of Fungi.]

The requirements of Mould-fungi as regards mineral matters have already
been referred to ; it will be sufficient, therefore, to note here that essentially the
same substances are needed by them as by higher plants. The only difference
is that the Fungi require only one of the alkaline earths, calcium or magnesium,
whilst higher plants require both. In order to study the sources of carbon
employed by our ordinary moulds we may select a nutritive solution which, in
addition to minerals, contains nitrate of ammonia to meet the demand for
nitrogen, adding to the solution different bodies containing carbon, and
introduce a few spores of Aspergillus niger or of Penicillium glaucum. Accord-
ing to the way in which the fungus grows we may readily draw conclusions as
to the nutritive value of the source of carbon supplied. We are able to
determine, for example, that sugar is an excellent nutrient, but that many acids,
such as formic or oxalic, are very inferior nutrients or have no nutritive value
at all. Exhaustive studies on this question have been carried out by PASTEUR
(1860 and 1862), NAGELI (1879 and 1882), and REINKE (1883). These investi-
gators have shown that an extraordinarily large number of carbon compounds
may serve as nutrients to Fungi, e. g. carbohydrates, alcohols, organic acids,
fats, amido-compounds, peptones, &c. We give below a summary by way of
showing how varied in character these compounds are ; the materials are
arranged in descending order of their value as nutrients : —

NAGELI (1882) gives the following series a propos of Fungi : — i. sugar ;
2. mannite, glycerine, leucin ; 3. tartaric acid, citric acid, succinic acid, aspa-
ragin ; 4. acetic acid, ethyl-alcohol, quinic acid ; 5. benzoic acid, salicylic acid,
propylamin ; 6. methylamin, phenol.

PFEFFER (Phys. 1, 372), as a result of later experiments, rearranged the series
as follows: — i. sugar ; 2. peptone ; 3. quinic acid; 4. tartaric acid ; 5. citric
acid ; 6. asparagin ; 7. acetic acid ; 8. lactic acid ; 9. ethyl-alcohol ; 10. ben-
zoic acid ; n. propylamin ; 12. methylamin ; 13. phenol ; 14. formic acid.

THE NUTRITION OF HETEROTROPHIC PLANTS 179

DUCLAUX (1885, 1889) finds the following substances of special value to
Aspergillus: — i. dextrose; 2. cane sugar; 3. lactose; 4. mannite ; 5.
alcohol ; 6. acetic acid ; 7. tartaric acid ; 8. butyric acid.

LABORDE (1897) has compared Aspergillus with another fungus (Eurotiopsis
gayoni) and found that the latter was unable to make use of cane sugar and
tartaric acid, although it accepted lactic acid which was quite unsuitable for
Aspergillus. Finally WENT (1901) obtained the following series for Monilia
sitophila : — carbohydrates, acetic acid, mannite, glycerine, lactic acid, malic
acid, ethyl-alcohol, ethyl acetate, tartaric acid ; a number of other acids are
poor nutrients, while formic and benzoic acids are of no use at all. [Certain
Bacteria are able to exist with the aid of the traces of volatile organic com-
pounds occurring in the air (BEIJERINCK, 1903). According to certain older
experiments of ELFVING this is true also of many Fungi.]

There is nothing to be gained by the citation of additional examples, since
a comparison of the results obtained by different authors is at present un-
fortunately not possible, seeing that certain factors, of which we shall speak
by and by, have not been studied with sufficient completeness. Thus, e. g.
the nutritive value of a certain carbonaceous substance may depend on the
age of the fungus, for it is not infrequently the case that during germination
more exacting demands are made on such substances than later on ; Aspergillus,
e.g., germinates very badly in the presence of lactose and mannite, whilst
a somewhat older plant thrives quite well in the presence of these bodies.
In the second place, the chemical reaction of the substratum has to be noted.
In this relation there is a noticeable difference between Fungi and Bacteria ;
the former prefer weak acid solutions, the latter weak alkaline. In both cases,
however, an excess of free acid as well as of free alkali inhibits development.
The quality of the nitrogenous material also has an influence on the nutritive
value of any particular carbon compound. Thus glucose is the best source of
carbon for Monilia sitophila when peptone is used as the source of the supply
of nitrogen ; but if aspartic acid be used instead of peptone, cane sugar is
found to be far more valuable than glucose (WENT, 1901). As may be easily
understood the concentration of the nutritive solution is of importance, but
Fungi have a wonderful capacity for adapting themselves to high degrees of
concentration, as elsewhere occurs only in the case of germinating pollen-
grains (CORRENS, 1889. [MoLiscH, 1893.]) The high osmotic activities of
concentrated sugar solutions were alluded to earlier. Fungi germinating
in such solutions must develop a much higher osmotic pressure than usually
occurs in plant-cells, otherwise plasmolysis would be induced. ESCHENHAGEN
(1889), who carried out experiments on this subject in the Leipzig Institute,
obtained the following values for the maximum degree of concentration (in
weight per cent.) which ordinary Fungi could tolerate : —

Glucose. Glycerine.

Aspergillus niger 53 43

Penicillium glaucum 55 43

Botrytis cinerea 51 37

From these numbers we may calculate how great the pressure in the
interior of the cells must be, since the osmotic value of the cell-sap in a turgid
cell must always exceed that of the external solution.

[RACIBORSKI (1905) has obtained even greater values, for he has shown
that Aspergillus glaucus and a species of Torula could grow in concentrated
salt solution ; Torula germinated in a saturated solution of lithium chloride,
that is to say, in a fluid which gives the highest osmotic pressure of all
neutral salts.]

The adaptation to high degrees of concentration is apparently in many
cases effected by the formation of unknown osmotically active bodies in the cell

N 2

i8o METABOLISM

(HEINSIUS, 1901), in other cases, by the entry of the nutritive solution ; and in
Bacteria and Cyanophyceae especially, the protoplasm appears to be extra-
ordinarily permeable. In addition to its osmotic effect the nutritive solution
may have as well a poisonous action when certain variable concentrations are
reached. A 10 per cent, solution of alcohol is in general injurious to Fungi,
while a 2-4 per cent, solution is usually nutritive ; the maximum for butyric
acid lies much lower, i. e. about 0-4 per cent. In such cases it is conceivable
that the different organisms behave in different ways, and a certain form might
be able to tolerate a gradually increasing concentration, which would be fatal
if suddenly applied (compare MEISSNER, 1902).

Lastly, let us glance at the influence of temperature. As THIELE (1896)
has shown, the temperature maxima suitable for the growth of Penicillium lie
at variable heights according to the food materials provided ; development
ceases about 31° in the presence of grape sugar, with formic acid at about 35°,
and with glycerine at about 36°. Formic acid has, therefore, a greater nutritive
value at high temperatures than glucose, whilst at ordinary temperatures it is
nearly the worst, the glucose proving itself the best source of carbon.

All the points above indicated, and many others as well, must be
taken into account in any renewed investigations if exact results are to be
obtained as to the comparative nutritive value of the different compounds of
carbon. Though definite results have by no means been obtained in all cases,
still, from the researches already carried out, it may be concluded that a large
number of compounds may act as nutrients to Fungi, but that their value is
very unequal. The nutritive value of these compounds depends obviously on
the nature of the compound itself, but the special peculiarities of the organism
which it nourishes are also of importance. This latter condition becomes es-
pecially apparent when we compare ordinary Fungi, which we may term
omnivors, on account of their ability to nourish themselves with the most varied
food materials, with specialists, that is to say, such forms as are compelled,
during their life, to use certain definite substances as food. Thus Mycoderma
aceti thrives well on alcohol and acetic acid, substances which are of little
value to other Fungi. Bacillus perlibratus, according to BEIJERINCK (1893),
grows exceedingly well in acetic and malic acids, but seems to be unable to
assimilate tartaric acid, whilst this latter substance is, as a rule, a better food-
stuff and is especially acceptable to Bacillus cyanogenus. Many similar
examples might be adduced, and we shall meet with other cases of
'specialism' in the course of our studies.

At the same time it must be remembered that there are many organic
substances which are useless even to the most thoroughly omnivorous types.
NAGELI carried out experiments (1879) with the view of finding what part
was played by the constitution of the compounds in determining whether
they could be assimilated or not. It appeared that carbon could be assimi-
lated whether it occurred in the combination CH2 or CH ; in the combination
CHOH it proved injurious, and when present as CO or CN it was found to be
quite useless (NAGELI, 1879, 401). There are, however, many exceptions
known to this rule (REINKE, 1883, DIAKONOW, 1887, BEIJERINCK, 1901) ; thus,

NH C-OOH

for example, urea CO < XTTr2 and oxalic acid I are capable of nourish-

NH* C-OOH

ing certain organisms, and we may yet, by careful research, be able to show
that carbon may be assimilated when in combination with nitrogen. At present
we are forced to the conclusion that the constitution of the compounds
is not nearly so important as NAGELI supposed. That this is so is shown
by the fact that Fungi are able to obtain all their organic food just as
well from methane derivatives (glucose) as from benzol derivatives (quinic
acid). On the other hand, as the results of careful observation, it has been
shown that Fungi possess an extraordinary capacity for distinguishing sub-

THE NUTRITION OF HETEROTROPHIC PLANTS 181

stances which our ordinary chemical reagents fail to differentiate; that
of two bodies which have an entirely similar constitution and differ
only in the spacial arrangement of their atoms, the one is assimilated easily,
the other with difficulty or not at all. A well-known and typical example
of the behaviour of such ' stereoisomeric ' bodies has been given by PASTEUR
(1858, 1860). He cultivated Penicillium in optically inactive racemic acid and
showed that it was resolved into dextro- and laevo-tartaric acids, and that the
dextrotartaric acid was used up first. Numerous similar examples have been
discovered since then (PFEFFER, 1895), and it has been shown that many,
though not all, organisms prefer definite optically active substances. There is,
for example, a bacterium which behaves in exactly the reverse way to Peni-
cillium, and which prefers the laevotartaric acid (PFEFFER, 1895), while Bacillus
subtilis appears to have no preference for either. Similarly BUCHNER (1892)
has observed that fumaric acid forms a good nutrient for Aspergillus and
Penicillium, while the stereoisomeric maleic acid is known to be rapidly
poisonous. Reference should also be made here to lactic acid and many of the
glucoses, which we shall take a later opportunity of discussing.

Interesting as these conclusions are we do not obtain by their study any
deeper insight into the reasons for the unequal metabolic value of nearly
related bodies nor for the similar treatment of very different bodies; and
yet elucidation of these facts must be obtained if we are to reach a clear under-
standing as to the mode of assimilation of food materials. Meanwhile our
experiments with stereoisomeric bodies offer us valuable suggestions in other
respects. They show, for example, how well the power of selection is developed
in Fungi. Aspergillus is able to distinguish not only between dextro- and laevo-
tartaric acids but also between entirely distinct substances. Out of a nutritive
solution containing much glucose and some glycerine, it selects first of all
the more valuable food-material, viz. the glucose. It may be said indeed that,
in the presence of glucose, glycerine is not employed at all. The converse,
however, does not hold good ; the slightest traces of dextrose are greedily
absorbed, although glycerine be present in quantity. Similarly, PFEFFER
(1895) has shown that glycerine is excluded from metabolism in the presence
of peptone and lactic acid in the presence of dextrose.

Let us glance now at the requirements of heterotrophic organisms for
nitrogen. In the nutritive solutions we have employed hitherto we have in
general presented the nitrogen in the form of nitrate of ammonia, and we saw
that the requirements for nitrogen were met in this way, and that proteid was
undoubtedly manufactured. We have now to ask whether this is the only and
the best form in which one may offer nitrogen to Fungi. One very important
question is whether nitrate will act without ammonia, and whether Fungi, like
autotrophic plants, prefer a nitrate to an ammonium salt. As a matter of
fact, research has shown that different Fungi and Bacteria behave in totally dif-
ferent ways in relation to nitrogen, so that they have to be arranged in several
groups (compare BEIJERINCK, 1890, FISCHER, 1903, p. 96 ; [BENECKE, 1904]).

1. Nitrate Organisms. These thrive in the presence of nitrates quite as
well as, if not better than, along with other compounds. To this group belong
the Fungi Alternaria tenuis, Mucor racemosus, Aspergillus glaucus (LAURENT,
1889) ; and among Bacteria : foecal Bacteria (JENSEN, 1898), Bacillus pyocyanus,
and Bacillus ftuorescens. Some employ nitrites, e. g. Bacillus perlibratus
(BEIJERINCK, 1893) and a fungus described by WINOGRADSKY (1899).

2. Ammonia Organisms. These develop in the presence of nitrates but
thrive much better with ammonia. To this series belong, e. g. Eurotiopsis,
Aspergillus niger, yeast and Bacillus subtilis.

3. Amide Organisms. Bacillus perlibratus, Bacillus typhi and Rhizopus
oryzae grow better with asparagin than with ammonia. Other acid amides
and ammo-acids appear to operate in a similar manner.

182 METABOLISM

4. Peptone Organisms. Scarcely any growth takes place with asparagin
or ammonia ; nor can proteid replace peptone. Examples : Bacillus anthracis^
Bacillus proteus, lactic acid Bacteria (BEIJERINCK, 1901).

5. Proteid Organisms. Micrococcus gonorrhoeae and Bacillus diptheriae
require proteid and are unable to live in peptone or other nitrogenous substrata.
They certainly exist in nature as parasites only and, strictly speaking, do not
belong to this category.

6. Mention must also be made of organisms which use the free nitrogen of
the air in preference to other nitrogenous material. We shall refer to them
elsewhere.

The types which have been indicated show that among Fungi and Bacteria
there are all sorts of transitional forms, from those which are able to assimilate
nitrogen just like autotrophic green plants to suchas are dependent on previously
elaborated proteid. We may if we choose, therefore, designate the nitrate and
ammonia organisms as autotrophic so far as their relation to nitrogen is concerned,
and term the others heterotrophic. But just as we found that the requirements
of the plant, re carbon, often depended on the combination in which nitrogen
was presented, so conversely the nitrogen requirements depend on the
source of carbon. Thus A. FISCHER (1897, p. 53) found that Bacillus coli,
Bacillus subtilis, and Bacillus pyocyanus could use nitrate in presence of glucose ;
but if glycerine were substituted for glucose, Bacillus pyocyanus alone thrived ;
the others used ammonia exclusively as a source of nitrogen when glycerine
formed the source of carbon. When proteid or peptone was supplied as nutri-
tive nitrogen, an additional supply of a special source of carbon was found to be
unnecessary ; frequently the same was found to be true of asparagin. A com-
parative research on the nitrogen requirements of different Fungi would be of
special value since the results hitherto obtained are exceedingly fragmentary,
and similar researches are needed on the nitrogen requirements both of omnivors
and of specialists.

Nor are we able to answer comprehensively the problem as to the best
combinations of carbon and nitrogen, although it is generally held that a nutri-
tive solution containing peptone and glucose acts best. On the other hand,
CZAPEK (1902) has shown in the case of Aspergillus that the amino-acids are
preferable to peptone in presence of glucose. Taking into account the state-
ments in the literature, however, it cannot be said that any law of general
applicability has as yet been formulated. BEIJERINCK (1891) found, for
example, that peptone alone (as a source of carbon and nitrogen) was a better
medium for Bacillus cyaneofuscus than asparagin and glucose, and WENT (1901)
demonstrated in the case of Monilia that when glucose formed the source of
carbon, peptone was preferable to all other substances ; with asparagin it reached
only a third of the increase in weight gained when peptone was supplied, while
leucin was found to be far inferior to potassium nitrate. Ammonium salts of
acetic acid, tartaric acid, &c., were found to be specially unsuitable combina-
tions of carbon and nitrogen, although many Fungi for long maintained their
growth in them.

The ubiquity of Fungi and of biologically related plants is due to their
capacity for living on the most varied organic materials, together with their
remarkable power of adapting themselves to highly concentrated nutritive
solutions. The general occurrence of dead vegetable tissues renders the exist-
ence of Fungi possible in most situations, and so we see that on dead leaves, twigs,
and fruits, a covering of Fungi soon appears, provided only the general condi-
tions be sufficiently moist. Dead animals also, as well as animal excrement,
form a suitable medium for the growth of Fungi, so long as the reaction be acid,
as is generally the case in plant debris ; when the reaction is alkaline, on the
other hand, Bacteria are the dominant organisms. Under the influence of
these microscopic forms a decomposition of the debris of the higher organ-

THE NUTRITION OF HETEROTROPHIC PLANTS 183

isms takes place (which we will consider in greater detail later on) which fre-
quently leads to the formation of humus. Humus again forms the home of
a large number of Fungi and also of Phanerogams, which, like Monotropa and
Neottia, are heterotrophic in their mode of nutrition owing to their possessing
no chlorophyll. Leaving on one side for the moment the Phanerogams, whose
complicated nutritive relations we shall consider later, there are numberless
Basidiomycetes at least, of which it may be said that they obtain all the organic
materials they require from humus, although what precisely these substances
consist in we are quite ignorant. The peculiar humus substances which are
soluble in alkalis cannot, according to REINITZER (1900), at least in most
Fungi, act as a source of carbon, although they may serve as a source of
nitrogen (compare NIKITINSKI, 1902) ; REINITZER, who experimented with
Penicillium, holds that possibly certain ' specialists ' may obtain their supply
of carbon from humins.

We do not know precisely what other organic substances occur in humus
besides humins, and although we are unable to extract any useful nutritive
materials from it by means of ordinary chemical media, it is quite likely that
many of the Fungi are able to do so by excreting enzymes with dissolving
capacities. These enzymes are among those already studied, viz. diastases
and sugar-splitting enzymes, also cytase and protease. (Many other enzymes
occur in Monilia ; compare WENT, 1901). On the enzymes occurring in Fungi
a number of interesting observations have been made, two of which only we
shall refer to here. In the higher plants we found cytases occurring as
a rule only when required to dissolve reserve cellulose ; the cellulose of ordi-
nary cell-walls in the presence of these enzymes remains as originally formed
completely untouched ; it is not dissolved and reabsorbed before the fall of
the leaf, and thus when the leaves and branches fall a large amount of organic
material is lost. In many Fungi, however, not merely those which live as
' specialists ' on wood, such as Merulius lachrymans, the dreaded ' dry rot ', and
other wood destroyers, but also in Fungi in general, the power of dissolving cell-
walls has been definitely proved to exist. This capacity is obviously developed
in many cases only to enable the fungus to enter the interior of the cell ; the
dissolution of the cell- wall is subsidiary to the chief object, viz. to reach the
cell contents, starch, &c. In other cases the fungus apparently lives chiefly on
the cellulose, and is even able (CZAPEK, 1899) to make use of lignified walls,
effecting, by means of a special enzyme, a splitting off of the cellulose from
the etherial .substance combined with it (hadromal, compare p. 70). The
cellulose is assimilated, but the hadromal remains unused. In this capacity
possessed by these Fungi we have one mode of destroying cellulose in nature ;
in Lecture XVII we shall have to speak of another method of achieving the
same result. Were it not for such decompositions the surface of the earth
would soon be entirely covered with thick layers of cellulose.

Another observation which may be referred to deals with the excretion of
diastase. So far as seedlings are concerned a controversy still exists as to
whether diastase may be excreted from living cells, since it is often assumed
that it cannot pass through the cell-wall (compare p. 156, Lecture XII). It
cannot be doubted that in the case of Fungi and Bacteria such an exudation
of diastase through the cell-wall does take place. WORTMANN (1882), and
especially PFEFFER (1896) and KATZ (1898), have shown that the production
of diastase is not a constant characteristic of certain Fungi, but that it can
be induced or inhibited by external conditions. In the abundant presence
of various sugars but not of all suitable nutritive sources of carbon, no
diastase is formed; in Penicillium, for example, a 2 per cent, solution
of sugar is sufficient to inhibit its formation. Bacterium megatherium behaves
in a similar manner, while in the case of Aspergillus a 30 per cent, solution of
sugar only retards, but does not inhibit, the formation of diastase. The

184 METABOLISM

formation of the enzyme is often, though by no means always (WENT, 1901),
regulated by the need for it ; when the product of the activity of the enzyme,
or perhaps only a body like it, is present in the plant, the enzyme is not
required. Doubtless the formation and dissolution of starch, for example, is
regulated in higher plants in this way, and similar self-regulating processes
are likely to occur, other than those associated with metabolism.

Without entering further into a discussion of the other enzymes (on which
an extensive modern literature exists, e.g. FERMI and BUSCAGLIONI, 1899, SAN-
GUINETTI, 1897, MALFITANO, 1900, KOHNSTAMM, 1901, BRUNSTEIN, 1901), we
can easily see how greatly the possession of such a secretion capable of bringing
about extra-cellular solution facilitates the distribution of Fungi in nature,
where suitable nutritive solutions are not always to be obtained ready to hand.
They possess not only dissolving but also splitting enzymes as well.
Disaccharides must be split up before they can be used, for if an organism
such as Bacillus perlibratus (BEIJERINCK, 1893) possessed no sugar-splitting
enzymes it could thrive only on dextrose and levulose, but would be debarred
from using maltose, cane sugar and lactose.

The heterotrophic plants we have hitherto been considering are sapro-
phytes, i. e. they exist in nature on the dead remains or the excreted meta-
boh'c products of the animal and plant worlds. In a certain sense we may
describe the biological group of plants known as carnivors as an intermediate
class between the saprophytes which we have been studying and the parasites
yet to be discussed.

These much investigated and well-known carnivorous, or, as one may
more specifically term them, insectivorous plants have excited the greatest
interest not merely among botanists but the general public as well, on account
of the contrivances by which they obtain possession of their nutriment and by
the methods they use in digesting it. But from a purely physiological point
of view they are so gradually transitional to other types that they might be
classed equally correctly under saprophytes or under autotrophic plants. We
cannot enter here into a description of the morphological characters of these
plants ; we may refer to Figs. 33-35 and to the special treatises on the subject,
especially those of GOEBEL (1891-3), and only remark that in order to catch
small insects essentially three types of apparatus are employed, viz. (i) pitchers,
or trap-like cavities, such as the pitchers of Nepenthes (Fig. 33), Sarracenia and
Cephalotus, or the bladders of Utricularia (Fig. 35) ; (2) closing traps, that is,
organs which catch insects by active movements, Dionaea (Fig. 34) and Aldro-
vanda ; (3) sticky hairs (Drosera, Fig. 156, Lecture XXXVIII), Drosophyllum,
Pinguicula. Combinations of these contrivances also occur.

In certain simple cases, such as the bladders of Utricularia9 the animals
remain for a long time alive, die at last of hunger and decompose in
situ as the result of the action of Bacteria. We may assume that their
excrement at first and, later, the products of the decomposition of their
bodies are used by the plant as sources of nitrogen, so that a truly car-
nivorous habit does not here exist. A similar state of affairs appears to
hold good for Sarracenia and Cephalotus, and hence we may group these
plants in the same category with others which possess water reservoirs,
especially the Bromeliaceae already mentioned and Dischidia rafflesiana,
with its remarkable water pitchers ; in all these cases animals are invariably
present in the water reservoirs, their products of decomposition being in course
of time absorbed by the plant. The typical carnivorous plants are distinguished,
however, by the fact that they secrete proteolytic enzymes, for the most part
along with acids, by means of which they are able to dissolve proteid materials.
Further differences exist between the different types, inasmuch as in some
species the protease and the acid are always being secreted, whilst in other cases
one or both substances are produced only in response to a stimulus, especially

THE NUTRITION OF HETEROTROPHIC PLANTS

185

the chemical stimulus resulting from the presence of a digestible substance.
As yet, however, we know very little either about the enzyme or the acid.
According to one account, formic acid is declared to be present, but more recently
this has been denied. VINES'S (1897-1902) discovery that Nepenthes excreted
a tryptic ferment has been lately questioned by CLAUTRIAU (1900), who thought
he had proved the presence of a pepsin. It is true that minute particles of meat,

Fig. 34. Leaf of Dionaea musciputa. When the hairs on the
upper side of the leaf are touched the two half blades instantly come
together. (From the Bonn Textbook.)

Fig. 33. Pitcher of Nepen-
thes^ partly cut open to show
contents. The basal region is
occupied by a fluid excreted
from special glandular hairs in
which the animals which fall
into the pitcher are digested.
From the Bonn Textbook.)

Fig- 35- Utrtcularia vulgaris. A^ portion of the leaf with blad-
ders ; B, portion of a leaf with bladder ; C, bladder in longitudinal
section (x 28) ; v, trapdoor; a, wall ; /, central cavity, (frrom the
Bonn Textbook.}

fibrin, &c., are dissolved rapidly by Nepenthes and by Drosera, and that, too,
certainly without the help of Bacteria. Many of the secretions of carnivorous
plants have, in addition, antiseptic characters, so that micro-organisms are
quite unable to exist in them.

The products resulting from the dissolution of proteid are absorbed either
by the secretory hairs which produce the enzyme or by other special trichomic
structures. Digestion and absorption take place often very rapidly. Thus
DARWIN (1876) observed that small cubes of white of egg were dissolved in the
course of one or two days, and that the gelatinous fluid resulting was com-
pletely absorbed in about three days.

186 METABOLISM

The question whether carnivorous plants are benefited by being fed on
insects has often been discussed. It is a well-established fact that they can
not only exist but also thrive without any such nutriment. On the other
hand, it has often been shown that such food materials produce a favourable
result when given in sufficient amount. In the case of BUSGEN'S (1888) re-
searches, for example, the increase in growth of fed Utricularia shoots was double
that of the unfed ; and the same author (1883) showed that Drosera exhibited
remarkable results after being nourished on meat from the seedling stage right
up to the formation of seed. The dry weight of the plants which had been fed
was one and a half to three times that of the unfed ; the fed plants had three
times as many inflorescences, and formed five times as many capsules. There
can be no doubt that carnivorous plants are autotrophic so far as the acquisition
of carbon is concerned ; all of them have abundant chlorophyll and can thrive
without receiving any proteid food. The favourable action of the latter cannot
well depend on the gain in organically combined carbon ; it is more probable
that it originates from the nitrogen supplied or certain nutritive salts. It is
possible that feeding with insects is beneficial simply because a larger amount of
combined nitrogen and materials of the ash is introduced in this way into the
plant than could be obtained from the soil only. It is even more probable,
however, that the quality of the materials which are absorbed by the leaves
is of significance ; for instance, the obtaining of organically combined phos-
phorus or organic nitrogen may be the end specially aimed at. In the latter
case, which, without more exact proof may be regarded as the point of chief
importance, we must regard peptones as especially valuable. It is, in fact,
perfectly obvious that carnivorous plants are peptone-feeders, or in other words
that they are better nourished when the nitrogen is in the form of peptone
than in that of nitrate or ammonia. This idea is well worthy of investigation ;
meanwhile, we must depend on analogies and it is important to remember that
there are peptone organisms not only among Fungi, which are also dependent
on carbon in the organic form, but also among green autotrophic plants.
BEIJERINCK (1890) and ARTARI (1899) have shown that certain Lichen-Algae
(compare Lecture XIX) prefer peptone to any other compound of nitrogen.

The biological position of carnivorous plants is at present somewhat in-
definite, still it is fairly well settled that their proper place is among what
may be termed ' nitrogenously heterotrophic ' plants. As already remarked,
we have another series of heterotrophic plants to consider, namely parasites.
These forms are united with saprophytes by many transitions. Fungi are known
(compare DE BARY, 1884) which live as a rule saprophytically, but which still
have the power of penetrating living organisms and of taking their nourishment
from them. As examples of such ' facultative parasites ' may be taken Peni-
cillium glaucum and other Mould-fungi which enter into wounds in ripe fruit and
cause putrefaction in them ; 'further, Sclerotinia sclerotiorum may be mentioned,
as a fungus which can carry out its entire cycle of development as a saprophyte,
and does so in nature not infrequently, but which, after attaining sufficient
vigour, can live also as a parasite on many plants. The converse condition
is also known, viz. Fungi, which usually live as parasites, but which can
nourish themselves saprophytically (' facultative saprophytes/ e. g. Phytoph-
thora omnivora and many Bacteria), and research has certainly shown that
very many, if not all, parasites can grow and increase without the aid of
their usual hosts. The probability of such a result is very varied in different
degrees of parasitism.

The lower stages of parasitism include such Fungi as appear to attack
many different species of plants, as, for example, the already mentioned Phy-
tophthora omnivora, which is parasitic on Fagus, Sempervivum, Oenothera and
other plants, and Sclerotinia sclerotiorumt which is apparently able to attack
all succulent parts of .plants. These parasites are distinguished from ordinary

THE NUTRITION OF HETEROTROPHIC PLANTS 187

saprophytes by their power of entering into the host plants, killing their cells,
and robbing them of their nutritive contents. If these omnivorous parasites
avoid individual plant species one must consider it as due to their want of
ability to enter them and not that they cannot find in them food materials
suited to their wants. It has indeed been proved that facultative saprophytes
make no demands on any definite nutritive material. It is otherwise with
Fungi which are limited to a single family, genus, or species, which are par-
ticular in the selection of a host, often indeed confining themselves to a single
species ; these types represent a higher grade of parasitism. As examples we
may cite Cordyceps militaris, a parasite on certain insects ; many Uredineae
and Ustilagineae on specific representatives of some plant families ; Cystopus
portulacae, which occurs only on Portulaca; Uromyces tuberculatus, only on
Euphorbia exigua\ Laboulbenia baeri only on the common house-fly. Un-
doubtedly this limitation to one or a few organisms which act as hosts depends
on the fact that these hosts are not only specially suited for the entrance of
the parasites, but chiefly that they provide some peculiar and special nutrient
material required by the fungus, although we cannot say exactly what its
nature is, or at best can only guess.

The number of parasites among Fungi is very great. The Fungi are,
indeed, generally speaking, so far as they do not exhibit special features of
which we shall speak later, typically heterotrophic plants. There are fewer
heterotrophic plants, and more specially parasites, among Phanerogams, but the
latter exhibit so many variations that they claim special mention. First of all,
we have Lathraea and Orobanche, which, owing to their want of chloro-
phyll, remind one of Fungi, and in many tropical forms, e. g. the RafHe-
siaceae, the likeness to Fungi appears even in the structure of the vegetative
organs. The use of carbon-dioxide by these plants is naturally quite impossible ;
in relation to the acquisition of carbon, nitrogen and minerals, they are com-
pletely dependent on their host plants, and generally speaking, they possess no
organs with which to absorb materials from the soil. How much they are
dependent on their host plants is especially well seen in their germination, which,
in_the case of Lathraea and Orobanche, takes place only when the seed is placed
in immediate proximity to the root of the host-plant ; in such cases there must
be some definite substance given off by the root which permits of germination
taking place (Lecture XXV). In the same group of parasitic Phanerogams occurs
also Cuscuta. This plant cannot live without a host, although isolated shoots
may become green (PEIRCE, 1894). It may be assumed that the chlorophyll
arising in this way is functional, but that the products of assimilation are
insufficient in quantity to maintain the plant in life. We must look upon the
capacity for forming chlorophyll as an indication that Cuscuta has been evolved
from chlorophyll-bearing plants ; like Lathraea, it in all probability quite lost
the power of forming chlorophyll at a very early period in its phylogenetic
history. That is certainly not essential since a whole series of Phanero-
gams have not lost it in their gradual transition to a parasitic mode of life,
as, for example, many Scrophulariaceae (Euphrasia, Rhinanthus, Bartsia,
Tozzia), Santalaceae (Thesium) and Loranthaceae (Viscum, Loranthus). Among
these the Rhinantheae have been most closely studied. With the exception
of Tozzia, they are in their germinating stage independent of the presence of
a host plant and can often carry on their development for a certain time
without any host at all. Tozzia has progressed farthest in parasitism and is
most dependent on outside help ; the other extreme is occupied by individual
species of the genus Euphrasia (E. odontites, E. minima), which can reach the
flowering and fruiting stage without a host, while others, e. g. E. rostkowiana,
although able to germinate without the aid of a host, develop only into dwarf
forms. These green parasites are not very particular as to what host they
select ; one reason for this is that their seeds, if sown sufficiently closely, hold

i88 METABOLISM

on to others of the same species, as L. KOCH (1888) was the first to show,
and that one of several seeds bound together by haustoria develops at the
expense of the rest.

HEINRICHER, to whom we owe exhaustive researches on the Rhinantheae
(i897,onwards) has endeavoured to prove that these parasites decompose carbon-
dioxide by means of chlorophyll in the normal way. This cannot be proved
with certainty, and researches are especially wanting as to whether the pro-
ducts of assimilation so formed are quantitatively sufficient for their needs
(compare what has been said above as to Cuscuta). It is possible that the chloro-
phyll function may be present but in a weakened form, that it is insufficient for
the plant's requirements, and that the plant must have recourse to ready formed
carbohydrate. On the other hand, one may explain the parasitism of these plants
by saying that they are dependent on their hosts only for nitrogenous material,
perhaps in the form of proteid or amide bodies, or for the materials of the ash.
HEINRICHER, on account of the quantity of nitrates in parasites, decides in
favour of the latter possibility and concludes that these green parasites employ
their hosts as a source of unelaborated sap only. The question is a purely
experimental one and the decision will depend on the results of such experi-
ments.

The common mistletoe is still imperfectly known so far as the physiology
of its nutrition is concerned. Owing to the fact that the union between host
and parasite is limited to the water-carrying vessels, we are led to believe that
Viscum takes only water and inorganic salts from its host ; and such a relation is
more probable in a plant that lives high up on trees than in forms like Euphrasia,
which has roots in the soil and is partly, at least, provided with roothairs.
Viscum may be conceived as a plant, originally epiphytic, which has surmounted
the difficulty of the deficiency of water and salts — against which most epiphytes
have to contend — by attaching itself to the vascular systems of other plants.
This conception has not, however, been substantiated, and here also experi-
mental research is necessary.

Exhaustive morphological and developmental studies on these interesting
phanerogamic parasites are to be found in the following works : —

Orobanche : KOCH (1887) ; Lathraea : HEINRICHER (1895) ; Cuscuta :
KOCH (1880), PEIRCE (1894).

Rhinanthaceae : KOCH (1889 and 1891), HEINRICHER (1897, 1898, 1901) ;
Loranthaceae : PITRA (1861).

The difference between autotrophic and heterotrophic plants, it must once
more be clearly pointed out, lies solely in the mode of absorption of nourish-
ment, and consequently we may speak of unicellular green organisms
only as autotrophic, for there the whole plant is concerned ; on the other
hand, in the case of the higher plants only certain parts are autotrophic, the
leaves especially, whilst others, as, for example, the roots, are completely
heterotrophic. So far as regards the further alteration of the organic com-
pounds of carbon and nitrogen, it is quite immaterial whether these are used
for constructive purposes in the regions where they are made or whether they
be translocated in an already prepared condition. There is no essential
difference between the metabolism of heterotrophic and of autotrophic forms.

As in green plants, so in Fungi, the nutritive substances are employed in
the construction of the body, are stored in reserves or, when of no more use,
are transformed into waste products ; that is to say, we have here also to
distinguish (i) plasta ; (2) reserves ; (3) translocatory materials ; (4) excreta.
On the whole the cell of the fungus is constructed out of the same kind of materials
as that of the Phanerogam, and although there are deviations in individual cases,
e. g. in the occurrence of chitin in their cell-walls, still we need not go further
into the matter, since, both in these as in the higher plants, we are unacquainted
with the conditions of origin of the different constituents of the cell. The

THE NUTRITION OF HETEROTROPHIC PLANTS 189

principal agreement with autotrophic plants lies, in the first instance, in the
nature of the reserve substances. In addition to nitrogenous we also find
non-nitrogenous reserves in Fungi, and among these the fats are especially
widely distributed ; on the other hand, since chromatophores are entirely
absent, Fungi form no starch. In regions where temporary or more per-
manent storage of carbohydrates might be expected to take place we
frequently find starch replaced by glycogen, a substance which occurs in
animals also. In yeast-cells glycogen (LAURENT, 1890, MEISSNER, 1900) is
formed often in considerable quantity from the sugar present in the nutritive
solution, and also apparently from various organic acids. Its accumu-
lation in organs which are capable of growing in length with great activity in
a short time, e. g. the stipe of Phallus (CLAUTRIAU, 1895), is especially remark-
able ; during this growth glycogen is altered just in the same way as starch
would be under similar conditions in the stems of Phanerogams, in order to
provide material for the formation of cell-walls.

Glycogen in its composition is closely related to starch, but it is soluble
in water. Its large molecule makes it incapable of diffusing through either
protoplasm or cell-wall, and it is for that reason well adapted to act as a reserve
substance. It cannot wander from cell to cell ; it must first of all be trans-
formed into sugar by means of some enzyme related to diastase. It cannot be
employed directly as a nutrient by yeast, since the enzyme cannot be excreted
from the cell.

It is unnecessary for us to discuss the other metabolic processes in the
fungus cell since, as we have said, they agree entirely with the corresponding
processes in autotrophic Phanerogams.

Bibliography to Lecture XV.

ARTARI. 1899. Bull, naturalistes de Moscou, No. i.

[ARTARI. 1904. Jahrb. f. wiss. Bot. 40, 593.]

DE BARY. 1884. Vgl. Morph. u. Biol. d. Pilze. Leipzig.

BEIJERINCK. 1890. Bot. Ztg. 48, 766.

BEIJERINCK. 1891. Ibid. 49, 705.

BEIJERINCK. 1893. Centrbl. Bakt. 14, 834.

BEIJERINCK. 1901. Archives neerland. II, 6, 212.

[BEIJERINCK and DELDEN. 1903. Centrbl. Bakt. II, Abt. 10, 33.]

[BENECKE. 1904. Lafar's Handb. d. techn. Mykologie, i, 303-429.]

BRUNSTEIN. 1901. Bot. Centrbl. (Beihefte) 10, i.

BUCHNER. 1892. Ber. d. chem. Gesell. 1161 (quoted by Pfeffer, 1895).

BUSGEN. 1883. Bot. Ztg. 41, 569.

BUSGEN. 1888. Ber. d. bot. Gesell. 6, LV.

CLAUTRIAU. 1895. Acad. de Belgique, Cl. d. sc. (Koch's Jahresbericht, 6, 51).

CLAUTRIAU. 1900. Mem. couronn., in 8°, Acad. Belgique, 59.

CORRENS. 1889. Ber. d. bot. Gesell. 7. 265.

CZAPEK. 1899. Zeitschr. f. physiol. Chem. 27, 141.

CZAPEK. 1902. Beitr. z. chem. Phys. u. Path, i, 538 ; 2, 557 ; 3, 47.

DARWIN, C. 1876. Insektenfressende Pflanzen (German edition by Carus).

Stuttgart.

DIAKONOW. 1887. Ber. d. bot. Gesell. 5, 380.
DUCLAUX. 1885. Compt. rend. Soc. biolog.
DUCLAUX. 1889. Annales Instit. Pasteur, 3, 97 and 413.
ESCHENHAGEN. 1889. Einfl. v. Losungen versch. Konz. auf Schimmelpilze.

Diss. Leipzig.

FERMI and BUSCAGLIONI. 1899. Centrbl. Bakt. II, 5, 24.
FISCHER, ALFR. 1 897. Vorlesungen uber Bakterien. Jena.
FISCHER. 1903. Ibid., 2nd ed. Jena.
GOEBEL. 1891-93. Pflanzenbiol. Schilderungen. Marburg.
HEINRICHER. 1895. Beitr. zur Biol. d. Pfl. 7, 315.

HEINRICHER. 1897, onwards. Jahrb. f. wiss. Bot. 31, 77 ; 32, 389; 36, 665 ; 37, 264..
HEINSIUS v. MAYENBURG. 1901. Jahrb. f. wiss. Bot. 36, 381.

190 METABOLISM

JENSEN, Hj. 1898. Centrbl. Bakt. II, 4, 401.

JOST. ^ 1895. Jahrb. f. wiss. Bot. 27, 403.

KATZ. ' 1898. Jahrb. f. wiss. Bot. 31, 599.

KOCH, L. 1880. Die Klee- u. Flachsseide. Heidelberg.

KOCH, L. 1887. Entwicklungsgesch. d. Orobanchen. Heidelberg.

KOCH, L. 1889. Jahrb. f. wiss. Bot. 20, i.

KOCH, L. 1891. Ibid. 22, i.

KOHNSTAMM. 1 90 1. Bot. Centrbl. (Beihefte) 10, 90.

LABORDE. 1897. Annales Instit. Pasteur, n, i.

LAURENT. 1889. Annales Instit. Pasteur, 3, 368.

LAURENT. 1890. Koch's Jahresb. iiber Garungsorg. i, 54.

LAURENT. 1898. Compt. rend. 127, 786.

[LAURENT. 1904. Revue gen. de Bot. 16, 14.]

MALFITANO. 1900. Annales Instit. Pasteur, 14, 60 and 240.

MEISSNER, CURT. 1902. Akkommodationsf ahigkeit d. Schimmelpilze. Diss. Leipzig.

MEISSNER, R. 1900. Centrbl. Bakt. II, 6.

[MOLISCH. 1893. Sitzb. Wien. Akad. 102, I, 423.]

NAGELI. 1879. Ernahrung d. nied. Pilze. Bot. Mitt. 3, 395.

NAGELI. 1882. Unters. iiber nied. Pilze. Munich and Leipzig.

NIKITINSKI. 1902. Jahrb. f. wiss. Bot. 37, 365.

PASTEUR. 1858-60. Compt. rend. 46, 617 ; 51,298.

PASTEUR. 1860. Annales Chim. et Phys. Ill, 58, 323.

PASTEUR. 1862. Ibid. Ill, 64, 106.

PEIRCE. 1894. Annals of Bot. 8, 53.

PFEFFER. 1895. Jahrb. f. wiss. Bot. 28, 205.

PFEFFER. 1896. Ber. Sachs. Gesell. Wiss. (Math. -phys. Cl.), 513.

PITRA. 1 86 1. Bot. Ztg. 19, 53.

[RACIBORSKI. 1905. Bull. Acad. de Cracovie, Math.-nat. Cl. 461.]

RAULIN. 1869. Annal. Sc. nat. v, n, 91.

REINITZER. 1900. Bot. Ztg. 58, 59.

REINKE. 1883. Unters. aus d. bot. Labor. Gottingen, 3, 13.

SANGUINETTI. 1897. Annal. Instit. Pasteur, n, 264.

THIELE. 1896. Temperaturgrenzen d. Schimmelpilze (comp. Pfeffer, Phys. I, 373 .

VINES. 1897-1902. Annals of Botany, n, 563 ; 12, 545 ; 15, 563 ; 16, i.

WEHMER. 1895. Beitr. z. Kenntnis einh. Pilze, Jena, Heft u, p. 86 (Koch s

Jahresbericht, 1895).

WENT. 1901. Jahrb. f. wiss. Bot. 36, 6n (also Centrbl. Bakt. II, 8, 544).
WINOGRADSKY. 1899. Centrbl. Bakt. II, 5, 342.
WORTMANN. 1882. Zeitschr. f. physiol. Chemie, 6, 287.
ZUMSTEIN. 1899. Jahrb. f. wiss. Bot. 34, 149.

LECTURE XVI
RESPIRATION

So far we have been studying certain of the chemical processes which go on
in the plant from one point of view only ; we have dealt with the phenomena
of assimilation, i. e. the construction of complex compounds out of simple ones, of
organic substances out of inorganic, and also glanced at the alterations which
products of constructive metabolism undergo when they become reserves, plasta,
&c. ; but we drew special attention to the fact (p. 124) that another series of
processes took place in the plant which resulted once more in the formation of
simple bodies from complex. This statement we must now emphasize. Through-
out the entire plant and at all times what may be termed dissimilation is
going on — a process which partly, at least, undoes what assimilation has done.
On examining a leaf which has been assimilating all day, but which has been
prevented from getting rid of the products of assimilation we find that it does
not contain at nightfall as much carbon in the organic form as we should expect
from the amount of carbon-dioxide decomposed, nor does the plant as a whole

RESPIRATION 191

at the end of summer contain as much organic material as would be equivalent
to the sum of the amounts assimilated during the several days of the annual
period of metabolic activity. The difference between the total amount assimi-
lated and the total amount dissimilated is the increase in dry weight — the
net result of normal plant growth. Plants may be easily cultivated under
conditions where assimilation is prevented or greatly retarded (e. g. in the
case of autotrophic plants grown in darkness, or of heterotrophic plants in
absence of nutrients), and under such circumstances destructive metabolism
still goes on and continued growth now results in a diminution in dry weight.

This is admirably shown by a study of seedlings which have been grown
in the dark, though at first sight such a conclusion is by no means obvious.
The seedlings grow from day to day, and roots and shoots increase markedly
in volume, but that increase is entirely due to absorption of water, and the dry
weight, and more especially the organic material, decreases daily. The following
summary of one of BOUSSINGAULT'S experiments shows this clearly (DETMER,
1880, p. 247) :—

Material.

46 wheat grains
10 peas

Dry weight of seeds.

1-665 gr.
2-237 gr.

Dry weight of seedlings

several weeks old, grown in

darkness.

Loss.

0-952 gr.
i.i6igr.

A comparison between assimilation and dissimilation is more readily made
in a fungus than in one of the higher plants, for we have only to determine how
much nutritive material (e. g. sugar) the fungus has absorbed, how much dry
substance it has formed from it, and how much it might have formed. As
a basis for this last calculation we reckon that a fungus can construct about
2 gr. of dry weight for every gram of cane sugar absorbed, instead of which we
find that only 0-4 gr., or 0-5 gr., or even less, is produced. PFEFFER (1895, 257)
and KUNSTMANN (1895) have termed the numerical relation between the sugar
used up and the fungal substance formed the ' economic coefficient'. Theo-
retically the minimum value of this coefficient is about J, but in reality it has
always been found to be greater than unity. KUNSTMANN (1885) gives it as
from 1-13 to 3-38, and ONO (1900) obtained a value as high as 6-1, so that we
must not consider the coefficient as in any sense a constant, nor look on the
plant as always an economical worker. The coefficient increases, for example,
with the progressive development of the fungus and with elevation of tempera-
ture. Among other external influences poisons must be specially noted, the
stimulating e&ects of which in weak doses we have already drawn attention to.
Thus it has been shown by ONO that the addition of a 0-003 per cent, to a 0-03
per cent, solution of zinc sulphate reduced the economic coefficient, in the case
of Aspergillus, from 6 (or 4 in other experiments) to about 2-8. The chemical
stimulus resulting from the addition of such substances induces an economy
in the expenditure of food materials. The deficit appearing from day to day
under all conditions is the result of dissimilation.

No organism can remain in existence without constantly losing weight
from the dissimilation or destruction of organic substance. We may term
this katabolic process respiration, whatever be the nature of the final pro-
ducts, or we may reserve this name for those destructive changes which are
exhibited by most plants under ordinary conditions and which result in the
formation of carbon-dioxide and water from organic materials. Of these
products of decomposition the most obvious one is carbon-dioxide ; the
production of water is much less easily demonstrated. Both carbon-
dioxide and water may be obtained by decomposition of organic materials
such as starch, sugar, &c., not merely in the course of respiration in the
organism but also by ordinary combustion. Hence we may conclude that
respiration is a combustion process, and every experiment goes to show

I92 METABOLISM

that oxygen is essential to the maintenance of respiration. Respiration may,
therefore, be also described as an oxidation process, standing in marked con-
trast to carbon assimilation, which we have learned to regard as a process
of reduction.

It will be necessary for us to study, in the first instance, the methods of
demonstrating respiration so that we may appreciate to what extent respiration
occurs in the vegetable kingdom. As a proof of its occurrence we shall employ,
as a general rule, the excretion of carbon-dioxide, a gas which may be measured
quantitatively and qualitatively without any difficulty. Place, for example,
a handful of germinating seeds in a flask, closed by means of a rubber stopper
through which passes a glass tube, and keep the tube closed for a few hours ;
then open the tube under lime water — the resulting cloudiness in the fluid
demonstrates that a certain amount of carbon-dioxide was present in the flask.
In place of lime water caustic potash may be employed, as this substance readily
absorbs carbon-dioxide and replaces the gas in the flask by way of the glass
tube. Perhaps the simplest, although a more indirect, method of all for
demonstrating respiration is based on the fact that oxygen is used up in pro-
portion to the carbon-dioxide given off. If we place some germinating seeds,
young leaves, buds, &c., at the bottom of a tall glass cylinder closed by
a stopper, and if after several hours we insert a burning taper into the jar
after careful removal of the stopper, we shall see from its immediate extinction
that the enclosed air has been deprived of most of its oxygen through the
activity of the vegetable structures within.

It is obvious that both the method by absorption of carbon-dioxide by
caustic potash and that by precipitation of carbonate of lime on the addition of
lime water furnish us with a means of determining quantitatively how much
carbon-dioxide is produced during respiration, still the use of completely closed
spaces in such experiments is to be avoided, since respiration itself, under such
conditions, becomes abnormal owing to the rapid decrease in the amount of
oxygen present. It is preferable, therefore, to place the plant experimented
on in a vessel through which a continuous stream of air may be driven. This
air is deprived of all its carbon-dioxide before entering the vessel and becomes
once more charged with that gas within it ; the amount added may be easily
determined as the gas leaves the vessel at the other end. Into the purely
chemical details of the experimental method it is unnecessary for us to enter.
The amount of oxygen used up may also be employed as a measure of the amount
of respiration taking place, as well as the amount of carbon-dioxide produced.

The first conclusion we arrive at from a study of comparative estimates
of the intensity of respiration is that different plants, different members of the
same species, and even the same organ of an individual plant in different stages
of development exhibit the widest possible variations. Certain biological groups,
such as oily and shade-loving plants, are remarkable for the feebleness of their
respiration, while on the other hand, many Fungi exceed the warm-blooded
animals in respiratory activity. Flowers, embryonic organs, germinating seeds,
buds, &c., appear to respire more vigorously than full-grown roots, stems, or
leaves, assuming of course that external conditions remain constant. It will
be advisable at this stage to illustrate these statements by a few tables.

According to AUBERT (1892, p. 375) the following plants absorb hourly the
following amounts of oxygen (in ccm.) per gram of fresh weight, at a tempera-
ture of 12° to 15° C. :—

Cereus macrogonus 3-00 Picea excelsa 44-10

Opuntia cylindrical 680 Lupinus albus 73-7°

Opuntia maxima JS'S0 Tulipa europea 89.60

Phyllocactus grandiflorus 28.70 Faba vnlgaris 96-60

Sedum album 56-60 Mirabilis jalapa 120-00

Sedum acre 72.45 Triticum sativum 291-00

Temperature.

Fresh weight
of buds.

Dry weight
of buds.

CO2 in
24 hrs.

CO2 per i gr.
of dry weight.

15° C.

9-0 gr.

2.0 gr.

70 ccm.

35 ccm.

,,

10^0 ,,

1.75 „

60 „

34 „

,,

7.0 ,,

1-25 „

60 „

48 „

„

4.0 „

0.70 „

46 „

66 „

RESPIRATION 193

The succulent plants in the first series, it will be noted, respire on an average
far less actively than the others.

As examples of excessive respiratory activity, GARREAU'S (1851) data,
obtained from germinating seeds, may be next quoted : —

Plant Temoerature Fresh weight Dry weight CO2 in CO2 per t gr.

of seeds. of seeds. 24 hrs. of dry weight.

Lactuca sativa 16° C. 4.5 gr. 0-40 gr. 33 ccm. 82.5 ccm.

Valerianella olitoria „ 4.0 „ 0-20 „ 25 „ 125 „

Papaver somniferum „ 5-8 ,, 0-45 „ 55 „ 122 „

Sinapis nigra „ 8.5 „ 0.55 „ 32 „ 58 „

Lepidium sativum „ 2.5 „ 0-25 „ 12 „ 48 „

The same investigator gives the following values for buds : —

Plant.

Syringa

Sambucus nigra
Ribes nigrum
Tilia europea

These numbers cannot be directly compared with those obtained by SAUS-
SURE (1804) (compare SACHS, 1865, p. 277) for flowers and floral organs, be-
cause that author estimated the volumes of oxygen absorbed and reckoned the
volume of the organ in question as unity. Has results are, all the same, of great
interest because he investigated not only the flowers but also the leaves of the
same plants in darkness.

Oxveen used uo bv Oxy£en used UP by the Oxygen used up by

Plant' flowers "n 24 hrs y reproductive organs the foliage leaves

in 24 hrs. in 24 hrs.

Cheiranthus cheiri n«o 18-0 4-0

Polyanthus tuberosus 9.0 — 3-0

Tropaeolum majus 8-5 16.3 8-3

Passiflora serratifolia 18.5 5.25

Cucurbitamelopepod 7.6 n — 7 (anthers)

Cucurbita melopepoo 3.5 4 — 7 (stigmas)

Ilex aquifolium 0.86

Viburnum tinus — 2-23

Juglans regia — 4.4

Populus alba 4—6

Finally, a few examples may be quoted by way of demonstrating the
variations which occur during development, and the first of these is of interest
as giving the absolute amount of respiration. The inflorescence of Arum used
up the following amounts of oxygen (in ccm.) in successive hours (GARREAU,
1851) : —

ist hour
2nd „
3rd „
4th „
5th „

In 18 following hours
JOST

Experiment i.

39

57
75

100

50
20

Experiment 2.

75
95
125
85
55
25

Experiment 3.

45
70

95

140

85

35

al 341

irs 184

460
230

470
300

I94

METABOLISM

If we express these results graphically we obtain a curve similar to that
obtained for many other physiological processes. Fig. 36 represents such
a curve, based on RISCHAVI'S experiments on germinating wheat, where the
abscissae represent days and the ordinates the amounts of carbon-dioxide pro-
duced daily in mg.

The examples quoted above furnish us with an approximate estimate of the
variations in respiration but are not adapted to exact comparative study, because
in some cases the oxygen absorbed is determined, in others the carbon-dioxide
produced, the calculations being based either on volume or on weight and be-
cause in some cases the fresh weight, in other cases the dry weight or the
volume of the parts are used to found estimates on. Strictly speaking none of
these methods are quite accurate, for we shall see that it is the living protoplasm
that is really the seat of respiration. The point of special interest is to determine
whether or not differences exist in the amount of respiration taking place in the
protoplasm of these organs, but unfortunately we have no data as to the amount
of protoplasm, either volumetric or gravimetric, to serve as starting-points
for such a comparison. All we know is that the amount of protoplasm present
in young organs is relatively much greater than in mature organs, and that so
far as we at present are aware, the respiratory variations at different develop-
mental stages maybe thus, at least partly,
accounted for. It is, however, also extremely
probable that a given amount of protoplasm
may respire with varying intensity accord-
ing to its condition. It may be sufficient
at present to draw attention to the two
chief vital conditions in which protoplasm
occurs, viz. the active and the passive,
the former being exhibited during the
vegetative period, the latter during the
summer or winter resting period. Under
constant external conditions the resting

Curve of carbon-dioxide excretion (in protoplasm of tubers, bulbs, trCCS, &C.,

exhibits marked differences to the same
protoplasm in the active state, evidenced

Fig. 36.

g.) of 40 wheat-seedlings at a temperature of

* C. After RISCHAVI (1876).

by the greatly diminished intensity of respiration, but so long as the necessary
external factors are present no protoplasm entirely ceases to breathe.

Proof of this continuous respiration is not always easy to establish since it
may be completely masked by other processes. Thus, as we have already seen,
cells containing chlorophyll decompose carbon-dioxide in sunlight. Such cells,
even though they be respiring can still give off oxygen, or may, if respiration and
assimilation be equally active, fail to show any evidence of gaseous exchange.
In fact as the light decreases in intensity the amount of oxygen given off also
decreases ; later on, it ceases altogether, and finally an evolution of carbon-
dioxide manifests itself instead. This is, doubtless, most simply explained
by assuming that respiration and assimilation go on concomitantly and quite
independently of each other. Although there are no good reasons for assuming
that reduction and oxidation go on in the same cell, yet it is very difficult to
prove this exactly, since it is possible that respiration, which can be easily demon-
strated to occur in a green leaf in the dark, is masked when the leaf is exposed to
light. Observations made on non-green tissues and organisms do not aid us
much in this relation. For long, attempts have been made to study these two
antagonistic functions separately and not without a certain amount of success.
CL. BERNARD (1878) was the first to inhibit assimilation by means of chloro-
form vapour, and AD. MAYER (1879) noted that a similar result might be ob-
tained by using prussic acid. It has been shown generally that assimilation may

RESPIRATION 195

be inhibited more readily than the respiratory function by the employment of such
poisons ; for EWART (1896) was able to inhibit assimilation for a certain time
by the use of ether vapour, although respiration continued and the organism
remained alive. Such experiments would, however, have increased value if it
were possible by etherization to inhibit assimilation completely whilst leaving
respiration entirely unaffected. BONNIER and MANGIN (1886) have attempted
to do this, but their results, taken in relation with those of other observers, are
open to criticism, and may indeed have been obtained rather by good fortune than
otherwise. It has often been observed that weak etherization accelerates
respiration (ELFViNG,i886; JOHANNSEN, 1896; MORKOWIN, 1899), whilst strong
etherization, by killing the cells, inhibits it. That respiration may continue
constant in leaves placed in a narcotic condition is possible, but, as we have said,
it is only by chance that such a result is obtained, and experiments carried out
with leaves of the same kind do not confirm BONNIER and MANGIN' s results.

Another method of proving directly that respiration takes place in green
cells exposed to light, which was for long believed to be effective, was that
employed by GARREAU (1851). GARREAU showed (by means of baryta water) that
demonstrable traces of carbon-dioxide always escaped from illuminated branches,
and he believed he was dealing in that case with carbon-dioxide formed in the
process of respiration and escaping from the chloroplasts. BLACKMAN (1895)
showed, on the other hand, that this excretion of carbon-dioxide was very
improbable, and that it was incapable of demonstration if one experimented
exclusively with chlorophyll-containing cells and rejected all peduncles and stems
not possessing green colour, a precaution which GARREAU failed to take.

Although direct proof of continuous respiration in illuminated green
cells cannot be obtained, indirect evidence is available. We may observe
not infrequently in green cells during active assimilation a continuance of proto-
plasmic movement and growth, two phenomena which are impossible in the
absence of respiration. In general, it is correct to assume that respiration takes
place in light to as great an extent as in darkness. The assimilatory activity
of a foliage leaf must be estimated not only by direct measurement of the
amount of carbon-dioxide taken up from without but also by estimating the
amount of carbon-dioxide which is produced during the same time by respira-
tion, but which does not escape from the plant because it is at once employed
in assimilation (compare Lecture X, p. 124).

Since, also, the respiratory gaseous exchange is, under normal conditions, far
less intense than the assimilatory gaseous exchange, it follows that in the long run
a plant organ may suffer from scarcity of oxygen or from the presence of injurious
quantities of carbon-dioxide. Carbon-dioxide, when in sufficient accumulation,
undoubtedly interferes with the essential functions of the plant, so that removal
of the gases arising in the course of respiration may be considered as absolutely
necessary. The removal of these is a simple matter in the assimilating leaf. If
an accumulation of carbon-dioxide has taken place in it during the night, in the
morning it is at once removed on the commencement of assimilation. The
abundant intercellular spaces with their openings, the stomata, are well adapted
for the promotion of thorough aeration. The gaseous exchange is conducted with
greater difficulty in colourless subterranean organs ; but here also the individual
cells, by means of intercellular spaces, are, generally speaking, favourably
situated for giving off and taking up gases. Since these organs, however, have
no direct exit passages the gases must either travel long distances to reach the
aerial stomata or must escape by diffusion through the cuticle. Without doubt
the cuticle of subterranean organs offers much less opposition to such a diffusion
than does that of the foliage leaf ; we have already seen how permeable it is
to water, and it would appear to be equally permeable to carbon-dioxide. So
far also as the oxygen is concerned, that gas, owing to vigorous partial pressure,

o 2

196 METABOLISM

will easily pass through the outer walls of the cells. The matter is not so certain
in the case of water plants because the oxygen surrounding them has only
a low tension. An investigation of the intercellular space system in all plants,
whether aerial, subterranean, or submerged, teaches us that accumulation of
carbon-dioxide and deficiency in oxygen never reach a degree worth considering,
and hence that the means at the disposal of the plant are always sufficient for main-
taining a gaseous exchange. Carbon- dioxide to the extent of 5 per cent, and
oxygen as low as 8 per cent, are only seldom met with in intercellular spaces,
and PFEFFER and CELAKOWSKI have shown that in the interior of individual
cells oxygen is never wanting. PFEFFER (1889) studied Rotifera living in the
cell-sap of Vaucheria, which moved actively under normal conditions, but whose
movements ceased when oxygen is prevented from entering. CELAKOWSKI (1892)
studied the protoplasmic streaming in cells of Tradescantia which had been
absorbed by the plasmodium of a Myxomycete, and found that these movements
continued inside the plasmodium ; the cells must, therefore, have been abun-
dantly supplied with oxygen.

Having now gained some acquaintance with the general occurrence of
respiration we have to inquire next as to the substances which undergo respira-
tion and the products resulting therefrom, products which naturally are related
to those arising from ordinary combustion. In many cases it may be shown that
starch and sugar disappear during respiration. If these were completely burnt
we must expect carbon-dioxide and water as the final products. From the formula

C6HI005 + 602 = 6C02 + 5H20
it may be seen that for every volume of oxygen taken in one volume of carbon-

CO
dioxide must be produced, and in many cases the respiratory quotient -Q-*

has actually been found to be unity. The contemporaneous formation of water
may also be demonstrated. SAUSSURE (1804, p. 17) long ago remarked that
germinating seeds lost more weight than one would expect from the amount of
carbon-dioxide formed, and he thought that this was due to loss of water ' which
previously was united with the substance of the seed '. LASKOWSKY (1874) in-
vestigated the origin of the water by exact methods and found it to be about
equal to the amount which was to be expected from the formula given above.

It would be quite wrong to expect unity as the value of the fraction -~p

in higher plants, on the assumption that carbohydrates were exclusively used up
in respiration. The average value for the fraction is the result of a number
of processes, each of which varies from unity, being sometimes greater, some-
times less.

The higher plants do not lend themselves readily to such experiments, because
it is difficult to say in most cases what substances are undergoing combustion. It
is quite otherwise with Fungi ; in their case we have it in our power to supply the
organism sometimes with one kind, sometimes with another kind of material.
On this subject we owe much to PURIEWITSCH'S (1900) thorough researches on
Aspergillus, and the following table gives a summary of his results : —

Relation of Carbon-dioxide to Oxygen in Aspergillus.

Nutrient. i% 1.5-2% 3% 5% » % 15-17% 20-25%

Dextrose 0.9 0-9 1-06 1-18 0-73

Cane sugar 0-87 0.96 1-02 0-83

Raffinose 091 0-66

Starch 0-68 0-55

Glycerine 0-77 0-78 0-69 —

Mannite o«66 — 0-49 065

Tannin 0-91 — 0-50 0-43

Tartaric acid 1-59 1-52 1-78 1.6 (7 %) —

Lactic acid 0-69 0-89 0-98(4%) —

RESPIRATION 197

It is not possible to deduce from these experiments any regular dependence
of the quotient on the quantity or constitution of the material used up in
respiration, still these results are full of interest as showing how extraordinarily
variable the quotient is, whose value in the majority of higher plants examined —
the exceptions we have still to study — lies close to i. This fact, also established
byJPuRiEwiTSCH, is especially important, viz. that in an individual experiment
with a definite mycelium, which was placed in various nutritive solutions in
succession, the fluctuations in the amount of carbon-dioxide excreted run by no
means parallel with those in the amount of oxygen taken in (average values are
given above) ; while the fluctuations in the oxygen absorbed were limited (up
to 35 per cent.), those of the carbon-dioxide excreted varied within far wider
limits (28 to 120 per cent.). The two processes which in chemical combustion
follow each other so closely that we may consider them as simultaneous, are
frequently in physiological combustion widely separated. Physiological com-
bustion is by no means a simple process, many intermediate reactions lie between
the absorption of oxygen and the excretion of carbon-dioxide, and these vary

CO

under different conditions. As a rule, the quotient -^~ is less in value than

unity, some oxygen being stored in the plant, so that we may conclude that the
final products of the combustion of respiratory material are not in this case
carbon-dioxide and water, but are, at least in part, other bodies as well, and
organic acids suggest themselves at once to us as probable products, more
especially as the occurrence of these bodies in Fungi has long been known.

Oxalic acid is very frequently formed, and an exhaustive study by
WEHMER (1891) has made us acquainted with the details of its manufacture.
Among Fungi, Aspergillus niger is known to form oxalic acid in great quantity,
and WEHMER' s experiments were carried out mainly on this plant. The chief
results which he obtained are summarized in the following table : —

Nutrient. Weight of the fun. Weight of the oxalic acid formed,

1.5 g. in each case. estimated as a calcium salt.

Tartaric acid o-issg. o-oo

Citric acid 0-240 g. o-oo

Ammonium tartrate t 0-030 g. 0-767

Potassium tartrate 0-032 g. 0-550

Ammonium citrate 0-056 g. 0-390

Dextrose 0-228 g. 0-278

It appears that the formation of the acid is not necessarily connected-with
the growth of the fungus ; it arises only when the substratum gives no acid
reaction and when the fungus is cultivated in sugar, proteid, glycerine, oil, and
salts of organic acids. No oxalic acid is formed if the nutritive substance be
a free acid, and the addition of phosphoric or hydrochloric acid to the nutritive
solution inhibits its formation. More recently, WEHMER (1897), and also
EMMERLING (1903), have obtained other results with Aspergillus niger, so that
probably there are several physiological forms of this fungus. The formation
of oxalic acid in Aspergillus has probably only a biological significance. The
fungus grows well in an acid substratum, and if the substratum be not acid it
makes it so, so that associated organisms are excluded. A continuous forma-
tion of acid would in the long run prove in j urious to A spergillus itself ; as a matter
of fact, it ceases to produce any more when the substratum contains above
0-3 per cent., but if the acid formed be neutralized the fungus can be made to
produce more. In one experiment, for example, 1-253 g- of anhydrous oxalic
acid was produced from 1-5 g. of sugar, whilst 2-25 g. might have been produced
by its complete transformation ; the 1-5 g. of sugar was altered as follows : —
0-8318 g. was oxidized into oxalic acid, 0-290 g. was employed in the archi-
tecture of the fungus, and the remainder (0-3782 g.) oxidized into carbon-dioxide.

198 METABOLISM

In a corresponding experiment with ammonium tartrate only about half the
amount of oxalic acid possible was formed, whilst free tartaric acid was com-
pletely oxidized into carbon-dioxide and water. Since, however, it is obvious
that the respiratory material is used up to a considerably less extent in an
incomplete oxidation such as that which takes place when oxalic acid is formed
than when carbon-dioxide is produced, it becomes a question whether this in-
complete utilization of respiratory material does not make itself felt in the growth
of the fungus. The increase in the dry weight of the fungus is, however, the
same whether oxalic acid be formed or not ; the oxalic acid lost has no great
nutritive or respiratory value (compare Lecture XVII). Further, under certain
conditions, the oxalic acid formed may be still further used for respiratory
purposes by the fungus itself. [WEHMER (1891, a) shows that at high tempera-
tures (above 30° C.) the oxalic acid is always oxidized by the fungus.]

Just as Aspergillus (and Penicillium) form oxalic acid, so Citromyces
(WEHMER, 1894) manufactures citric acid and can itself make use of it again.
Citromyces glaber can acidulate its substratum up to 4 per cent, of citric acid ;
it can withstand as much as 20 per cent, of citric acid, although it is very
sensitive to the presence of inorganic acids.

In addition to proving that the formation of the acid is useful to the fungus
in acidifying its substratum, WEHMER' s research is also specially important
as showing that the formation of the acid is not due to a deficiency of oxygen,
as was formerly thought to be the case in this and other instances.

Indeed a formation of acid takes place in almost all plants, and although
this occasionally perhaps occurs during the process of synthesis it is associated,
for the most part, with the process of respiration. BENECKE (1903, Bot. Ztg.
61, 79) has shown clearly that in the higher plants also, where it is very widely
distributed, oxalic acid is produced under the same conditions as in Fungi. Since
it is possible to grow certain plants, such as maize, both with and without oxalates
it is manifest that here also oxalic acid is not an essential product of meta-
bolism. Generally speaking, however, the formation of organic acids during re-
spiration is quantitatively less than that of carbon-dioxide, and it is only amongst
succulents that one finds organic acids produced in such large quantities that
the formation of carbon-dioxide is, at least at first, completely inhibited. It has
long been known that the amount of acid in the leaves of these plants increased
greatly at night, but it is to the comprehensive researches of AD. MAYER (1875-
87), G. KRAUS (1886), WARBURG (1886) and AUBERT (1892), that we owe
a knowledge of the details of the case. In darkness these plants absorb oxygen
without giving off equal quantities of carbon-dioxide ; the atmosphere sur-
rounding them decreases in volume. Malic acid occurs in the Cactaceae,
isomalic acid in the Crassulaceae, oxalic acid in the Mesembryanthemaceae,
and the formation of the acid takes place so freely that one may detect its

presence by tasting the leaves. The extreme case is when -^ = o, that

is to say, when no carbon-dioxide is formed at all. In continued darkness and
at higher temperatures the coefficient increases in value but never reaches unity.
Continued formation of these acids would lead to serious injury, which the
plant avoids by gradually having recourse to normal respiration and the forma-
tion of carbon-dioxide when a certain limit has been reached. This fact shows
that we have here to deal with a special peculiarity of succulents, and that the
formation of acids cannot depend upon an insufficiency of oxygen. Possibly,
as in the case of Fungi, succulents may obtain certain advantages from the forma-
tion of acid : that is indeed true, but the purpose of the formation of the acids is
naturally very different in the two cases. The acids break down in sunlight, not
only when they are exposed in pure solutions, but also more especially in the
presence of certain accelerating bodies which act catalytically ; then carbon-

RESPIRATION 199

dioxide is formed which can at once be assimilated. While in ordinary
plants the respiratory products escape from the plant, in the case of succulents
they are retained in the leaves and carbon-dioxide arises just at the moment
when it may be again use$ up. Obviously it is a matter of great difficulty to
provide fleshy leaves with carbon-dioxide from the air. The acquisition of
carbon-dioxide depends on the existence of wide-open stomata and abundant
intercellular spaces, and these are features which accelerate transpiration ; but
the succulents live under conditions which forbid copious transpiration, and
hence they do not possess these adaptations for vigorous gaseous exchange. We
need only refer in a word to the fact that these leaves were shown in the
lecture on carbon assimilation also to be peculiar, since one can, on conceivable

CO

grounds, establish in their case a value for -^—2 in assimilation, which varies

*J»

very considerably from the ordinary (p. no). In the morning, at the commence-
ment of assimilation, the leaves give off far more oxygen than they absorb
carbon-dioxide, indeed they can continue to give off oxygen in an atmosphere
free from carbon-dioxide as long as the carbon-dioxide arising from their own
activity is at their disposal.

In succulent plants, as in Mould-fungi, there are special features connected
with the formation of acids which are not to be explained from the chemico-
physiological point of view. The formation of acids has the same general
significance as the complete combustion of organic materials attained elsewhere,
but it has a subsidiary biological meaning differing widely from normal
respiration. There are many down-grade metabolic products of general
occurrence in plants which serve ecological purposes only, and it is quite likely
that organic acids are similarly to a certain extent, regulatory, in order to render
turgidity of cells possible. For example, in the formation of oxalic acid from
glucose a convenient medium is produced capable of inducing osmotic pressure
in the cell up to three times the normal.

Let us now return to the discussion of the value of the respiratory fraction

CO

-~r* . We have seen that variations from its typical value ( = i) might be the
U2

result of the formation of unusual respiratory products ; but these variations
may also be due to the varying constitution of the materials employed in
respiration.

We have already established the fact that large quantities of fat are stored
in many seeds, materials which are very much poorer in oxygen than carbohy-
drates. When these seeds germinate the fats undergo combustion, and SAUSSURE
showed long ago that during the process an absorption of oxygen took place which
does not correspond to the amount of carbon-dioxide given off ; the quotient

-Q- is less than unity. BONNIER and MANGIN (1884) found, for example, in
Linum, the following values for -~~ on successive days; 0-30, 0-34, 0-39, 0-40,

^2

0-63, 0-64. The vigorous absorption of oxygen, leading to an increase in the
dry weight (DETMER, 1880, 335) takes place especially in the first days of
germination ; later on, when the fats are gradually altered into carbohydrates

CO
the value of -~ gradually approaches unity. He observed, for example,

that in a seedling 3-5 cm. in length the value of the fraction was 0-81, an
amount as great as that seen only in plants which are beyond the seedling
stage (e. g. in Pinus ; BONNIER, 1884, p. 240).

On the other hand, when fatty oils are formed from carbohydrates in ripening
seeds, an increase in the respiratory coefficient is naturally to be expected :

200 METABOLISM

as a matter of fact GERBER (1900) found it to be as much as 471 in Ricinus, i. e.
almost five times as much carbon-dioxide was formed as oxygen absorbed.

The fact that sugar may be formed by oxidation in destructive metabolism
is worthy of special attention, because it shows better than any other illus-
tration that a physiological classification of materials cannot be made to
agree with a chemical classification, since one and the same substance, in the
present instance sugar, can constitute at once a product of assimilation in
constructive metabolism and a respiratory product in destructive metabolism.
Further, we do not always meet with sugar in the germination of oily seeds, as
for example, in the onion ; it is wanting in Cannabis, where it is true it is formed
but very quickly changed into starch. Starch and sugar serve later on equally
well as supporters of respiration and as constructive materials for the manu-
facture ol cell-walls.

Carbohydrates and fats can to a certain extent replace each other in the
plant as respiratory material. This is known to be the case also in the animal
world, but there the two substances are not sufficient to maintain life, which is
dependent on a constant destruction and oxidation of proteid, followed by the
excretion of such nitrogenous waste as hippuric acid, urea and uric acid. The
question thus arises whether proteid is also destroyed in the plant during
respiration and whether that takes place of necessity. It may be easily shown
that peptone can act as a respiratory material, in Fungi especially, and it will
be sufficient to quote here an experiment of WEHMER'S (1892) on Aspergillus.
This fungus thrives remarkably well on peptone as a source of nitrogen and sugar
as a source of carbon, but there are no obvious indications that the peptone is
used as respiratory material also. The fungus is, however, able to supply all
its wants both as to carbon and nitrogen when peptone alone is supplied to
it. Under these circumstances functions carried out by sugar must be undertaken
by the peptone, and it appears that a part of the nitrogen of the peptone is
transformed into ammonia and excreted. The insight we thus obtain into the
process of respiration is only somewhat limited, since the nitrogen of the ammonia
produced in this reduction cannot be compared with the carbon of the carbon-
dioxide produced in the oxidation of carbohydrates. Ammonia is a secondary
product of peptone respiration, which comes off free when the carbon of the
peptone is turned into carbon-dioxide in the process of combustion. The
appearance of products of reduction in respiration will be dealt with later, at
present we need only note how extremely varied a mould fungus can be in its
metabolism : ammonia, the same material which in combination with sugar
serves as a source of nitrogen, is also given off as a worthless waste product
when peptone is supplied as the single organic nutrient.

The other question, as to whether some proteid must always be respired, is
not so easily answered. This was more often referred to the decomposition of
proteid especially noticeable in seedlings but which has been clearly demonstrated
elsewhere and which leads to the formation of amido-compounds. We have con-
sidered these bodies as products of the hydrolytic decomposition of proteid, and
have assumed that the formation of these crystallizable and easily diffusible
substances was necessary, since proteid as such is not well adapted to trans-
location through the plant body. It is quite possible, however, that the amido-
compounds arise in respiratory metabolism and are produced from proteids by
oxidation. This view, at the present moment, can neither be proved nor disproved,
still when the condition of things in animals is taken into account we must regard
it in a certain sense as probable. It is easily understood, according to the earlier
statements made, that the products of the decomposition of proteid in plants are
not excreted. The amido-compounds which are formed under suitable conditions
are capable of being again made use of for the regeneration of proteid ; and this
fact increases the difficulty of completely solving this problem. If the amido-

RESPIRATION 201

compounds were indeed the final products of respiratory metabolism, we might
say that the green plant differs essentially from the higher animal in being able
to make use both of its nitrogenous and non-nitrogenous metabolic products
as nutritive materials once more. Since animals do not possess this power,
they may in that respect be contrasted with plants.

Respiration, like all other functions of the organism, depends on the ex-
ternal factors of which we have already had occasion to speak. Light, according
to KOLKWITZ (1899), produces a feeble rise in respiration — at least in Fungi —
although it is impossible to say whether its action is purely chemical (by decom-
position of certain organic acids) or whether its influence is more far reaching and
affects the protoplasm. Since in other cases, however, a reduction in the respira-
tion has been observed when the plant is illuminated, the question cannot be
considered as in any way settled. More recently, MAXIMOW (1902) has only
partially confirmed the results obtained by KOLKWITZ. One thing at least is
certain, that light has no essential influence on respiration ; heat, on the other
hand, is of fundamental importance. Although the curves expressive of the
dependence of the majority of physiological processes on temperature resemble
very closely that of assimilation (p. 124), exhibiting between the minimum and
maximum a pronounced optimum, this last datum is not quite determined for
respiration. PFEFFER holds the view that increase in the intensity of
respiration is concomitant with increase of temperature until the latter begins to
influence injuriously all the vital processes. The plant must be permanently
injured by temperatures which bring about a diminution of respiration. If,
however, we accept the results arrived at by ZIEGENBEIN (1893) we must
acknowledge the probable existence of an optimum temperature for respiration.
ZIEGENBEIN finds that the intensity ofjespiration (measured by the amount
of carbon-dioxide excreted (in mg.) percent, of fresh weight) depends on the
temperature in the manner illustrated by the following table : —

10° 20° 30° 35° 40° 45° 50° 55° 60°

Potato tubers 1-17 2-22 4-62 7-85 10-24 12-22 11-14 10.30 2-71

Viciafaba (seedlings) — — 55-2 78-72 65-1 57-8 20-8

Abies excelsa (shoots) — — 185-0 2o6>4 198-4 168-9 33*3

Although one is compelled to hold that all temperatures above 45° are cer-
tainly injurious, and that the observed reduction in respiration at 50°, &c.,
must be regarded as the result of the plant's pathological condition owing to the
excessive temperature, it is difficult to believe that a temperature of 40°,
which causes a diminution of respiratory activity in the last two objects experi-
mented on, operates in this way. ZIEGENBEIN, at least, fails to offer exact proof
that this was not the case. Should an optimum temperature be established for res-
piration later on, it must lie in each case extremely near the maximum. Further
evidence as to the optimum temperature is given by KUNSTMANN (1895) for
Fungi, and STOKLASA (1903) for beetroot. The position of the minimum has
been often worked out and it has been found that it lies considerably below
p° C, in the case of lichens, for example, about — 10° C. QUMELLE, 1892). The
increase in respiration near the maximum temperature is often very marked ;
it reaches, for example, according to CLAUSEN'S experiments (1890) on germin-
ating wheat, to as much as eleven times, and in the case of lupins to sixteen
times what it is at o° C. The fact that respiration is accompanied by a pro-
duction of heat need only be casually referred to here, since that phenomenon
will require closer consideration in another place (Lecture XXXI).

In regard to the influence of materials on respiration we may consider
first of all water, which has no specific influence on respiration, but is important
only in so far as it is one of the general vital conditions. Respiration
ceases entirely in seeds and perfectly dry parts of plants, in mosses, lichens, &c.,
which can tolerate complete desiccation, continuing alive during drought without

202 METABOLISM

exhibiting any obvious metabolism ; minute quantities of water enable the for-
mation of carbon-dioxide to commence at once (KoLKWirz, 1901). The substances
which are used in respiratory metabolism are naturally of greater significance,
and if they be present in insufficient quantity respiration ceases, as, for example,
when plants are kept for a long time in the dark. In Fungi, such as Aspergillus
(KosiNSKi, 1901), which accumulates no reserves, the withdrawal of the nutritive
solution makes itself evident at once in a reduction of respiration ; so long,
however, as the organism remains alive respiration does not cease entirely,
and if nutrients are supplied to the plant after cessation of the period of starva-
tion respiration at once increases. Under normal nutritive conditions, however,
the amount of respiration is by no means proportional to the quantity of
respiratory materials present, and this fact is of the utmost importance in
a theoretical consideration of respiration.

Generally speaking, an increase in respiration may be noted if the plant
be subjected to injurious influences. Thus, for example, small doses of
certain poisons act in this way, and perhaps we may correlate this result with
the increased growth which results from the addition of such stimulants (p. 88).
Similar results are produced by anaesthetics and antipyretics QACOBI, 1899), as
has been already noted (p. 195). Carbon-dioxide acts in a like manner if it accu-
mulates in excessive amounts, and the same result is produced as an after-effect
of high temperatures, high atmospheric pressure, wounds, &c. (RICHARDS, 1896).

In conclusion we may note the effect of oxygen, the gas which is most con-
cerned in respiration. It is worthy of note that respiration is independent
within wide limits of the percentage of oxygen present in the air ; the partial
pressure of oxygen may be reduced or increased considerably in comparison with
the normal without at once influencing respiration. Hence the presence or absence
of indifferent gases, such as nitrogen, appears to be without significance, respiration
takes place in pure oxygen just as in ordinary air, in which the oxygen is reduced
to one-fifth of its volume ; in both cases, the partial pressure of oxygen is, however,
the same (i. e. one atm.) Only after the pressure of pure oxygen is raised to 2-5
atmospheres, does there ensue a marked increase in respiration followed soon
after by a marked diminution until death takes place ( JOHANNSEN, 1885). The
fact that death always takes place under higher oxygen pressures is not due to
increase of respiration, since far greater increase can be reached by other means,
e. g. higher temperatures, without any evil effect following. Why a fatal effect
should result from an increased access of oxygen we do not know ; only this much is
known that the different types of plant-life behave very differently in this relation,
since all transitions occur between such plants as we have as yet alone studied and
organisms which become injured by pressures of oxygen far below that present
in ordinary air (compare Lecture XVII).

Respiration, as already stated, is at first not affected by a re-
duction of oxygen pressure, and STICK (1896) showed that not until the
proportion of oxygen in the air was as low as 2 per cent., or even less, was there
any diminution in the amount of carbon-dioxide given off. Experiments on this
question are not at all easy to carry out because, as has long been known, carbon-
dioxide continues to be given off for a considerable time after the complete
exclusion of oxygen. In some plants (Vicia faba, Ricinus, &c.) this excretion
is not less intense than when oxygen is supplied ; in the majority of cases how-
ever it reaches only one- third to two-thirds of this value, and varies in the individual
plant, according to its developmental condition. The carbon-dioxide produced
in this oxygenless respiration arises from the same materials as are consumed in
ordinary respiration ; still it cannot arise from simple combustion but from
the splitting of organic bodies, resulting in the formation of both reduced and
completely oxidized bodies. Oxygen atoms wander within the molecules of
the respiratory materials in so-called intra-molecular respiration. When glucose,

RESPIRATION 203

for example, breaks down and all the oxygen present becomes used up in the forma-
tion of carbon-dioxide there remains over, besides carbon-dioxide, a completely
reduced body composed of carbon and hydrogen ; if all the oxygen be not so
used up, a body poor in oxygen as compared with glucose is formed, such as
alcohol, which, as a matter of fact, always appears in intra-molecular respiration,
and which collects often in considerable quantities ( LECH ARTIER and BELLAMY,
1874, MAZE, 1900). Thus BREFELD (1876) found J per cent, of alcohol in the
leaves of ivy and Corylus after seventeen days ; in grapes, after several weeks,
J-2 per cent. ; in cherries, after four weeks, 1-8-2-5 percent. ; and in pea-seedlings,
after three months, as much as 5 per cent. [According to DUDE (1903, Flora,
92, 205) the plants used by BREFELD, at least partly, can withstand the with-
drawal of oxygen for only a very short time (hours or days). MATRUCHOT and
MOLLIARD (1903) have shown that alcohol is formed by higher plants when
micro-organisms are completely excluded. In LECH ARTIER and BELLAMY'S
experiments Fungi or Bacteria doubtless co-operated in bringing about
the result.] If seeds of Vicia faba are kept for two days under water one can
recognize the presence of alcohol by the smell on rubbing. In addition to
ethyl-alcohol other substances also appear during intra-molecular respiration,
i. e. higher alcohols, acids, aromatic compounds, and even hydrogen, but as to
their proportional amounts nothing is known. By means of such special
decompositions of organic substance the plant parts remain long alive, the
most resistant, it maybe, for months, while others die in a few days or hours ; the
amount of carbon-dioxide produced is also, asmight be expected, very variable, and
in extreme cases may be seven to ten times the volumeof theplantpart concerned.

These last-mentioned phenomena, more especially, throw considerable light
on the problem of the factors concerned in respiration [compare BARNES, 1905].
We have described respiration as a case of combustion, and this we were very well
entitled to do from the products it gives rise to ; one may arrive easily, however,
at a totally incorrect conception of the causes of respiration by such a mode of
expression. In ordinary combustion an oxidizable body is oxidized, during
which process it absorbs the oxygen of the air, and this combustion may go on
at normal or at supernormal temperatures. Physiological combustion or respira-
tion takes place at temperatures so low that a direct union between sugar,
starch, &c., and oxygen is inconceivable. Further, the oxygen cannot be the
cause of the oxidation, since, necessarily, an alteration in the intensity of
respiration should be observed simultaneously with an alteration in the amount
of oxygen present in the cell, which, as we have already said, is not the case.
Further, we have seen that respiration is within wide limits independent of the
amount of materials capable of being oxidized, and hence we must also conclude
that this is not the cause of physiological combustion. Should we thus be driven
to assume the presence in the protoplasm of a substance which can oxidize more
vigorously than ordinary oxygen, viz. the so-called * active oxygen ', we find
ourselves at the same time face to face with a serious difficulty . ' Active oxygen '
once present must attack all oxidizable bodies in the plant, whilst it is character-
istic of respiration that only some substances are oxidized. It is difficult to see
how the cell -wall could resist the attack of ' active oxygen ' if sugar and starch
be oxidized by it. Further, PFEFFER (1889) succeeded in bringing direct evi-
dence against the occurrence of vigorously oxidizing substances in the cell. He
showed that one might introduce into the cells of many plants dilute solutions
of peroxide of hydrogen without injuring them, and that the colouring bodies
occurring naturally in the plant, as well as chromogenic substances artificially
introduced, suffer a change of colour which does not take place in nature.

If further proof be necessary that physiological combustion is not so
simple as ordinary combustion we need only refer to the facts stated
above, which show that not infrequently in respiration combustion is

204 METABOLISM

only partial and reaches the formation of organic acids only and not to the final
products, water and carbon-dioxide, although the amount of oxygen necessary
to burn the substance completely is without doubt available. A certain light
is shed on the cause of respiration by a consideration of intra-molecular respira-
tion. If we assume with PFEFFER (1889) that normal and intra-molecular
respiration have a genetic connexion, that intra-molecular succeeds normal respira-
tion when oxygen is deficient — and this assumption is the most probable that
occurs to one — then we are forced to admit the decomposition of organic bodies as
the primary phenomenon in both processes. The decomposition must result in
the formation of an oxidizable body, which in the presence of free oxygen takes
it up, but which in its absence satisfies its requirements so far as oxygen is con-
cerned from other compounds containing it. Carbon-dioxide must arise from it in
all cases, although the intermediate products must differ according as oxygen is
present or not. We must not assume that identically the same products are

E resent in normal respiration as in intra-molecular, and that these, e. g. alcohol,
ecome afterwards oxidized ; that such an assumption cannot be correct is
shown by the proportion of carbon-dioxide to oxygen in any selected case.

It is not known what the bodies are which undergo this hypothetical
primary decomposition. They may be the substances which we noted disap-
pearing in mass during active respiration, the carbohydrates ; but they may
be proteids or even protoplasm itself. The latter view, which has been advanced
by PFLUGER, and from the botanical side has been vigorously upheld by DETMER
(1883), cannot be exactly proved, still one cannot deny its inherent probability.
The abundant consumption of carbohydrates would be explained by this theory,
for these would serve to regenerate the broken down proteid or protoplasm ;
if they be present in insufficient quantity, as in seedlings of Leguminoseae
grown in the dark, the regeneration of the broken down proteid is carried
only as far as asparagin ; if they be entirely wanting, as in the case of a fungus
nourished by peptone only (p. 200), nitrogenous loss takes place, while ammonia
is formed. PFEFFER (1885, 656) has, however, advanced a serious objection
to this idea, inasmuch as he has shown that intra-molecular respiration comes
at once to an end in the absence of carbohydrates (compare DIAKONOW, 1886).

It has recently been suggested that the immediate cause of this supposed
decomposition is an enzyme. HAHN (1901) believes he met with such an
enzyme in the sap squeezed out of the bulbs of Arum, which caused sugar to
disappear without the addition of oxygen. He did not isolate it, however, nor
did he examine its properties in detail. On the other hand, in dead plant parts
and in expressed sap oxydases have frequently been found, that is, bodies which
act as carriers of oxygen and which colour guaiacum resin blue. That these
bodies are somehow connected with respiration is not impossible, still proof
must be forthcoming that they exist already in the living plant, which is certainly
not the case in many. When these bodies have been more fully investigated we
shall be in a better position to answer the question as to whether they are to be
considered as enzymes and whether they are to be considered as hydrolytic as well
as oxidizing. At all events, the mode of action of these two types of enzyme must
be fundamentally distinct. The recent literature on the subject of oxydases
includes papers by RACIBORSKI, 1898 (a criticism of whose views is given by
MOLISCH, 1901; VINES, 1901); HUNGER, 1901; BEHRENS, 1901. [The literature
on the subject of the occurrence and function of oxidizing enzymes in the plant
has, during the past few years, increased in quantity out of all proportion to the
finality and clearness of the explanations offered. We may refer to CZAPEK, Bio-
chemie, II, 464-481, as also to CHODAT and BACH (1904) and RACIBORSKI (1905).]

Respiration is, as already stated, a process of universal occurrence in
organisms, one too which is absolutely essential, since when it ceases and,
generally speaking, when also oxygen is withdrawn, the more important functions

RESPIRATION 205

of the organism come to a standstill — growth and the phenomena of movement
as well as the transport of food material (compare Lecture XIV, p. 169) from
cell to cell, the movements of protoplasm and of entire organs. We must not
forget, too, that oxygen is also an essential food-stuff of the plant, and hence
we have for the first time to deal with an element as a nutritive material,
whilst the nutrients hitherto spoken of were compounds. We are still far from
a complete understanding of the significance of respiration in the mainte-
nance of vital phenomena, but we can at least claim to possess an ap-
?roximate conception of it by studying the energy relations of the process.
n burning wood or coal energy is liberated which, as every steam-engine
demonstrates, is capable of doing work. A transformation of the energy origin-
ally present in the material must take place ; it must be changed from the
potential into the kinetic condition. Similarly, in the physiological combustion
of starch or sugar in the plant-cell, kinetic energy is evolved, obviously essential
for carrying on the manifold activities of the organism. When organic materials
are broken down in the process of intra-molecular respiration energy is also
released, although no free oxygen be added, just as when in the breaking down
of certain chemical compounds a re-arrangement of their atoms only takes
place without the addition of any other element. In the higher plants the energy
arising from intra-molecular respiration is insufficient to carry on all the vital
phenomena ; in our next lecture, however, we shall get to know of organisms
where this is so. The heat of combustion of the respiratory materials gives
us an approximate idea of the amount of energy released in respiration. If
these materials be oxidized down to the final products, water and carbon-dioxide,
we thereby obtain bodies which have no heat of combustion, and the whole of
the energy is released by respiration. When, however, organic acids or alcohol
arise as primary products it is only the difference between the heat of com-
bustion of the materials produced, and the sum of the heats of combustion
of the final products that is available for carrying on the work of the plant.
In the production of heat which appears to be inseparable from respiration
(compare Lecture XXXI) we have the evidence required to show that the
chemical energy of the respiratory materials is transformed, still we must
remember that the heat so produced must obviously be reckoned as lost to
the plant. If the production of heat were the chief end of respiration then
respiration might be compensated for by heat introduced from without, and
we could reduce it by heating the plant ; this, however, is quite incorrect, for
with every increase in temperature respiration also increases. Under these
circumstances it is almost inconceivable that, as RODEWALD thinks (1888), the
total energy of the respiratory materials in certain cases appears as heat. One
would not expect this to be generally true ; one would rather expect, that in
addition to the heat produced in physiological combustion, other forms of
energy would appear which might be of service in the plant economy.

We cannot close this lecture without summarizing, if only in a sentence,
the history of our position in relation to this subject. The fact that in illumi-
nated green parts respiration is masked by assimilation makes the demonstration
of universal respiration extremely difficult. Although SAUSSURE had a clear
perception of the fact that respiration was continuous in chlorophyll-bearing
regions exposed to light, we should not have been indebted to SACHS (1865) for
the first expression of the modern conception of the matter, if LIEBIG had not
explicitly denied respiration in plants. The service rendered by SACHS lay
essentially in correcting the phraseology previously in use, seeing that GARREAU
(1857), had already shown the probability of the existence of respiration in all
green parts of plants ; before SACHS'S time it was customary to speak of a ' day
and night respiration', but SACHS, by introducing the terms ' assimilation ' and
' respiration ' for these two antagonistic processes, did away with many un-
ending misconceptions.

206 METABOLISM

Bibliography to Lecture XVI.

AUBERT. 1892. Revue gen. 4, 203.

[BARNES. 1905. Bot. Gaz. 39, 81.]

BEHRENS. 1901. Centrbl. Bakt. II, 7, i.

BERNARD, CL. 1878. Le9ons s. 1. phenom. de la vie. Paris, i, 278.

BLACKMAN. 1895. Phil. Trans. R. Soc. B, 186, 502.

BONNIER and MANGIN. 1884. Annales Sc. nat. vi, 19, 217.

BONNIER and MANGIN. 1886. Ibid, vii, 3, 5.

BREFELD. 1876. Landw. Jahrb. 5, 327.

CELAKOWSKI. 1892. Flora, 76, 194.

[CHODAT and BACH. 1904. Arch, des Sc. phys. et nat exper. 17, 477.]

CLAUSEN. 1890. Landw. Jahrb. 19, 893.

DETMER. 1880. Phys. d. Keimung. Jena.

DETMER. 1883. Lehrb. d. Pflanzenphysiologie. Breslau.

[DIAKONOW. 1886. Ber. d. bot. Gesell. 4, 2.]

ELFVING. 1886. Oefversigt Finsk. Vet. Soc. Forh. 28.

EMMERLING. 1903. Centrbl. Bakt. II, 10, 273.

EWART. 1896. Journal Linn. Soc. 31, 408.

GARREAU. 1851. Annales Sc. nat. in, 15, i.

GERBER. 1900. Congr. internat. de bot. Paris. Compt. rend. p. 55.

HAHN. 1901. Ber. d. chem. Gesell. 34, 3355.

HUNGER. 1901. Ber. d. bot. Gesell. 19, 374.

"[ACOBI. 1899. Flora, 86, 289.

[OHANNSEN. 1885. Unters. aus d. bot. Inst. Tubingen, i, 686.

[OHANNSEN. 1896. Bot. Centrbl. 68, 337.

[UMELLE. 1892. Revue gen. 4, 269.

IOLKWITZ. 1899. Jahrb. f. wiss. Bot. 33, 128.

KOLKWITZ. 1901. Ber. d. bot. Gesell. 19, 285 ; Blatter f. Gerstenbau etc. 1901.
KRAUS, G. 1886. Abh. Naturf. Gesell. Halle, 16.
KOSINSKI. 1901. Jahrb. f. wiss. Bot. 37, 137.

KUNSTMANN. 1895. Verh. zwischen Pilzernte u. verbr. Nahrung. Diss. Leipzig.
LASKOWSKY. 1874. Versuchsstationen, 17, 219.
LECHARTIER and BELLAMY. 1874. Compt. rend. 79, 1006.
LIEBIG. 1862. Die Chemie in Anwendung auf Agrikultur u. Physiologic, 7th ed.

Braunschweig, i, p. 29.

[MATRUCHOT and MOLLIARD. 1903. Revue gen. d. Bot. 15, 193.]
MAXIMOW. 1902. Centrbl. Bakt. II, 9, 193.
MAYER, AD. 1879. Versuchsstat. 23, 335.

MAYER, AD. 1875-87. Versuchsstat. 18, 410; 21, 277; 30, 217; 34, 127.
MAZE. 1900. Annales Instit. Pasteur, 14, 350.
MOLISCH. 1901. Stud, iiber Milchsaft etc. Jena. p. 63.
MORKOWIN. 1899. Revue gen. n, 289.
ONO. 1900. Journal Coll. Sci. Imp. Univ. Tokyo, 13, i, 141.
PFEFFER. 1885. Unters. aus. d. bot. Instit. Tiibingen, i, 636.
PFEFFER. 1889. Abh. math.-phys. Kl. Kgl. Sachs. Gesell. d. Wiss. 15, 449.
PFEFFER. 1895. Jahrb. f. wiss. Bot. 28, 257.
PFLUGER. 1875. Archiv f. Physiol. 10, 251.
PURIEWITSCH. 1900. Jahrb. f. wiss. Bot. 35, 573.

RACIBORSKI. 1898. Ber. d. bot. Gesell. 1 6, 52 and 119. Flora, 85, 362.
RICHARDS. 1896. Annals of Botany, 10, 531.
RISCHAVI. 1876. Versuchsstationen, 19, 321.
RODEWALD. 1888. Jahrb. f. wiss. Bot. 19, 281.
SACHS. 1865. Experimentalphysiologie. Leipzig.
SAUSSURE. 1804. Chem. Unters. iiber die Vegetation. (German ed. by Wieler,

Ostw. Klassiker, 15 and 16. Leipzig. 1890.)
STICK. 1891. Flora, 74, i.

STOKLASA. 1903. Beitr. z. chem. Phys. u. Path. 3, 473.
VINES. 1901. Annals of Botany, 15, 18 1.
WARBURG. 1886. Unters. bot. Inst. Tubingen, 2, 53.

WEHMER. 1891. Bot. Ztg. 49 ; Abstract by Author in Bot. Centrbl. 1894, 57, 104.
WEHMER. 1892. Bot. Centrbl. 51, 337 (Rev.).
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WEHMER. 1897. Centrbl. Bakt. II, 3, 102.
ZIEGENBEIN. 1893. Jahrb. f. wiss. Bot. 25, 563.

207

LECTURE XVII
FERMENTATION

THE destructive metabolism which is an essential concomitant of vitality
in so far as it consists in the complete combustion of organic materials with
the production of carbon-dioxide and water, we have termed respiration. In
the present lecture we propose to treat of fermentation, by which we mean
an active metabolism where the oxidation is incomplete or where, instead of
oxidation, a decomposition of an entirely different kind takes place. Respiration
and fermentation have in common the formation of final products having less
heat of combustion and more limited stores of energy than the materials from which
they arise. In the formation of these final products energy must therefore be
released, and it is this energy which the organism makes use of in some unknown
way to carry out its vital activities. As has been shown in the last lecture, respira-
tion and fermentation are not two essentially different processes, for since we
found that in the respiration of Fungi, for example, a number of organic acids,
such as oxalic, malic, &c., arose as products of incomplete combustion, we are quite
entitled to term this process fermentation, and speak of oxalic acid fermenta-
tion, malic acid fermentation, and so on, after the chief products produced.
The nomenclature of fermentation is not as yet quite consistent since it is some-
times based on the nature of the chief, or to be more accurate, the most noticeable,
product — for in alcoholic fermentation carbon-dioxide is as much the chief
product as alcohol is — sometimes also after the material that is fermented. Thus
by butyric acid fermentation one understands a process in which butyric acid
is the most prominent product which arises, but by cellulose-fermentation we
mean a process which results in the destruction of cellulose. As an example
of another type of fermentation which takes place without the presence of
oxygen, may be taken that in which alcohol is formed as a result of intra-mole-
cular respiration. We have already established the fact that this alcoholic
fermentation stands partly, at least, in lieu of respiration ; so long as intra-
molecular respiration continues, the plants we have hitherto been considering
cannot develop their full vital capacities — their growth, for example, comes to
a standstill — but still they remain alive and regain their ordinary powers after
being transferred to normal conditions and after the addition of oxygen. If,
however, one places these organisms in an atmosphere free of oxygen — which
may be easily done in the case of Fungi — without at the same time permitting
the conditions necessary for the formation of alcohol, and if one gives them in
place of an easily fermentable sugar another equally nutritive source of carbon,
e.g. quinic acid, lactose (DIAKONOW, 1886), they rapidly die. [DIAKONOW'S
work has not, however, received complete confirmation, for NABOKICH (1903)
and KOSTYTSCHEW (1904) have shown that peptone, quinic acid, and milk-
sugar may also support intra-molecular respiration, but nothing like so well as
sugar.] Similarly, oily seeds cannot respire intra-molecularly so well as
starchy seeds (MAZE, 1900; GODLEWSKI, 1901) because the alteration of fat into
carbohydrate is manifestly impossible without oxygen. There is no ground for
distinguishing the process of alcohol formation which takes place in intra-
molecular respiration from alcoholic fermentation so called, especially since
GODLEWSKI has demonstrated that alcohol and carbon-dioxide occur in the same
proportions as in fermentation, and that they also develop in seedlings of Pha-
nerogams from sugar supplied artificially. Still the term 'alcoholic fermenta-
tion ' always suggests in the first instance a definite organism, viz. yeast, because

208

METABOLISM

the alcohol formed in the manufacture of wine, beer, and brandy is almost
entirely a product of the activity of this organism.

Yeasts belong for the most part to the genus Saccharomyces, a genus rich
in species and in varieties (Fig. 37), Ascomycetes of the simplest structure
increasing by budding. Under certain culture conditions they do not exhibit
any capacity for forming alcohol and behave just like other Fungi, respiring
organic substance into carbon-dioxide and water. Yeast behaves in this way
when grown in a nutritive solution in which peptone serves as a source both of
carbon and of nitrogen, or in solutions which in addition to some appropriate
source of nitrogen, contain quinic acid or lactose to supply the carbon required.
As might be expected the yeast under these conditions dies at once if oxygen
be withheld. If the milk-sugar be replaced by cane sugar, alcohol is formed
whether oxygen be present or not. It is obvious that fermentation begins only
when an appropriate fermentable substance is present, so that we must first
of all inquire which substances are fermentable and which not. Yeasts can form
alcohol only from carbohydrates, and at the same time they exhibit a remarkable
capacity for distinguishing between bodies nearly related, as we have elsewhere

Fig- 37- Yeast Fungi, a. Saccharomyces cerevisiae ; b. S.pasteurianus\\\ ; c. S. elltpsotdcus I ;
d. ellipsoideus II. (After FISCHER, Vorles. Q. Bacterien. and ed.)

discovered. The proof of this fact, as far as yeasts are concerned, we owe chiefly
to E. C. HANSEN (1888) and E. FISCHER (1898), who have established the fact
that individual species and varieties behave in entirely different ways.

Fermentable carbohydrates are recognized by possessing three carbon
atoms, or a multiple of that number, and are directly fermentable trioses,
hexoses and nonoses, whilst the more complicated di-, tri-, and polysac-
charides must be first of all hydrolysed and transformed into hexoses before they
can be fermented. In nature, only the hexoses and those higher sugars which
may be split into hexoses are to be considered as forming the material of fer-
mentation. Amongst these we may distinguish aldohexoses and ketohexoses,
the former exhibiting four and the latter three asymmetric carbon atoms. These
asymmetric carbon atoms are indicated (in the following formulae) by heavy
type, and the first of these we may draw attention to is ordinary grape sugar
i.e., ^-glucose : —

OH OH H OH OH H2OH
C — C

A

Let us now consider in this formula the four H and OH groups
united to the four asymmetric carbon atoms to be arranged in all possible
configurations, we shall then have sixteen ' stereoisomeric ' hexoses and
these are all optically active ; eight rotate polarized light to the right
and the other eight rotate it to the left. The enantiomorphs, dis-
tinguished by their different rotatory powers, are characterized by the
interchange of H and OH groups around the asymmetric carbon atoms;

FERMENTATION 209

thus, for example, the enantiomorph of ^-glucose, i. e. /-glucose, has the follow-
ing configuration : —

OH H OH H H H2OH

i_i_«U--c— c--c

OH H OH OH

Many, but not all, of these sixteen different bodies are known, and it has
been found that only those which rotate polarized light to the right, and even
not all of these, are fermentable. Besides ^-glucose, d-mannose and ^-galactose
alone among the hexoses are fermentable, i. e. those whose asymmetric carbon
atoms are united to the H and OH groups in the following way : —

H OH OH OH H H OH

II! , , , I I I I

_c — c — and a-galactose — o — c — c — c —

H H H OH OH H

In contrast to ^-glucose it is the first carbon atom in d-mannose and the
third in d-galactose which is exchanged, but these exchanges do not prevent
fermentation ; if, however, as in d-talose, the first as well as the third carbon
atoms are exchanged : —

H H H OH

_i_ c— c— c-

r t i i

OH OH OH H
then fermentation is impossible.

Of all the known ketohexoses only one, d-fructose ( = laevulose), is ferment-
able ; its configuration is as follows : —

H2OH O H OH OH HaOH

c_cL_cL- c— c— c

OH H H

So far as regards the behaviour of its three carbon atoms it agrees entirely
with d-glucose, and hence may be explained the fact that it is nearly as easily
fermentable as that sugar ; mannose also closely resembles glucose, and ex-
perience shows that it is more easily fermentable than galactose, whose configura-
tion differs more widely. In individual cases we have not as yet learned what
factor determines this capacity for undergoing fermentation. Further, all
Saccharomycetes do not behave in the same way towards galactose ; 5.
pasteurianus I causes it to ferment almost as quickly as it does the three other
hexoses ; 5. ellipsoideus induces fermentation in it only slowly, S. productivus
and S. apiculatus dp not cause it to ferment at all.

The disaccharides are, as already mentioned, not directly fermentable,
they must first of all be hydrolysed into hexoses by means of enzymes. Thus
ordinary beer and wine yeasts give off enzymes which break down cane sugar
very quickly outside the cells into equal parts of dextrose and laevulose. One
of these, invertase or saccharase, we have met with before. The products
of decomposition are, for the most part, not acted on with equal rapidity;
many yeasts consume the dextrose first, some attack the laevulose first. This
may depend on a difference in fermentative power, but probably other
considerations may play a conspicuous part, e. g. powers of diffusion (KNECHT,
1901). In certain cases, Monilia Candida for example, we may assume a direct
fermentation of cane sugar, for it disappears during fermentation without any
invert sugar ( = dextrose + laevulose) appearing in its place. Careful research
has shown that in this case also an invertase comes into play ; but since it
cannot diffuse out of the cell its activity is intra-cellular only.

JOST P

210 METABOLISM

A large number of yeasts are able to hydrolyse not only cane sugar but
maltose also, and to induce fermentation in the resulting products (= two
molecules of glucose). Others (S. marxianus, ludwigii, exiguus) attack cane
sugar only, others still (S. apiculatus, Schizosaccharomyces octosporus) maltose
only. Maltase must, therefore, be looked on as a distinct enzyme from
saccharase. Its recognition was rendered all the more difficult because it was
at first to be obtained only from the dead cells by drying, since it was obviously
unable to penetrate the living protoplasm. A third disaccharide, lactose (milk-
sugar), is broken up by yet other yeasts, which together can also hydrolyse
maltose and saccharose. Similarly a fourth natural disaccharide, trehalose, is
acted upon by certain other yeasts.

We need not discuss the artificial disaccharides nor the fermentable tri-
saccharide, rafnnose, since they add nothing essentially new to the facts already
known. Finally, as to the polysaccharides, e. g. starch, we already know that
they also are transformed into sugar by an enzyme, diastase ; in general, however,
organisms which can produce alcoholic fermentation are unable to use starch,
although there are certain Fungi, members of the Mucorineae — though certainly
not Saccharomycetes — such as, for example, Mucor alternans, Amylomyces
rouxii, which can do so, and these during the last few years have been
employed technically for the purpose.

There is another question which we need only glance at in passing, and
that is whether the possession of a definite enzyme and the 'power of producing
fermentation are constant characters of the organism or whether they may be
induced to appear in it by culture methods and change of habit. The latter
view is held by French investigators, and although, according to the results of
the work carried out in the Carlsberg Laboratories, these results do not appear
to be entirely reliable (KLOCKER, 1901), still it must be said that critical evidence
against such a possibility does not exist, since experience has taught us that
micro-organisms may be easily affected by culture methods, acquiring or losing
certain characteristics according to circumstances.

As already noted, sugar is broken down into carbon-dioxide and alcohol
during alcoholic fermentation, both of which appear in about equal amounts ;
thus 100 parts of cane sugar in one of Pasteur's experiments, giving by hydro-
lysis 105-26 g. of invert sugar, gave rise to 51-0 g. of alcohol and 49-1 g. of
carbon-dioxide ; the remainder, about 5 g., went to the nourishment of the yeast
and the formation of certain secondary products. The proportional relations
of carbon-dioxide and sugar is about what one would expect if a molecule of
cane sugar, on taking up water, broke down into four molecules of carbon-
dioxide and four of alcohol : —

= 53-8 g. + 51-5 g.

It is quite possible that the process of fermentation consists in a decom-
position of sugar of this nature. As a matter of fact, however, there are always
secondary products formed which, in the older analyses at least, must have
been due to the presence of foreign organisms in the fermenting fluid ; still some
of these products, especially glycerine and succinic acid, appear during the action
of perfectly pure yeast. The amounts of these substances vary, in the first place,
according to the species of yeast employed, and in the second, according to
the nature of the food, not only of the fermentable carbohydrate but also of
the nitrogenous constituent. LABORDE (1899) found that 2-5-775 g. of gly-
cerine were formed from 100 g. of fermenting sugar as the result of the action
of different yeasts, and WORTMANN (1892) also showed that very variable
amounts of glycerine were produced by different wine yeasts. Succinic acid
comes next to glycerine in amount and reaches about 0-5 g. per 100 g. of sugar ;
the amount is also liable to considerable variation. Since both these secondary
products are formed by yeast under conditions which exclude the formation of

FERMENTATION 211

alcohol (UDRANSKI, 1889), we have obviously to do with a special kind of
metabolism whose significance is unknown but which has nothing to do
with alcoholic fermentation (WORTMANN, 1898). Into the question of the
occurrence of other secondary products, such as aldehyde, volatile acids, &c.,
we need not enter ; these are doubtless formed also in pure yeast fermentations,
but they are not actually products of fermentation. There are also other
bodies present which give definite characters to the fermented liquor, but
these are not due specially to the yeast but to the fermentable material ; in the
case of wine they are derived from the grape.

The decomposition of sugar into alcohol and carbon-dioxide is remarkably
complete ; this is best seen by a comparison of the structure of ^-glucose with
the products of fermentation : —

OH OH H OH OH HaOH H3 Ha H3 H3

C—C — C — C — C — C = C— C + CO3 + CO2 + C — C
H OH H H OH OH

It has not hitherto been possible to carry out this decomposition by purely
chemical means, and hence it was for long thought that the living protoplasm
alone had the power to bring it about. Recently, however, E. BUCHNER (1897
onwards) has succeeded, by vigorous friction and under high pressure, in obtain-
ing a sap from yeast, which can induce the formation of alcohol from all the
carbohydrates that the yeast acts on. BUCHNER' s view is that fermentation is
a purely chemical process, carried out by definite substances present in the
expressed sap. BUCHNER'S explanation has not remained unchallenged ; doubts
have been expressed in the first place whether the decomposition of the sugar
effected by the expressed sap is identical in character with that produced by the
living protoplasm, and, secondly, it has been suggested that the activity of the
expressed sap is due to living yeast protoplasm contained in it. If that be so,
yeast protoplasm must have certainly extraordinary powers of resistance, since
after precipitation with alcohol and acetone the redissolved precipitate still
retains its activity. Recently, ALBERT (1901) has met these objections by kill-
ing the yeast in alcohol and ether, when, notwithstanding this treatment, he
found that its power of fermentation was unaffected ; the dead cells were able
to form alcohol from sugar, whether in the natural or in the crushed state.
The fermentation in the former case is intra-cellular as in the case of living cells ;
the sugar is split up inside the cells, for the fermenting substance cannot pass
through the cell-wall. This, however, is not extraordinary, for it has been
established that many enzymes are incapable of passing through the cell- wall.
BUCHNER unhesitatingly considers this body as an enzyme and calls it 'zymase'.
It must, however, be remarked that zymase has not much in common with the
enzymes we have previously studied, for these act by relatively simple methods,
e. g. hydrolysis, a power possessed by inorganic agents also. In addition to
hydrolysing enzymes we have also oxidizing enzymes to take note of. Zymase
differs from all of these in being much more thorough in its action and in
possessing the power of breaking down the sugar molecule into new combina-
tions of carbon, hydrogen, and oxygen. [According to BUCHNER'S (1905)
observations it would appear that lactic acid is not improbably an inter-
mediate product in alcoholic fermentation.]

But zymase differs from hydrolytic enzymes in another respect. The
duration of its activity at summer temperature is very slight ; it is very rapidly
destroyed, and in order to obtain large quantities of alcohol much expressed sap
and concentrated sugar solutions must be employed, whilst in the case of ordinary
enzymes small amounts are able to produce great changes. This distinction
may not, however, be of great importance since it is possible that the activity
of zymase may be inhibited by the enzymes in the expressed sap ; still the

p 2

212 METABOLISM

difference in the activity of enzymes and zymase appears to us to be sufficient
to separate these bodies from each other. Since also it is possible that we may
find substances associated with other fermentative processes analogous in their
action to zymase, we had better employ the name 'zymase' as a collective term
for all substances produced by organisms which have the power of inducing
fermentative decompositions, and designate BUCHNER'S zymase by the term
alcoholase. There can be no doubt that the determination of zymases is one of
the greatest advances as yet made in the study of the theory of fermentation ; it is
therefore worthy of mention that E. TRAUBE (1858) believed ' enzymes ' to be
the cause of fermentation, although he was unable to separate them from the living
substance. The successful isolation of alcoholase has as yet not been effected.

Although the existence of alcoholase has been proved it by no means follows
that alcoholic fermentation is to be regarded as a vital phenomenon, even
although zymases as well as enzymes generally, in their origin and activity, are
entirely dependent on organisms. This is all the more apparent when we
remember the dependence of fermentation on external conditions. We will
confine ourselves to the consideration of two factors only, temperature and
oxygen. With regard to temperature we may note that an optimum of 25° C.
is^ necessary for fermentation while the optimum activity of zymase lies much
higher. We must go more carefully into the question of the effect of oxygen
on fermentation. Doubtless, when zymase operates in a test-tube it is quite
immaterial whether oxygen be present or not. From our experience of Fungi
and higher plants also, we must expect yeast to produce alcohol only in the
absence of oxygen ; but in the cases mentioned the formation of alcohol takes
place only in intra-molecular respiration and is due to the presence of a zymase ;
at least, STOKLASA (1903) has found such an enzyme in beet, although
GODLEWSKI (1901) was unable to determine its presence in seeds which pn>
duced alcohol. [MAZE (1904) has shown that STOKLASA'S arguments in favour
of general distribution of zymase are not valid.] The formation of zymase in
beet must come to an end in the absence of oxygen. Yeast, however, behaves
quite otherwise ; it forms alcohol in presence of oxygen as easily as in its absence.
[According to WEHMER (1905) species of Mucor behave in the same way.] This
fact, obviously of so much importance to the theory of fermentation, deserves
more detailed investigation, for it is impossible to believe that alcoholic fer-
mentation is quite independent of oxygen.

Yeast, in addition to its power of inducing fermentation, can also respire in
the ordinary way, and if oxygen be present in the fermentable medium part
of the sugar will be respired and the rest fermented. A definite quantity of
yeast, however, generates all the more alcohol the less normal respiration is
permitted, and if oxygen be entirely absent the whole of the sugar disappearing in
the course of metabolism will be fermented. Since the energy evolved from
the respiration of the sugar is much greater than from its fermentation one
comprehends that more sugar will be used up in the latter case. In GILTAY
and ABERSON'S experiments (1894) i g. of yeast (dry weight) transformed 7 g.
of sugar in presence of oxygen and 14 g. in its absence in the same time. The
maximum formation of alcohol in no way corresponds to the optimum vital
conditions of yeast. Growth and increase of yeast are largely dependent on the
presence of oxygen ; when it is withdrawn vegetative activity ultimately ceases,
although fermentation still goes on. There is, unfortunately, no unanimity
among the different authorities as to how far increase of the yeast is possible in
the absence of oxygen ; according to some authors, it ceases entirely, according
to others (BEIJERINCK, 1894), twenty to thirty times the original mass may be
produced. Whichever view is correct, growth is always limited when alcoholic
fermentation only occurs, while it is unlimited when respiration begins. Since,
however, the amount of alcohol produced in a unit of time depends obviously
on the amount of yeast present, one cannot say a priori whether a minimum
amount of yeast will give in the long run more alcohol in a nutritive fluid con-

FERMENTATION

213

taining oxygen or in one in which none is present. Experiments and practical
experience have shown that in presence of a limited amount of oxygen yeast
increases so vigorously and fermentation is so little retarded that the maximum
amount of alcohol may be formed. If a very small quantity of yeast be added to
a suitable nutritive solution from which air has been fully excluded the organism
at first greedily absorbs the oxygen dissolved in the solution ; it even makes
use of combined oxygen, such as the oxygen united with haemoglobin, although
it is unable to decolourize indigo-carmine. As the yeast increases and the
alcoholic fermentation begins, bubbles of carbon-dioxide make themselves
apparent in the fluid ; these, however, become rapidly smaller and finally cease.
The addition of a minute quantity of air at once increases the intensity of the
fermentation in a remarkable manner, and bubbles of carbon-dioxide once more
make their appearance (DUCLAUX, 1900). If oxygen be permanently excluded,
the yeast in the end dies, even though food-materials be still present (BEIJERINCK,
1894). In the long run, alcoholic fermentation proper is also promoted by the
presence of small quantities of oxygen. The statements in the literature on
this whole question are, however, more contradictory than in any other depart-
ment of physiology.

It will perhaps help us to reach an accurate conception of the behaviour of
yeasts if we consider here two terms suggested by PASTEUR (1861 and 1863), and
generally employed since his time. Such forms as could go through their normal
development only in presence of oxygen he termed aerobic, and anaerobic such
as could get on without oxygen, or such as were injured by that gas. In
studying typical aerobic plants we saw that a certain partial pressure of oxygen
was injurious to them also. This oxygen pressure lies in their case far above
that of normal atmospheric air, whilst that affecting typical anaerobes lies far
below it. The extremes are connected by transitions which, on the one hand,
from their increasing liability to injury from oxygen, and on the other in their
increasing need for that gas, shade quite gradually into each other. Since
nothing is known as to the injury done to yeast by oxygen, we can only
characterize it by its oxygen requirements. It differs obviously in this point
both from typical aerobes and from anaerobes, since it can live and multiply
for a long, though not unlimited, time without respiration.

We have not as yet solved the chief question before us, viz. why yeast
forms alcohol not merely when its respiration is interfered with but under all
conditions ? From the point of view of energy we have here to deal with a loss,
and the question is whether or not gain in another direction does not counter-
balance this loss ? To WORTMANN (1902) we owe a very probable hypothesis ;
he considers the alcohol as a protection employed by the yeast against associated
micro-organisms, since yeast itself can tolerate 10-18 per cent, of alcohol while
all other organisms occurring in saccharine fluids are injured by 4-10 per cent,
of alcohol. This theory reminds us of the formation of acids by Fungi, not with
the object of gaining energy but for certain biological reasons. The difference
between the formation of acids by Aspergillus and the formation of alcohol
by yeast lies in this, that Aspergillus aims at acidifying the substratum,
only in order that it may be made unfavourable for other organisms ; it ceases
forming acids before it is itself injured by this product of its own metabolism,
while yeast, on the other hand, goes on producing alcohol until it is itself killed
by the alcohol, and apparently almost all fermentative actions are similarly
brought at last to a standstill by the products of fermentation. This fact does
not seem to us to be quite in accord with WORTMANN'S hypothesis. If our
definition is to be maintained and we are to speak of fermentation only where
destructive metabolism precedes a gain in energy, then alcoholic fermentation as
well as acid ferment at ion by Fungi must be excluded from this category of actions.
We must also inquire what other fermentative processes exist, since the poisonous
effect of the chief product has apparently a definite significance apart from the

214 METABOLISM

gain in energy in these other cases also. We must extend our original definition
and say that fermentation is dissimilation which is not carried, as in respiratory
combustion, to the final stages. Under these circumstances we must also
regard the formation of organic acids in succulents as a case of fermentation.
Attempts to bring other fermentative actions within the scope of our
observations land us in a region where the data are most contradictory — more
indeed than need be. Up to the time of PASTEUR'S first important investiga-
tions we knew nothing of what is nowadays known as a 'pure culture'; before
that, investigators experimented with cultures usually containing several
organisms and they were consequently unable to determine what part each
played in the production of the final result; and nowadays, when the need for
pure cultures has become generally recognized, the effort to achieve practical
results in the physiology of fermentation has increased to such an extent that
important scientific questions requiring solution have been quite ignored. Hence
there has arisen a mass of literature which can only be surveyed by specialists.
Under these conditions it is to-day more than ever difficult to give an outline of
the present position of our knowledge of this subject. [LAFAR has performed
an extremely valuable service in giving in his Handbook of Technological
Mycology (Jena, 1904, onwards) an exhaustive account of all the important
fermentative processes used in the Arts. This work, which is still in course of
publication, may be referred to for further details.]

Alcoholic fermentation, that is to say the formation of ethyl-alcohol, is
not confined to yeast ; it may be induced also, as we have seen, under certain
conditions by Mould-fungi as well as by higher plants, but
om"y ky a relatively small number of Bacteria. It is true
that certain Bacteria have the power of forming higher
alcohols ; thus, for example, fusel oils (a mixture of
Pr°pyK butyl-, and especially amyl-alcohols), which are
developed during the manufacture of brandy, and par-
ticularly of potato spirit, are due apparently to the action
Fig. 38. Bacillus We™. °f Bacteria ; pure yeasts give rise to none of the higher
After BEIJERINCK. x 900. alcohols. BEijERiNCK (1894) has carefully studied a
bacterium which induces the formation of propyl- and

butyl-alcohols, and to this form we may devote some little attention since it is in
many respects an interesting contrast to yeast. Bacillus butylicus (Granulobacter
butylicum, Beij.) (Fig. 38), as it is named, is an elongated rod of considerable
size ; it contains a large amount of a carbohydrate, the so-called granulose,
coloured blue by iodine, and finally forms endospores in more or less spindle-shaped
swellings. In nature this bacterium occurs with great constancy on the fruits of
certain species of barley and consequently also in the meal formed from them.
If such meal be made into a mucilage by cooking for a short time, the spores,
which resist, at least for a few minutes, this high temperature, quickly begin to
germinate, and the organisms increase rapidly in number. At the same time the
starch is changed, by a diastatic enzyme excreted from the cells into maltose,
and this later on is in part used in constructing new organisms and in part is fer-
mented. The fermentation consists in the development of hydrogen and carbon-
dioxide in varying proportions, while butyl-alcohol is also developed. The net
amount of this characteristic product is not very great, amounting as it does only
to about 1-3 per cent, of the meal. According to more recent research propyl-
alcohol is also formed (BEIJERINCK, 1889 ; Archiv. neerland. II, 2, 402, note).

Bacillus butylicus — apart altogether from its specific zymatic capacities —
differs, however, from yeast in one noticeable point, namely, in its relation to
oxygen ; it is strongly anaerobic. In order to obtain butyl-alcoholic fermenta-
tion the greatest care must be taken to exclude oxygen from the nutritive sub-
stratum, since if sweetwort be used as the culture medium small quantities of
oxygen are found to be directly injurious. BEIJERINCK removed most of the

FERMENT A TION 215

oxygen by pumping it out and by passing hydrogen through it, the remainder
he got rid of by adding an easily oxidizable body (sodium hydrosulphide). When
free oxygen is completely absent unlimited growth of the bacterium takes place
and, at the same time, very active fermentation. But whenever the least traces
of oxygen are present the organism begins to assume a somewhat different
appearance from the strictly anaerobic form and forms no spores. BEIJERINCK
(1894 and 1899) denies that we have here to deal with a vital process which goes
on entirely without oxygen and without normal respiration, although he succeeded
in carrying out seven cultures one after the other without oxygen being present,
and although he found an increase from one to many millions taking place —
not as in the case of yeast from one to twenty- or thirtyfold only. What led
him to this conclusion was specially the behaviour of the bacterium under a
cover-glass. Many motile Bacteria strive to reach the concentrations of oxygen
which suit them best, and collect in such regions ; if they be placed on a
slide and covered by a cover-glass only a very limited quantity of oxygen
can reach them, for the amount of oxygen dissolved rapidly decreases from
the edge of the cover-glass inwards. Genuine aerobes collect at the edge of the
preparation, genuine anaerobes, on the other hand, in the centre ; Bacillus
butylicus places itself at a certain distance from the periphery, where a low but
not a minimum pressure of oxygen exists. The organism thrives also on other
nutritive substrata, e. g. I per cent, solution of peptone with J per cent, of
starch paste, but induces fermentation only when air can enter easily ; only
in sweetwort can it form many generations one after the other without free
oxygen. Hence BEIJERINCK held that the sweetwort contained an oxygen
reserve combined in some way and yet accessible to the bacillus, and believed
that all Bacteria, even the so-called obligate anaerobes use small quantities
of oxygen, and thus that there are no organisms which are able to do without
free oxygen. Even if BEIJERINCK be right in his conclusions the distinc-
tion between aerobes and anaerobes, or as BEIJERINCK puts it, between
aerophiles and micro-aerophiles is still perfectly well marked, for the consump-
tion of oxygen in the latter is still far from being sufficient to render possible a
respiration capable of maintaining the necessary supply of energy ; fermenta-
tion must always assist in the process. It is possible that certain functions are
carried out only when respiration takes place, while other functions may be
performed at the expense of the energy supplied by fermentation.

The researches of CHUDIAKOW (1896) form a useful supplement to those
of BEIJERINCK, and they ought to be mentioned here, although they do not
deal with Bacillus butylicus, but with two butyric acid-producing Bacteria
termed Clostridium butyricum and Bactridium butyricum. Both these forms are
strongly anaerobic ; in their vegetative condition they are injured by a brief
exposure to ordinary air and are killed by longer exposure ; even their spores
cannot in the long run resist the action of oxygen, although small quantities of
oxygen do not produce any injury. Bactridium butyricum can develop quite well
in air with a pressure of 5 mm., and Clostridium at a pressure of 10 mm., and,
what is most noteworthy, the small quantities of oxygen present in the culture
are, under these conditions, completely absorbed and utilized by the organism.

CHUDIAKOW'S experiments do not lead us to the conclusion that the develop-
ment of the butyric acid Bacteria mentioned depends on the presence of combined
oxygen in a medium that possesses no free oxygen, in the same way as BEIJERINCK
asserted for Bacillus butylicus, for they grow on dextrose or saccharose, maltose,
starch, lactose, and mannite, in combination with peptone or asparagin, urea,
ammonium chloride (but not with nitrates), exactly like anaerobes. Since
again, the presence of combined oxygen in wort is (in BEIJERINCK' s researches)
problematical, the last word has not as yet been spoken as to the nature of
anaerobiosis ; yet it is extremely probable that the organisms concerned can
live without oxygen. On the other hand, there is always the possibility that

216 METABOLISM

these organisms live during anaerobiosis on stored oxygen ; at all events,
organisms are known which possess such a power (£WART, 1897). There are
certain Bacteria which render themselves to a certain extent independent of
the presence of oxygen by forming a pigment which has the capacity, even
when dissolved out of the cells, of absorbing oxygen, just as the haemoglobin of
the blood does, and of combining it in such a way that the oxygen is set free
again in any situation not provided with free oxygen, and, in the course of
a few hours, diffuses out of the cells. These organisms can, for a long time,
maintain normal respiration in a space free from oxygen, at the cost of reserves
of oxygen which they have accumulated. How far they make use of this power
has not as yet been determined. On the other hand, this power is of great
service to anaerobes, whether they are able to live always or only for a certain
time without oxygen ; for they can live in places where, owing to the activity of
aerobes, the oxygen has been used up, and thus nutritive material from which
other organisms are debarred will be accessible to them.

Having now learned from a study of 'the formation of ethyl- and butyl-
alcohol, and, earlier, of oxalic and other organic acids, that there are various
types of fermentation, which differ very essentially in their oxygen requirements,
we may now glance at some of the numberless other types of fermentation, and
chiefly at those by which the products of the animal and plant worlds are
transformed, through the fermentative agency of lower organisms, into simple
bodies which may serve once more as nutrients to the higher plants.

If we begin with the alcohol which arises from the fermentative activity of
yeast, we have first of all to notice that this substance is produced artificially
not only in brewing, wine-making, and distilling, but that it is generally to be
found in nature wherever sugar-containing sap occurs. Thus yeasts or other
Fungi always locate themselves on the outer surface of many fruits or in sap
expressed from plants by osmotic pressure, and in such situations they are able
to induce alcoholic fermentation. Among the chief products so formed there
is one, carbon-dioxide, a completely oxidized body, for whose further alteration
in the plant an expenditure of energy is necessary (p. 130), and which on that
account is of no value to any organism as a material for the support of metabolic
changes ; the other product, however, alcohol, is relatively poorer in oxygen
than sugar, and hence can serve as a source of energy to some types of organisms.
An earlier opportunity was taken of noting that alcohol may serve as a source
of carbon to many Fungi, and it may be concluded that it may serve to help
not only in the construction of the organism but also in respiration. Acetic acid
Bacteria (compare HOYER, 1898, HENNEBERG, 1898) oxidize alcohol completely
into acetic acid, approximately according to the following formula : —

C2H60 + Oa =

For this process oxygen in large quantity is obviously necessary. Since carbon-
dioxide (in the first instance at all events) is not formed, normal respiration
does not take place ; only when the alcohol is all decomposed, is acetic acid
further decomposed into carbon-dioxide, but doubtfully in the case of all acetic
acid Bacteria. At present we cannot say with certainty whether an acidifying
of the substratum is the primary object of acetic acid fermentation so as to
shut out associated organisms, or whether the object is merely to make use of
the chemical energy of alcohol. The latter seems to us to be somewhat improb-
able because there would appear to be no reason why the formation of acetic
acid should cease, and, in support of the former hypothesis, we must remember
that the acetic acid Bacteria are more resistant to that acid than other organisms.
Acetic acid Bacteria are furthermore not exclusively confined to alcohol as
a medium ; they can exist in other substances as well, all of which they are cap-
able of oxidizing. Thus they can transform higher alcohols into fatty acids, e. g.
propyl -alcohol into propionic acid, and butyl-alcohol into butyric acid. Some of

FERMENTATION 217

them can oxidize glucose into gluconic acid, mannite into laevulose, sorbite into
sorbose. In addition, certain acetic acid Bacteria are known to be able to form
oxalic acid from sugar and many other organic compounds — though scarcely
from alcohols (BANNING, 1902). Sugar is further a good source of carbon for
acetic acid Bacteria and can be used for growth purposes along with an appro-
priate source of nitrogen ; many acids also, e. g. acetic acid, serve as food-stuff,
while alcohol is employed only as a fermentable substance.

Although acetic acid Bacteria do not make any further use of that acid,
another widely distributed organism occurs in nature — known as Saccharomyces
mycoderma — which does do so, and by the three consecutive activities of yeast,
of acetic acid Bacteria and of Mycoderma, the sugar is finally transformed into
the same end products which arise from it in respiration in the normal plant.

The decompositions described are not the only ones which sugar and
related carbohydrates undergo owing to the action of microbes in nature ;
very often lactic acid or fatty acids arise. Bacteria which produce lactic acid
as a by-product have been described by the dozen, but only a few form this acid
in such quantity that we may speak in this case of lactic acid fermentation.
If, as in the case of Bacillus lactis acidi, the entire fermentation consists
simply in the splitting of one molecule of glucose into two molecules of
dextro-lactic acid, or, as in Bacillus acidificans longissimus, into two molecules
of laevo-lactic acid, there is no question of fermentation in the old sense of the
term, since no energy is released in the process and its significance can lie only
in the exclusion of associated organisms. There are other lactic acid Bacteria,
however, which give rise to other fermentation products.

Butyric acid is the only one of the fatty acids which arise during the
fermentation process which we need speak of here. Animal anaerobes produce
fatty acids also as fermentative products, e. g. propionic acid in Ascaris (WEIN-
LAND, 1901, Zeit. f. Biol. 24, 55). Butyric acid may be derived from sugar
as well as from lactic acid, and indirectly from the polysaccharides starch, inulin,
dextrin, &c. ; its development is due for the most part to the influence of
anaerobic Bacteria, as to which much more or less accurate information has been
accumulated. These forms are very like each other morphologically, and also
resemble Bacillus butylicus above described. According to BEIJERINCK (1894)
the latter is characterized by producing butyl-alcohol only, and never butyric
acid, while Bacillus saccharo-butyricus (Granulobacter, Beij.), which occurs in
similar situations to B. butylicus, develops from butyric acid, in addition to
butyl-alcohol, large quantities of carbon-dioxide and hydrogen. GRIMBERT
(cited by DUCLAUX, Trait6 de Microbiol. vol. IV) has described a bacterium
named Bacillus orthobutylicus, which, in addition to these bodies, forms acetic
acid also, as does a bacterium studied by PERDRIX (cited by DUCLAUX), termed
by him ' bacille amylozyme '. In the cases which have been accurately investi-
gated it has been shown that the proportion of fermentation products is by no
means constant ; thus in the case, e. g., of ' bacille amylozyme ', at the beginning
the amount of carbon-dioxide is far less than that of hydrogen, though, later
on, both occur in about equal quantities ; so too at the beginning of fermentation
only acetic acid is formed, but not in the later stages. No explanation has as yet
been given of the causes of these variations, nor of the appearance of such varied
products ; we must wait for the future to provide us with an explanation of how
the active enzymes are to be found and isolated ; especially if it be shown that
several different zymases occur in one and the same organism, whose activities
are affected in a variety of ways by external conditions.

As we have already said, polysaccharides, such as starch, can be used up by
certain butyric acid Bacteria ; some also are able to attack cellulose, e. g. the
bacterium studied by OMELIANSKI (1902) in WINOGRADSKY'S laboratory,
a bacillus of very small diameter (0-2 /x) which forms spherical spores in terminal
swellings, but which gives no blue reaction when treated with iodine. It may be

218 METABOLISM

cultivated anaerobiotically in a nutritive solution in which Swedish filter paper
forms the source of carbon and an ammonium salt the source of nitrogen, and
to which carbonate of lime is added to neutralize the acids which arise. The
cellulose becomes at first transparent, then in the course of a few months
entirely dissolves and is broken down into acetic acid, butyric acid, traces of
other fatty acids, carbon-dioxide, and water. In a definite case, 3-35 g. of
cellulose gave rise to 2-240 g. of acetic and butyric acids (in varying proportions),
0-972 g. of carbon-dioxide, and 0-014 g. of hydrogen. Obviously the cellulose
is first of all split into simpler carbohydrates ; but it is noteworthy that the
bacillus has not as yet been made to develop on any other medium than cellulose.

The cellulose, which is produced annually in enormous quantities by the
higher plants, and which once formed is no longer for the most part of any
further (metabolic) service to them, is again made available for metabolism, and
thus vast quantities of carbon, which otherwise would lie unused, become trans-
formed into humus, turf, and coal, once more to be employed in the support of
life. The bacillus mentioned is not the only one which acts in this way. It is
frequently stated that methane is also formed from cellulose, and the plentiful
occurrence of this gas in places where cellulose is undergoing decomposition is
evidence in favour of the correctness of this statement. In a word, OMELIANSKI
has succeeded in showing that the cause of this methane-fermentation of cellu-
lose is a bacillus, which appears to be like that described, but thinner and more
delicate. It grows in a culture solution like that in which the bacillus producing
hydrogen-fermentation thrives, but it ferments the cellulose into acetic acid,
butyric acid, carbon-dioxide and methane. OMELIANSKI, in one experiment
found that 2-0065 g. of cellulose gave rise to 0-1372 g. of methane, 0-8678 g. of
carbon-dioxide and 1-0223 g« of volatile acids. Thus about 50 per cent, of the
fermentation products are volatile acids, and among these there is about nine
times as much acetic acid as butyric acid.

The Bacteria causing hydrogen and methane-fermentation occur frequently
in conjunction in nature and it is extremely difficult to separate them. As
long as this separation is not effected the products of the fermentation set up
by both organisms appear in the culture, sometimes those of the one predomi-
nating, sometimes those of the other. We may suppose, therefore, that the
contradictory results arrived at in other fermentation experiments are often
due to impure cultures, and hence OMELIANSKI' s work is of extreme value from
the point of view of technique. [OMELIANSKI, 1904 a.]

In addition to cellulose, pectins are among the substances which go to the
construction of the cell-membrane. They also are not dissolved out by the plant,
but remain in the fallen leaves, twigs, &c., and are attacked by definite micro-
organisms in the soil or in water. We have to thank WINOGRADSKY (1895) and
BEHRENS (1902) for proof that certain Bacteria, apparently butyric acid Bacteria,
carry put ' pectin fermentation ' in nature, though we are not as yet accurately
acquainted with the products of fermentation. Many species of Mucor also
are able to dissolve pectins. This dissolution of pectin compounds plays a part
also in the technique of hemp and flax manufacture (' retting '), since the
isolation of the fibres of these plants is possible only after the dissolution of the
middle lamella by pectin-fermentation. [OMELIANSKI, 1904 b.]

On studying the final products of fermentation, e. g. cellulose-fermentation,
we find that they are partly fully oxidized products (carbon-dioxide), partly
products of extreme reduction (methane, hydrogen), partly intermediate sub-
stances (acetic acid, butyric acid, &c.). All the compounds which are incom-
pletely oxidized may still be made use of. Although the actual employment of
such energy as may be obtained by oxidation of methane or hydrogen is not as
yet known, substances like lactic, butyric, and many other organic acids are, on
the other hand, used for respiratory or fermentative purposes by numerous micro-
organisms. [The most thoroughly studied of these organic acid fermentations

FERMENTATION 219

is that of formic acid (OMELIANSKI, 1903).] The numberless bodies which arise
in consequence of these decompositions need not be referred to at present ; nor
need we treat of the various fermentation products of the higher alcohols,
e. g. glycerine, mannite, dulcite, &c. We need only note that none of the
products of such fermentations accumulate in nature, but that other organisms
are always at hand which have the power of breaking down these primary pro-
ducts, till finally the organic substances are transformed into simple inorganic
compounds which are once more available for the nourishment of higher
plants. Thus we begin to appreciate the fact that over the earth's surface
there exists a perpetual transformation, a continuous circulation of materials
which we will take a later opportunity of once more referring to (Lecture XIX).

But it is not only the simple organic bodies, such as carbohydrates, acids,
and alcohols, which are broken down by the fermentative power of micro-
organisms, when these bodies are removed from the living organism, but com-
plicated bodies, and indeed the most complicated of all, the proteids, undergo
the same fate. In the first place, these proteids are broken down more or less com-
pletely by various excreted enzymes, and the products so formed suffer appro-
priate fermentations. Although Bacteria are known which are able to produce
fermentation in proteids, and although much research has been carried out on the
resulting products, we are in no case so fully acquainted with the chemical and
biological conditions of such fermentations that we can present a complete
picture of the course of the fermentative process. Our remarks on the subject
must therefore be brief. When proteid is acted on by anaerobic Bacteria, we
term it putrefaction, and this is characterized by the appearance of evil-smelling
compounds (indol, skatol, &c.), but in the presence of air these substances dis-
appear. In nature these processes go on hand-in-hand as, for example, in the
putrefying bodies of animals or plants ; soon the aerobic forms have consumed
all the oxygen in the interior of the body, and the anaerobic forms then proceed
to carry put a further decomposition which we term putrefaction. Owing to
the activities of a series of living organisms following each other or living side
by side, the proteid molecules, after passing through numerous intermediate
stages, are finally broken down into a few simple bodies, viz. carbon-dioxide,
methane, hydrogen, ammonia, nitrogen, sulphuretted hydrogen and phosphoric
acid.

In the following lectures we will return to the consideration of certain of
these final products of proteid fermentation.

Bibliography to Lecture XVII.

ALBERT. 1901. Centrbl. Bakt. II, 7, 473.

BANNING. 1902. Centrbl. Bakt. II, 8, 395.

BEHRENS. 1902. Centrbl. Bakt. II, 8, 114.

BEIJERINCK. 1894. Archives neerlandaises, 29, i.

BEIJERINCK. 1899. Ibid. II, 2 ; Centrbl. Bakt. 1900, II, 6, p. 341.

BUCHNER, E. 1897. Alkoholische Caning ohne Hefezellen (Ber. d. chem. Gesell.
1897, P- H7 ; also numerous papers in the same Journal, from 1897 onwards.
Compare also the summaries in Bot. Ztg. 1898 onwards).

BUCHNER, E. and H. and RAPP. 1903. Die Zymasegarung. Munich and Berlin.

[BUCHNER and MEISENHEIMER, 1905. Ber. chem. Gesell. 38, 620.]

CHUDIAKOW. 1896. Zur Lehre von d. Anaerobiose. Review by ROTHERT, Centrbl.
Bakt. 1898, II, 4, 389.

DIAKONOW. 1886. Ber. d. bot. Gesell. 4, 2.

DUCLAUX. 1900. Traite de microbiologie, 3, 308. Paris.

EWART. 1897. Journal Linn. Soc. Bot. 33, 123.

FISCHER, E. 1898. Zeit. f. physiol. Ch. 26, 60-87.

GILTAY and ABERSON. 1894. Jahrb. f. wiss. Bot. 26, 543.

GODLEWSKI and POLZENIUSZ. 1901. Bullet. Acad. de Cracovie.

HANSEN. 1888. Rech. s. la phys. et la morphologic des ferments alcooliques, VII.
Action des ferments alcool. sur les diverses especes de sucre (Meddel. f . Carls-
berg Laborat. 2, Heft 5).

220

METABOLISM

HENNEBERG. 1898. Koch's Jahresbericht, 9, 249 and 251.

HOYER. 1898. Rev. in Koch's Jahresbericht, 9, 242.

KNECHT. 1901. Centrbl. Bakt. II, 7, 165.

KLOCKER. 1901. Centrbl. Bakt. II, 7, 152.

[KosxYTSCHEW. 1904. Jahrb. f. wiss. Bot. 40, 563.]

LABORDE. 1899. Compt. rend. Paris, 129, 344. (Koch's Jahresbericht, p. 98.)

MAZE. 1900. Annales Instit. Pasteur, 14, 350.

[MAZE. 1904. Annales Instit. Pasteur, 18, 535.]

[NABOKICH. 1903. Ber. d. bot. Gesell. 21, 467.]

OMELIANSKI. 1902. Centrbl. Bakt. II, 8, 193.

[OMELIANSKI. 1903. Ibid. II, n, 177.]

[OMELIANSKI. 1904 a. Ibid. II, n, 369.]

[OMELIANSKI. 1904 b. Ibid. II, 12, 33.]

PASTEUR. 1859. Annales de chim. et de phys. Ill, 58, 347.

PASTEUR. 1861. Compt. rend. 52, 344 ; 1863, 56, 416 and 734.

STOKLASA, JELINEK and VITEK. 1903. Beitr. z. chem. Phys. u. Path. 3, 460.

TRAUBE, E. 1858. Theorie der Fermentwirkungen. Berlin.

UDRANSZKI. 1889. Zeit. f. phys. Chem. 13, 539.

WEHMER. 1905. Centrbl. Bakt. II, 14, 556.

WINOGRADSKY. 1895. Compt. rend. Paris, 121, 742.

WORTMANN. 1892. Landw. Jahrb. 21, 901.

WORTMANN. 1898. Ibid. 27, 631.

WORTMANN. 1902. Weinbau u. Weinhandel.

LECTURE XVIII
SULPHUR AND NITROGEN BACTERIA

AT the conclusion of our last lecture we noted that the sulphur of the
proteid molecule was set free as sulphuretted hydrogen in the course of putre-
faction. There are, however, other processes in nature in which sulphuretted

hydrogen is also developed. In
many of these also micro-organisms
are concerned, such as in the re-
duction of sulphates, which is
carried out with tolerable com-
pleteness not only by the strictly
anaerobic Spirillum desulphuri-
cans (BEIJERINCK, 1895), but also
by other less exclusively anaerobic
forms (BEIJERINCK, 1900). We
are unable, however, to explain

Fig. 39. Sulphur-bacteria, a-c, Beggiatoa, x 1000. d, Chro- what Service SUCh reductions are
tnatium okenit. x ooo. f, Latxprocystis foseo-persictna, . ,-t "DA* rA J' A

x5oo. After FISCHER; vorlub. Bakt. and edit. to these Bacteria. [According to

DELDEN (1903) it consists in the

gain of oxygen from the sulphates, just as, in the other reduction processes
(p. 232), oxygen is obtained from compounds containing it.] In any case it is
not our purpose to enter into a detailed discussion of these processes ; what
we are interested in at the moment is merely the final product, especially
the sulphuretted hydrogen, and the further alterations it undergoes under
the influence of certain Bacteria, which for that reason have received the
name of ' sulphur-bacteria1.

We may select as our first example of sulphur-bacteria, the genus Beg-
giatoa (Fig. 39, a), which may be described briefly as a colourless Oscillaria, in
whose protoplasm plentiful aggregations of sulphur particles or drops may be
found. The presence of such large quantities of pure sulphur in its cells leads
us to conclude that the sulphur plays an important part in the life of the

SULPHUR AND NITROGEN BACTERIA 221

organism, and detailed research confirms this view. For long Beggiatoa was
believed to be the cause of the formation of sulphuretted hydrogen, until HOPPE-
SEYLER (1886) proved that this was not the case, but that Beggiatoa gave rise
to sulphur by oxidizing sulphuretted hydrogen (H2S + O = H2O + S). More
recently WINOGRADSKY (1887), in a classical monograph, traced out the pro-
cess in an exhaustive manner, and showed what its significance was in the
economy of the organism. [OMELIANSKI (1904) has also provided us with
a comprehensive exposition of the physiology of the sulphur-Bacteria.]

Beggiatoa occurs in nature in the mud associated with salt or fresh water,
wherever the water or mud contains a sufficient quantity of sulphates. The
part these sulphates play, however, is only to supply the material from which
other organisms may develop sulphuretted hydrogen. When sulphuretted
hydrogen is itself present in the water sulphates are quite superfluous. Beggiatoa
then appears in the sulphur-containing medium, and develops in it luxuriantly.
WINOGRADSKY proved that Beggiatoa was present in ever decreasing numbers
when the source of sulphur was gradually diminished, and that when the sul-
phuretted hydrogen was completely absent from the water, Beggiatoa also
disappeared.

These observations of natural conditions teach us how important sul-
phuretted hydrogen is for the maintenance of the life of Beggiatoa, but con-
vincing evidence and a more exact knowledge of this phenomenon are to be
obtained by cultures. If we attempt to cultivate Beggiatoa by the methods
employed for the majority of Fungi and Bacteria, presenting it with a solid
or fluid substratum rich in organic material, it dies off in a very short time.
If, on the other hand, we place a small quantity of Beggiatoa on a slide, cover
it, and keep supplying it daily with fresh quantities of water containing sul-
phuretted hydrogen (WINOGRADSKY used natural water obtained from the
Langenbriicken Baths, to which more sulphuretted hydrogen was added), it not
only remains alive, but increases so rapidly that large quantities have to be
removed in order to make room for the development of what is left. With the
aid of such a micro-culture in a healthy state of growth we are able to carry
out easily the following decisive experiments : —

1. The culture is treated twice daily with Langenbriicken sulphur-water,
which has been deprived of its sulphuretted hydrogen by being exposed to
air. The Beggiatoa soon loses its sulphur, and is unable to form more ; it
then gradually dies off.

2. If the culture, on the other hand, be supplied with the same water
containing sulphuretted hydrogen, Beggiatoa again develops rapidly.

The only difference between the two cultures is the absence in one of
them of sulphuretted hydrogen, and hence it follows that this substance is>
essential to Beggiatoa, and that from it Beggiatoa manufactures the sulphur
found among the cell contents. Since this is possible only by oxidation,
Beggiatoa is absolutely dependent on the presence of oxygen, although it
requires this element in quite definite quantities ; too little of it is as dis-
advantageous as too much. If the experimenter desires to regulate the amount
of oxygen supplied to the culture he will meet with insurmountable difficulties,
which at once disappear if the organism itself be allowed to regulate its own
supply of oxygen. Since Beggiatoa is a free motile form, it is able, just like
other motile organisms (p. 215) to find for itself the optimum concentration
of oxygen, provided all possible variations in oxygen tension occur between
the edge of the cover-glass and the centre. If we allow a drop of dilute sul-
phuretted hydrogen-water, free from Beggiatoa, covered with a cover-glass to
stand in a moist chamber, we see after a few hours that the formation of
particles of sulphur which takes place under the influence of air, is apparent
only for a distance of a millimetre from the edge of the cover-glass, while
the central region remains for long unoxidized, provided that, by fre-

222 METABOLISM

quent renewal of the fluid, we maintain a constant proportion of sulphuretted
hydrogen. If we now introduce under the cover-glass some active plants of
Beggiatoa, we can see the filaments rapidly wandering towards the edge of the
preparation, and forming a thick white border (visible to the naked eye) at a
distance of about i mm. from the edge of the cover-glass. Beggiatoa avoids
the periphery of the drop, where oxygen is abundant, and also the central
region, which contains no oxygen. On the fluid being renewed, however, the
filaments retreat more and more to the centre of the preparation as the
sulphuretted hydrogen gets gradually used up. When a Beggiatoa filament
has found the region of optimum oxygen tension, it is able by small changes of
position to reach zones either where the sulphuretted hydrogen is abundantly
present, or where oxidation of that gas can take place.

The behaviour of Beggiatoa in nature conforms to that which it exhibits
in a microscopic culture. In a mud swamp, just as in the culture, it always
attempts to find a region with an optimum oxygen tension, as it lives on the
surface of the culture, inhabiting places subject to overflow and avoiding
deeper hollows. Hence both oxygen and sulphuretted hydrogen play a part
in determining its distribution, since a definite and not excessive concentra-
tion of that substance is essential to its existence.

But Beggiatoa not only accumulates sulphur in its cells, but also dissolves
it out, and both processes may go on simultaneously, though it is not
possible to note this fact directly. The removal of sulphur from the cells
can only be determined if formation of the substance is prevented by
withdrawal of sulphuretted hydrogen. The amount of sulphur removed in
this way is enormous. When WINOGRADSKY supplied an active culture
every two or three hours during one day with fresh supplies of Langenbriicken
water containing sulphuretted hydrogen, the filaments became full of sulphur
by evening (Fig. 39, a), and after the supply of sulphuretted hydrogen
had ceased, the whole of the sulphur accumulated became dissolved out in
twelve to fifteen hours. Fig. 39, b, shows the filaments after remaining for
twenty-four hours without any sulphuretted hydrogen, while Fig. 39, c,
represents the same culture after an interval of forty-eight hours. According
to WINOGRADSKY' s estimates, the protoplasm of one cell uses up daily an
amount of sulphur equal to four or more times its own weight. Such quantities
as these render it impossible that the sulphur is devoted to the formation of
proteid or the synthesis of any other substances, since Beggiatoa grows relatively
slowly, only rarely doubling the length of its filaments in twenty-four hours.
As a matter of fact, it can be shown that the sulphur subserves an entirely
different purpose ; it is oxidized in the cell, and the sulphuric acid so formed
in turn attacks the carbonates taken into the cell from the water, and passes
back again to the medium in the form of calcium sulphate. Beggiatoa oxidizes
the sulphuretted hydrogen into sulphuric acid, and to some extent accumu-
lates the intermediate product, sulphur, as a reserve. If the supply of sul-
phuretted hydrogen be limited by offering to the plant extremely dilute solu-
tions, it is possible to grow these Bacteria without their showing any accumula-
tion of sulphur in their cells. Just as the formation of sulphur from sulphuretted
hydrogen may be diminished, so also may the formation of sulphuric acid
from sulphur, without the aid of the Bacteria ; while, however, the former
process appears to go on in the cells of Beggiatoa about as actively as in water,
the organism has obviously the means of greatly accelerating the formation
of sulphuric acid (possibly by means of an enzyme ?).

The oxidizing process above described, which, as we have said is absolutely
essential to the existence of Beggiatoa, is one which is eminently characteristic of
this organism, but which is absent in the vast majority of cases. Yet it is not the
only peculiarity of this remarkable plant. It possesses neither chlorophyll nor
any other allied colouring matter which would lead us to imagine that it was

SULPHUR AND NITROGEN BACTERIA

223

autotrophic ; it has always been considered as a heterotrophic form. In
WINOGRADSKY'S micro-cultures in which vigorous increase took place, the
Langenbriicken mineral water was the only nutrient supplied, and that fluid
contains nitrogen only as traces of ammonia and nitric acid, and only 0-0005 per
cent, of organic materials, i. e. in infinitesimal quantity. These are sufficient, as
we have seen, to support both life and growth, although they are qualitatively
not of a kind one would reckon as of nutritive value. According to FRESENIUS'S
investigations, the nutrients must, in part at least, consist of formic and pro-
pionic acids. When WINOGRADSKY employed solutions containing sugar,
peptone, asparagin, &c., he was unable to obtain as good cultures of Beggiatoa
as in Langenbriicken water, indeed, for the most part, the plants rapidly
succumbed to such treatment.

The general conclusion which WINOGRADSKY arrived at, as based on his
experiments on Beggiatoa, is as follows : — The oxidation of sulphuretted
hydrogen to sulphuric acid is a process in the course of which energy is set
free, and in Beggiatoa this energy takes the place of that released normally in
respiration. While ordinary organisms must devote organic substance, or
even part of their own bodies to respiratory purposes, Beggiatoa respires sul-
phuretted hydrogen, and thus saves its organic constituents. It is thus con-
ceivable that it makes such moderate demands on nutrients, both in quality
and quantity, that it employs these only for constructing its body and not
as a means of supporting vital processes (p. 229). WINOGRADSKY does
not deny the possibility of the existence of a normal respiration taking place
in addition to the oxidation of sulphuretted hydrogen, but he does not consider
it probable.

To Beggiatoa are related on the one hand the colourless sulphur-bacteria
(species of Thiothrix), which are precisely like it in all essentials (though WILLE,
1902, Biolpg-. Centrbl. 22, 257, has recently denied the occurrence of sulphur
in Thiothrix ; compare MOLISCH, 1903, Bot. Ztg. 61, 57), on the other hand,
there is a large number of so-called red sulphur-bacteria (Fig. 39, d, e) which
physiological research has shown to exhibit important differences from Beg-
giatoa, although no satisfactory conclusions have as yet been reached with
regard to them. They are distinguished in the first place by possessing a red
colouring matter (bacterio-purpurin) of varying tint, but its characters so
far as they are known afford no indication of its physiological significance.
These Bacteria are further distinguished from Beggiatoa by their mode of
occurrence, since they prefer water containing large amounts of sulphuretted
hydrogen, and are not injured even when that compound is present in a con-
centrated condition. They are, apparently at least, anaerobic, and avoid
situations where oxygen is abundant. Finally, they prefer light to darkness
and move towards it, whilst Beggiatoa avoids light. In common with the
latter they make use of sulphuretted hydrogen. Since they live in concen-
trated solutions of this gas, the question as to how they are able to obtain the
necessary oxygen must not be overlooked. According to WINOGRADSKY
(1888, b), they always live in association with other micro-organisms which are
provided with chlorophyll, and which therefore can decompose carbon-dioxide
and give off oxygen. The red sulphur-bacteria then absorb the traces of
oxygen present, and use it for oxidizing sulphuretted hydrogen. WINO-
GRADSKY, in fact, was able to cultivate red sulphur-bacteria only when he
associated them with green forms of this kind.

ENGELMANN (1888) offers another explanation of this phenomenon. He
has proved by means of the bacterium method previously described (p. 105)
that the red sulphur-bacteria decompose carbon-dioxide with the aid of light,
and especially of the ultra-red rays, and employ the oxygen so obtained in
oxidizing sulphuretted hydrogen. This view of the phenomenon may be
preferable, inasmuch as it gives a special function to the presence of bacterio-

224 METABOLISM

purpurin, making it a substitute for chlorophyll. Further, the need for sun-
light in the assimilatory activity of the sulphur-bacteria is thus explained,
while according to WINOGRADSKY'S view, its significance is only indirect. It
is to be noted, however, that ENGELMANN'S view is by no means established,
and that WINOGRADSKY'S criticisms (1888) still remain unanswered. It is to
be hoped that these interesting problems may soon receive a final investigation
and explanation.

WINOGRADSKY believed that another biological group of Bacteria should
be associated with the sulphur-bacteria, to which he gave the general name
of iron-bacteria. They are able to turn iron protoxide into iron oxide, and to
benefit by means of this oxidation just as the sulphur-bacteria do from the
oxidation of sulphuretted hydrogen. Unfortunately WINOGRADSKY has not
followed up his short preliminary note by a detailed treatise, and meanwhile
MOLISCH has failed to confirm his results ; hence at present we can present
no definite data on the subject of iron-bacteria. [Compare also RULLMANN,
1904.] For our present purpose, however, this is of little consequence, since
iron-bacteria play by no means so important a part in nature as do the
sulphur-bacteria. These latter work for the benefit of the higher plants, inas-
much as they enable the sulphuretted hydrogen formed in the process of
putrefaction or otherwise to become once more available for the nutrition of
the green plant.

We saw at the conclusion of the preceding lecture, that in addition to sulphur,
another even more important element was similarly transformed in the process
of putrefaction into a condition in which it was of no service to the higher
plant. We found that free nitrogen and ammonia were among the final pro-
ducts of putrefaction, and that the nitrogen was never used by the green plant,
and the ammonia much less frequently than nitrate. We have yet to answer
the question whether these materials, like sulphuretted hydrogen, undergo
alteration by the activity of definite micro-organisms, so that they become
once more available as food, and we will first deal with the question as far as
regards ammonia. We may note, to begin with, that all the ammonia does
not arise from the fermentation of proteid, but that there are many other
important sources of this substance.

Only rarely are katastates containing nitrogen formed in the plant which are
unable to undergo further elaboration (compare p. 200, the formation of ammonia
by Hyphomycetes) ; on the other hand, animals regularly give off nitrogen,
especially in urine, which contains it in abundance in the form of urea, uric acid,
and hippuric acid. It has long been known that these substances form un-
suitable sources of nitrogen for autotrophic plants, and it is all the more im-
portant to know that they may undergo alteration in the soil into nutrients
that are of value. In these processes micro-organisms also play a great part.
The best known is the alteration of urea into carbonate of ammonia, a process
which has often been termed ' urea fermentation '. This takes place according
to the formula : —

CO(NHa)2 + a H,O - COS(NH4)2

the process being thus a simple case of hydration, such as accompanies the
action of many enzymes, but without any splitting. If we limit our conception
of fermentation to such processes as result in a gain of energy, then the forma-
tion of carbonate of ammonia must be excluded from fermentative actions.
If, however, we consider the hypothesis referred to at p. 213, we may speak here
also of fermentation, since in all probability the significance of the process
in the organism is biological only. We can at least suggest that the markedly
alkaline reaction acts in the same way as acids and alcohol do (i. e. as a poison)
in preventing the presence of other concurrent organisms, since it is a fact

SULPHUR AND NITROGEN BACTERIA

225

that many organisms are injured by carbonate of ammonia, even in minute
doses. At all events, any further elaboration of the ammonia is unknown and
improbable, more especially it must not be assumed that ammonia arises from
the urea in the same way as it does from a pure peptone nutrient. Since
urea does not act as a source of carbon to uro-bacteria they cannot thrive on
it alone (BEIJERINCK, 1901) ; it supplies the nitrogen need only. As far as their
carbon requirements are concerned the individual forms behave very differently;
species which get on in acetic or oxalic acid are least exacting, but these form
only small quantities of carbonate of ammonia ; those which grow in tartaric
acid produce larger quantities, and those which live in malic acid still more.
The greatest amount of ammonia is formed by Urobacillus pasteurii and
Urococcus ureae, which use bouillon as a source of carbon, for in a thin sowing
they completely transform 10-12 g. of urea in 100 g. of fluid in the course of
a few days. The immediate factor in the formation of ammonia is an enzyme,
urease, as to whose occurrence or absence many contradictory statements have
been made. As the result of BEIJERINCK' s recent work, its existence can no longer
be doubted ; for this author was able to prove that uro-bacteria killed by the
action of chloroform acted as effectively on urea as living forms. He also estab-
lished the fact that the urease cannot diffuse out of the cells, so that previous state-
ments as to the solubility of urease are to be explained by assuming that some
minute Bacteria in the fluid had been overlooked. [MiQUEL (1904) holds that the
view taken as to the solubility of urease is correct.] Ammonia also arises
from uric acid ; we need not trace the fate of hippuric acid at present.

Ammonia, in whatever way it arises, is, as we have already said, absorbed
and retained in the soil, and undergoes, as we shall see, change into nitrous
and nitric acids. This process, known as nitrification, taking place everywhere
in arable soil, was previously considered as a simple oxidation due to inorganic
agencies. The observations of SCHLOSSING and MUNTZ (1877-1879) on the
dependence of nitrification on external conditions, and especially the effect
of temperature and anaesthetics on them, could only be explained by assum-
ing the action of lower organisms. From such a conception of the process
to the certain isolation of the operative Bacteria was a further and more dim-
cult step. Several authors attempted, with the aid of the usual bacteriologi-
cal methods (nutrient gelatine), to isolate the nitro-bacteria from the soil,
and not infrequently they succeeded in obtaining pure cultures, to which
they ascribed the power of inducing nitrification. But nitrification took place
within such circumscribed limits that it was impossible to avoid the suspicion
that the nitrate reaction obtained from the cultures was due less to the activity
of Bacteria than to the absorption of nitrates from the air. Nitrates occur
abundantly in the air, especially of laboratories, and it is well known that
nitrates are vigorously absorbed by alkaline fluids (BAUMANN, 1888).

The labours of WARINGTON (1888) and FRANKLAND (1889) have resulted
in an advance in our knowledge of this subject ; but it is due to WINO-
GRADSKY (1890, 1891) that the physiology of the nitro-bacteria has been
ultimately cleared up, and his work has the right to be considered as
one of the most important discoveries in physiology. It is essentially WINO-
GRADSKY'S description that we shall follow in our account of the phenomenon.

[WlNOGRADSKY, 1904.]

His experiences in dealing with the sulphur- and iron-bacteria had trained
this investigator perfectly for the study of the nitro-bacteria. He'had in those
cases to deal with highly characteristic types of organism, distinguished from
the mass of Bacteria by their varying behaviour to organic food materials.
The methods of isolation and culture of technical bacteriology which he had
employed in his experiments had completely failed ; might not these failures in
the case of the nitro-bacteria be explained by the fact that these organisms require

JOST Q

226 METABOLISM

special conditions of investigation, and cannot be treated in accordance with
ordinary methods ?

WINOGRADSKY started from the idea that nitro-bacteria, like sulphur-
bacteria, were injured by good organic nutrients, and therefore he attempted
to cultivate them first of all in a solution which, in addition to the necessary
minerals contained only potassium tartrate to serve as the source of carbon,
and ammonium chloride as the source of nitrogen and as nitrification material.
When small traces of natural soil, in which nitrification was known to occur,
were added to this fluid the results desired were not forthcoming, even
although all sorts of changes were made in the concentration of the nutrient
fluid. Since, however, the observations of HERAEUS (1886) had proved that
organic bodies had little effect, the nutritive solution previously used was
prepared without potassium tartrate. The result was in the highest degree
remarkable ; a vigorous nitrification at once began in the fluid, and hence the
line of further research was clearly indicated. After it had been shown that
a carbonate of an alkaline earth had a favourable influence, the following
nutritive solution was always used : —

Water (ZQrich Lake) looog.
Ammonium sulphate i g.

Potassium phosphate i g.

Basic magnesium carbonate S-Jog-

If 100 cc. of this solution after sterilization be impregnated with the smallest
possible drop of an old culture of a similar kind, a strong potassium nitrate
reaction may be obtained in the course of a few days, and in a fortnight all the
ammonia contained in the flask is found to have been transformed, while scarcely
any change can be distinguished in uninoculated control solutions. In such a cul-
ture very many typesof Bacteria and other micro-organisms occur, whose number
is reduced by repeated transference to similarly constituted nutritive solutions,
but finally it was found no longer possible to reduce in this way the number of
types present. Five organisms were still found in the thin scum which was
obvious on the surface of the fluid, and among these WINOGRADSKY sought for
the assumed organism which excited nitrification and absorbed oxygen greedily.
Each of these five organisms was investigated separately, but none of them were
found to be the cause of nitrification. The real agent was finally discovered in
another part of the culture, viz. on the sediment formed from the magnesium
carbonate, in the form of a bacterial zoogloea derived from motile oval Bacteria,
which for a short time at the beginning swarmed in the fluid. After the signifi-
cance of this bacterium was established, WINOGRADSKY had still the greatest
difficulty in obtaining a pure culture of it. Although his studies are extra-
ordinarily rich in lessons on methods, we will not enter into them further at
present, but rather confine our attention to the results he obtained with the
use of pure cultures, and study first of all nitrification itself, and then the
peculiar behaviour of the bacterium in relation to cultivation with carbon.

In the course of his studies, WINOGRADSKY soon noted that nitrification
was actively advanced if ammonia was added only in small quantity, and at
once replaced when used up. Consequently, he invariably added to his cultures
only 0-04-0-1 g. of ammonium sulphate at one time, and he was able to ob-
tain a very observable nitrification after the second addition. One culture, for
example, oxidized 860 mg. of ammonium sulphate in thirty-seven days, another
930 mg. in thirty days, i.e. on an average 4-93 to 6-6 mg. of nitrogen were nitrified
per day. What was surprising in these cultures was that the whole of the nitrogen
was not oxidized into nitrate, but that a part, variable in quantity, was always
changed into nitrite. G. and P. FRANKLAND (1890) also observed the formation of
nitrite in their cultures, while in the soil all the ammonia is altered into nitrate.
WINOGRADSKY was at first disposed to regard the formation of nitrite as

SULPHUR AND NITROGEN BACTERIA 227

a result of unfavourable culture conditions, and endeavoured to aid the
access of air by employing larger and more extensive layers of fluid. He
obtained indeed a very marked increase in nitrification (to as much as 22-7 mg.
of nitrogen per day) on spreading a culture, which had previously oxidized
9 mg. of nitrogen, over a surface four times as great in extent. But the effect
he expected did not take place, for instead of a decrease he obtained an increase
in the amount of nitrite. The cause of the formation of nitrate and nitrite
obviously lay deeper, and WINOGRADSKY was soon able to prove that
the formation of nitrite takes place first, and that only after all the ammonia
is used up a further oxidation of the nitrite to nitrate follows.

The next question to be answered was, did one and the same organism
carry out the formation both of the nitrite and of the nitrate — very much as,
according to many authors, the acetic acid-bacteria first turn alcohol into
acetic acid, and then respire the acetic acid — or was there a distinct organism
concerned in each of the two stages, one forming nitrite, the other nitrate ?

There were certain indications that in all probability the latter view
might be the correct one, because when a new culture was taken from one in
the stage of most active nitrite formation, only nitrite formation went on in
it. It was certainly possible that the organism had become altered, and had lost
its power of forming nitrate, but it appeared more probable that by a lucky
accident only the nitrite organism had been transferred to the new culture,
and that in the previous experiments at least
two organisms were operating one after the <»
other, by whose combined action the transforma- • ^
tion of ammonia into nitrate was effected. In his f <
•experiments WINOGRADSKY had now advanced
conclusive evidence of the existence of two *•
kinds of nitro-bacteria, one of which constructed
nitrites the other nitrates only. Both kept
to their specific functions, the one operating on Fig.40< Nitro.bacteria : a, from Zurich;
ammonia, the other on nitrite ; other kinds of *, from Java ; c, from Quito, x 1000.
nitrogenous bodies, such as urea, asparagin, pro- Aftcr FISCHER (Vorles' a> Bakt" *nd ed)-
teid, &c. (OMELIANSKI, 1899), cannot be nitrified at all, nor can the nitrate
organism oxidize either phosphorous or sulphurous acids (OMELIANSKI, 1902).

Only a few words need be said as to the morphology of these organisms
{WINOGRADSKY, 1892). The nitrite organism is apparently similar in appear-
ance in all European countries, viz. an oval, at some period motile, bacterium (Fig.
40, «). Those obtained from Java and other non-European countries resemble
the European type (Fig. 40, b), but in America other forms, related to the coccus
have been found. The nitrate-bacteria, so far as is known, are short rods (Fig. 40, c).

The nitrite- and nitrate-bacteria, as we have seen, behave very differently in
relation to the nitrogenous compounds which they can oxidize, but quite similarly
in relation to carbon. Both in fact not only require no organic carbon, but are
actually injured by it. Whence then do they obtain their carbon ? That is the
important question we have yet to answer. In his first experiments, in which
simple mineral solutions without any organic additions were used, WINOGRADSKY
took special care that all the materials used, the culture and vessels and fluids,
were absolutely free from organic impurities, and that no such substances
were permitted to enter from the air. In these solutions the nitro-bacteria
oxidized the nitrogen, and grew so markedly that he was able to determine
quantitatively and directly their gain in organic substance. Thus he found
that in four cultures carried on during about three months the organically
•combined carbon amounted to (in mg.) : —

No. ii. No. 12. No. 26. No. 30.

19-7 15-2 26-4 22.4

Q2

228 METABOLISM

The amount of organic carbon introduced along with the nitre-bacteria
was immeasurably small, so that practically all that was obtained must have
been produced in the culture.

This very small but absolutely certain increase of organic substance must
have taken place at the cost either of the carbon-dioxide of the air or of the
carbonate in the solution. As GODLEWSKI showed (1895), this latter source is
insufficient, and nitrification does not go on when the carbon-dioxide of the
air is shut out. The carbonate further can only be conceived of as a source
of carbon-dioxide to the m'tfnfc-bacteria, since the nitrous acid which appears
must of course decompose the carbonate ; in the further oxidation of the
nitrite, however, no carbon-dioxide would come off free.

WINOGRADSKY' s quantitative analyses have also proved what had already
been suspected by HERAEUS (1886) and HUPPE (1887), tnat tne nitro-bacteria
are able to form organic materials out of carbon-dioxide ; in other words, they are
in this respect, like green plants, autotrophic. We have already seen, that the
formation of organic material from carbon-dioxide is essentially associated
with expenditure of energy, and that the sunlight provides this energy for
carbon assimilation in the case of the green plant. The case is different with
the nitro-bacteria, for they assimilate the carbon-dioxide in the dark, if only they
be provided with ammonia or nitrite, which they oxidize with the aid of oxygen.
It would appear that the energy obtained by the oxidation of ammonia takes
the place of the energy of sunlight in the case of the green plant, and hence it
becomes explicable why WINOGRADSKY found a definite relation subsisting
between the amount of organic substances formed and the amount of ammonia
oxidized. On an average 35*4 mg. of nitrogen must be oxidized for every mg.
of organically combined carbon formed. The individual determinations vary
but little from this average : —

Nitrogen oxidized 722-0 506-1 928-3 815-4

Carbon assimilated 19-7 15-2 26-4 22-4

Proportion 33-6 33-3 35-2 36-4

These statements of WINOGRADSKY date from the time when he was
unacquainted with the differences between the nitrite- and nitrate -bacteria,
and the results he obtained were put down to their collective activity. He
would probably have considerably modified his views had he experimented with
pure cultures of one or the other, for they exhibit great differences in their capaci-
ties for oxidizing nitrogen. In a culture of the nitrite-bacterium the daily amount
of nitrogen oxidized gradually rises from 3-0 mg. on the fifth day, to 20 mg.
four weeks later, while the most energetic nitrate organism is capable of oxidiz-
ing not more than 10 mg. of nitrogen per day. It is only natural that the
energy, not only of oxidation but also of assimilation, in the two forms should
be quite different.

Further investigations are yet needed to elucidate fully the whole problem.
For example, we are as yet quite in the dark how carbon assimilation is effected,,
and what is the first product of assimilation. It is by no means essential
that the process should be carried out in the same way as in the green plant,
viz. by the decomposition of carbon- dioxide and the evolution of oxygen.
WINOGRADSKY, in fact, has advanced arguments against the possibility of
such a method. He observed that if oxygen is given off in this assimilation
process, it must be capable of maintaining nitrification just as well as respira-
tion in the green plant may be maintained by the oxygen released in carbon
assimilation. WINOGRADSKY, however, did not observe the quantitative
relation existing between the nitrogen oxidation and the carbon assimilation
in the nitro-bacteria, which differs entirely from that between respiration
and assimilation in the green plant ; in the case of the nitro-bacteria the

SULPHUR AND NITROGEN BACTERIA 229

oxygen tormed in assimilation is quite insufficient to supply what is wanted
for nitrification, while in a green plant assimilation results in the formation of
far more oxygen than can be used in respiration. It is possible that a splitting
off of oxygen from carbon-dioxide and a formation of carbohydrate does
occur in the nitro-bacteria, but, according to WINOGRADSKY, urea may also be
formed directly by the union of carbon-dioxide and ammonia, from which
might arise the other organic compounds found in nitro-bacteria. The be-
haviour of the nitro-bacteria in presence of urea, however, does not support
this hypothesis.

Respiration forms another important problem which still requires
elucidation. Do the nitro-bacteria remain content with oxidizing ammonia
into nitrous acid, or do they use up organic material they themselves have
manufactured ? This cannot be easily answered, but it is of interest in relation
to our general summary of respiration. PFLUGER and DETMER'S conception of
respiration as taking place in the protoplasm, that it is protoplasm that
undergoes respiration, and that the reserves are used to regenerate it has
already been discussed (p. 204). This regeneration can be effected by carbo-
hydrate, but not by ammonia. Were it possible to prove that the nitro-bacteria
respired no organic material, this hypothesis of PFLUGER would be definitely
established, but it is scarcely possible to bring forward such evidence
although it may perhaps be possible to do so in the case of other organisms.

Having now discussed the nitro-bacteria, the relations established for
the colourless sulphur-bacteria appear in altogether a new light. Not
only are we able to note a perfect analogy between the respiration
of ammonia on the one hand and sulphuretted hydrogen on the other, but
also the evil effects of supplying organic nutrients to Beggiatoa. It is in
the highest degree probable (WINOGRADSKY, 1890, p. 275) that both sulphur-
and iron-bacteria are autotrophic, and that they grow better when organic
materials are completely excluded than when they are present. One
wonders why this experiment has not long since been carried out. Then the
energy which is obtained by the oxidation of inorganic substances in the case
of sulphur- and iron-bacteria is perhaps used up only in the assimilation of
carbon, and just as in the case of the nitro-bacteria we have to inquire whether
or not a respiration of organic substance takes place here also.

Recently NATHANSOHN (1902) has published some observations on a new
group of sulphur-bacteria. Details as to the occurrence of these forms are
as yet entirely wanting, and studies on their physiological behaviour require
confirmation also. In consequence of their great importance we may add here
a few words on the subject of these Bacteria, without waiting for a confirma-
tion of NATHANSOHN'S work [OMELIANSKI, 1904]. They do not oxidize
sulphuretted hydrogen, but thiosulphate, and form therefrom sulphuric acid
and tetrathionic acid. Further, they are not sensitive, like nitro-bacteria and
Beggiatoa, to the addition of organic material, and hence it is possible to prove
that they are entirely unable to oxidize organic substances, e. g. sugar ; they pro-
duce no carbon-dioxide, and have no normal respiration. On the contrary,
carbon-dioxide is an essential food-stuff, since they form organic substance
from it. If NATHANSOHN'S experiments are correct, then these forms afford
proof of the fact that there are organisms which respire inorganic material only.

Let us now return to the nitro-bacteria. The formation of organic material
out of carbon-dioxide is not the final point of interest in their physiology.
It is of importance also to note their behaviour to such organic compounds as
are presented to them from without. As we have already remarked, the
culture of these microbes was retarded in WINOGRADSKY' s experiments by the
addition of potassium tartrate and gelatine. A short time previously WINO-
GRADSKY, in conjunction with OMELIANSKI (1899), investigated the effect of

330 METABOLISM

organic substances in the nitro-bacteria in an exhaustive manner, and their
results are summarized (in percentages) in the following table : —

Nitrite formers. Nitrate formers.

Glucose 0-025-0.05 02 0-05 0-2-0-3

Peptone 0-025 0-2 0-8 1-25

Asparagin 0-025 °-3 0-005 0.5-1-0

Glycerine >o-2 0-05 >i-o

Urea >o-2 0-5 >r-o

Sodium acetate 0-5 >i-5 x«5 3-0

Sodium butyrate 0-5 >i-s 0-5 i-o

Meat extract 10-0 20-40 10-0 60-0

Ammonia 0*0005 0-015

In the first column of each series are given the lowest percentages which
accelerate development, and in the second the doses which retard it. The
symbol > signifies ' more ', but not much more, than the dose following.

We may deduce several important conclusions from this table.

1. The different organic substances are by no means equally valuable to the
nitro-bacteria, but may operate directly as antiseptics. This antiseptic effect is
not less than that of carbolic or salicylic acids in the case of ordinary Bacteria. The
nitro-bacteria are much more autotrophic than green plants ; owing to the fact that
the latter are at least facultatively heterotrophic, they may get on with organic
substances supplied to them from without. We might certainly suppose that the
same was true of the nitro-bacteria, if the first assimilation product were known.

2. The very substances which form the best nutrients for ordinary Bacteria
and for heterotrophic plants inhibit nitrification most.

3. The nitrite-bacterium is much more sensitive to organic substances
than the nitrate-bacterium. On the other hand, the nitrate-bacterium is
astonishingly sensitive to ammonia, and that substance is more antiseptic to
it than is corrosive sublimate to other organisms.

All these facts are of significance, not only because they disclose to us
the remarkable differences and adaptations which occur in the organic world,
but also because they explain the role of nitrification in nature. The sensi-
tivity of the nitrite-bacterium to organic substances carries with it the con-
clusion that its development can commence only if all the organic materials
which are present in the soil (dead animals and plants, excrement, &c.) have
been completely decomposed by ordinary putrefactive organisms, so that
the carbon is present as carbon-dioxide, the nitrogen as ammonia, or even as
an element. The nitrate microbe is less sensitive to organic compounds, but
its development is inhibited by ammonia, and it can develop only after the
nitrite organism has operated first. The question now is whether the sharp
demarcation of nitrification from fermentation and putrefaction is of signi-
ficance in organic nature ; the answer to this is undoubtedly in the affirma-
tive. In many of the commonest fermentative processes, potassium nitrate
is reduced, whereby not only nitrite, but especially free nitrogen is formed
in large quantity. If nitrification sets in before the completion of fermentation,
the nitrates formed would be denitrified by these ferments instead of being avail-
able for the nutrition of the green plant. Such a denitrification does occur under
certain conditions, as we shall see in the next lecture. There we shall take
the opportunity of studying the combining of free nitrogen by organisms in
conjunction with its formation.

At the present moment we may note that the nitro-bacteria do not confine
their activity to arable soil where ammonia is presented to them in the manure,
but that they establish themselves on bare rock containing lime, and make
use of the traces of ammonia brought down by rain. They then decompose
the lime, and thereby render this mineral available for the higher plant as well
as by forming nitric acid.

SULPHUR AND NITROGEN BACTERIA 231

Bibliography to Lecture XVIII.

BAUMANN. 1888. Landw. Versuchsstationen, 35, 217.

BEIJERINCK. 1895. Centrbl. Bakt. II, i, i.

BEIJERINCK. 1900. Ibid. II, 6, 193.

BEIJERINCK. 1901. Ibid. II, 7, 33.

[DELDEN. 1903. Centrbl. Bakt. II, 9, 81.]

ENGELMANN, TH. W. 1888. PFLUGER'S Archiv, 42, 183.

FRANKLAND, G. and P. 1889. Zeit. f. Hygiene, 6, 373.

FRANKLAND, G. and P. 1890. Phil. Trans. Roy. Soc. B, 181, 107.

GODLEWSKI. 1895. Anzeiger d. Akad. Krakau.

HERAEUS. 1886. Zeit. f. Hygiene, i, 193.

HOPPE-SEYLER. 1 8 86. Zeit. f. phys. Chem. 10, 201.

HUPPE. 1887. Biolog. Centrbl. 7, 701.

[MIQUEL. 1904. In LAFAR'S Handb. d. techn. Mykologie, 3, 82.]

MOLISCH. 1892. Die Pflanze und ihre Beziehungen zum Eisen. Jena.

NATHANSOHN. 1902. Mitt. d. zoolog. Station Neapel, 15, 655.

OMELIANSKI. 1899. Centrbl. Bakt. II, 5, 473.

OMELIANSKI. 1902. Ibid. II, 9, 63.

[OMELIANSKI. 1904. LAFAR'S Handb. d. techn. Mykologie, 3, 214 and 234.]

[RULLMANN. 1904. In LAFAR'S Handb. d. techn. Mykologie, 3, 193.]

SCHLOSSING and MUNTZ. 1877-79. Compt. rend. Paris, 84, 301 ; 85, 1018 ; 86,

892; 89, 891, 1074.

WARINGTON. 1888. Centrbl. Bakt. 6, 498.

WINOGRADSKY. 1887. Ueber Schwefelbaktcrien, Bot. Ztg. 54, 493.
WINOGRADSKY. 1 888 a. Bot. Ztg. 46, 261.
WINOGRADSKY. 1888 b. Beitr. z. Morphologic und Physiologic der Bakterien, I;

Schwefelbakterien. Leipzig.
WINOGRADSKY. 1890-91. Recherches sur les organismes de la nitrification. An-

nales Institut Pasteur, 1890 : Mitt. I : 4, 213-231 ; Mitt. II : 4, 257-275 ;

Mitt. Ill : 4, 760-771 ; 1891 : Mitt. IV : 5, 92-100 ; Mitt. V : 5, 577-616.
WINOGRADSKY. 1892. Contrib. a la morphologic des organismes de la nitrification

(Archives d. Sc. biolog. St. Pdtersbourg i ).

[WINOGRADSKY. 1904. LAFAR'S Handb. d. techn. Mykologie, 3, 132.]
WINOGRADSKY and OMELIANSKI. 1899. Centrbl. Bakt. II, 5, 329.

LECTURE XIX

DENITRIFICATION AND NITROGEN FIXATION. SYMBIOSIS AND
METABIOSIS. CIRCULATION OF CARBON AND NITROGEN.

WE have already drawn attention on several occasions to the problems
connected with the circulation of nitrogen in the organic world. In Lecture XI
we established the fact that the green plant supported itself in the first instance
on nitric acid, and constructed proteid out of that substance ; in Lecture XVII
we found that on the death of the green plant the proteid was broken down
by micro-organisms in such a way that the nitrogen was finally transformed
for the most part into ammonia. When, owing to the combined activity of
nitrite- and nitrate-bacteria, the ammonia is changed again into nitric acid,
the circle is completed, and the nitrogen once more appears in a form which
green organisms can appropriate. The cycle is not, however, quite so simple
as this as regards all the nitrogen ; a complication appears when in certain
processes gaseous nitrogen is formed, and when in other cases free nitrogen may
be seen to undergo transformation into a combined form. Reference has been
already made to these processes, but some of them require further explanation.

We have already seen (p. 219) that free nitrogen as well as ammonia may
be produced during the decomposition of proteid. The conditions under
which this takes place have not as yet been fully determined, but the process

232 METABOLISM

of denitrification has received considerable attention. In spite of a voluminous
literature on the subject (compare LEMMERMANN, 1901, [JENSEN, 1904]) the
phenomena of denitrification are in several important aspects still obscure.
We speak of denitrification when nitrate is transformed into nitrite, nitrite
into ammonia, or finally nitrate or nitrite into free nitrogen. Since we have
seen that nitrification, that is, the transformation of ammonia into nitric
acid, is a source of energy to the organism, the reverse process can take place
only by the expenditure of energy, as when free nitrogen is formed from nitrate
or nitrite. Therefore denitrification obviously cannot be made to rank along-
side of fermentation where gain of energy is the significant point. A certain
likeness between fermentation, or, more accurately, respiration, and nitrification,
is perhaps, however, observable. According to JENSEN (1898, 1899) denitrification
occurs only in the absence of oxygen, and the denitrifying Bacteria are anaerobic
in the presence of nitrates ; when no nitrates, on the other hand, are present they
are strongly aerobic. The significance of denitrification rests solely in the fact
that there is gain of respiratory oxygen from the nitrates, and this view is
supported by an observation of MAASSEN (1901), that substances rich in
oxygen, such as chlorates, are able to inhibit the decomposition of nitric
acid. Probably these chlorates act in the place of the nitrate, giving up their
oxygen, and thus protecting, so to speak, the nitrates. Still various investigators
have shown that denitrification is also possible in the presence of oxygen, and this
fact does not at first sight appear to support the conception of the process just
advanced. When we remember, however, that yeast is also incapable of
developing alcohol when oxygen is abundantly present, we must admit the
possibility of the existence of Bacteria which habitually split off oxygen from
nitrogenous compounds, even if it be at their disposal in a free state. It may
be concluded from MAASSEN'S researches that certain Bacteria are always speci-
fically denitrifiers, while others develop such powers only under definite external
conditions. The number of the former type is apparently limited, while the
power of occasionally inducing denitrification appears to be widely distributed.

Denitrification is naturally a process of fundamental interest and impor-
tance to the agriculturist. In agricultural operations, as we have seen already,
large quantities of nitrogen are removed from the land in harvesting, and
manuring with nitrogen becomes one of the most necessary conditions of suc-
cessful agriculture. If this nitrogen be presented in the form of nitrate of potash
denitrification must be most carefully guarded against. It is not necessary
for us, however, to go into this question more fully ; we need only remark that
the land would indeed be in a parlous state, were there no process in nature
by which denitrification could be compensated for. The reconstruction of
nitric acid out of ammonia and nitrites we have already studied in the previous
lecture, but the free nitrogen of the air, which, owing to its chemical inertness,
is of no service to the organism (compare BUNGE, 1889) is by no means ex-
cluded from taking part in the circulation of material in living nature. The
combination of free nitrogen has been conclusively proved, and it is both
practically and theoretically a phenomenon of the very greatest importance.

Recently J. KUHN (1901) has provided us with a very interesting proof
of nitrogen combination in arable land. He was able to obtain out of a certain
field good and even increasing harvests after being sown for twenty successive
years with winter rye, without any nitrogenous manuring whatever. This
showed that more nitrogen was annually combined in the soil than was removed
in the process of harvesting ; and since the rye is itself incapable of bringing
about such a combination, and since further the amount of combined nitrogen
precipitated on the soil nothing like makes up for the loss by harvesting (com-
pare p. 136), obviously atmospheric nitrogen must have been combined to
a very large extent in the soil. We have again to thank WINOGRADSKY for a

DENITRIF1CA TION AND NITROGEN FIX A TION 233

very thorough elucidation of the details of the part played by certain Bacteria
in this process, BERTHELOT (1892) having previously shown that nitrogen
combination must be due to the activity of Bacteria.

WINOGRADSKY (1895) made use of the experience he had gained in culti-
vating sulphur- and nitro-bacteria, and began his studies on the organism
which combined nitrogen by preparing a nutritive solution, which, in addition
to dextrose, contained the usual salts, save that no nitrogen compounds were
present. He hoped by this ' selective ' mode of culture to provide the organism
he was in search of with all the conditions it required, and so isolate it, by
shutting out ordinary soil -bacteria which were unable to grow save in the
presence of combined nitrogen. His expectations were not disappointed.
After the culture fluid had been placed at the bottom of a glass jar and inocu-
lated with a small quantity of arable soil, a vigorous formation of butyric
acid soon began to take place, and irregularly spherical masses of zoogloeae made
their appearance. When the acid was neutralized, the fermentation continued
without intermission until the whole of the sugar was used up. Apart from the
products of fermentation, the fluid had undergone essential alterations, as was
shown by the fact that when fermentation had finished, Fungi appeared on the
zoogloeae, and that after these had destroyed the butyric acid, Algae introduced
themselves. None of these
organisms could exist in the
original fluid, seeing that it
was destitute of combined
nitrogen ; their appearance
after fermentation demon-
strated the fact that com-
bined nitrogen was present

afterwards, and Chemical ana- Fig. 41. Closiridium pasteurianum. /, Vegetative rods.

IVSis Confirmed this. 2> Sp°r<>genous spindle-shaped rods. ?, Burst spindles with spores.

ir- i . • 4> Germinating; spores. (After WINOGRADSKY.) From FISCHER

MlCrOSCOplCal investlga- (Vorles. u. Bakt. and ed.).

tion of the zoogloeae disclosed

the presence of two filamentous Bacteria, and a species of Clostridium (i. e. a bac-
terium which swells into a spindle form when spore formation takes place). Isola-
tion and cultivation of the filamentous Bacteria was easily effected, and it was
found that they were ordinary saprophytic forms which required extremely little
nitrogen, but were quite unable to cause it to enter into combination ; more-
over, they were found not to be the cause of butyric acid fermentation. Interest
thus became concentrated on the third form, Clostridium pasteurianum, which,
morphologically, must be ranked along with the butyric acid Bacteria pre-
viously referred to, capable like them of inducing butyric acid fermentation,
but which differs greatly from them in its behaviour to nitrogen. The isolation
of Clostridium presents many and great difficulties, and is only successfully
accomplished if it be sown on carrots in vacuo. If a pure culture on this
medium be once more placed in the original non-nitrogenous nutritive solution,
fermentation and nitrogen combination do not take place. Both processes
commenced at once when WINOGRADSKY added the two bacterial forms present
with Clostridium in the zoogloeae, or when he excluded oxygen entirely. The
significance of the three Bacteria was thus explained. When alone, Clo-
stridium pasteurianum is able to combine nitrogen ; it is strongly anaerobic
and thus, in a pure culture, is capable of growth only when oxygen is
quite excluded. In nature, however, it is able to live in the aerated upper
regions of arable soil if the two other Bacteria associated with it protect
it from the action of oxygen. These two Bacteria fulfil no specific function
in themselves ; they may be replaced by other appropriate organisms which
consume oxygen, e. g. Hyphomycetes. Not every organism, however, is

234 METABOLISM

capable of fulfilling this protective role in the same way. The organism
must first operate on the culture medium by absorbing the oxygen before
Clostridium can begin to combine the nitrogen ; the protecting organism must
also obtain combined nitrogen in the first instance from the nutritive solution ;
later on Clostridium provides for it in this respect. It would thus appear that
organisms which make very small demands on nitrogenous compounds will
be the most suitable forms to accompany Clostridium. This condition is
fulfilled by the two Bacteria present in the zoogloeae because such traces of
nitrogen as are unavoidably present as impurities in the reagents employed
are sufficient for the purpose, although WINOGRADSKY showed that a minute
addition at first of ammonia or nitric acid induced much more rapidly
both fermentation and nitrogen combination.

Working on his previous experiences, WINOGRADSKY has since then been able
to isolate Clostridium pasteurianum by a second method much more rapidly and
effectively. He added a trace of garden soil to a non-nitrogenous nutritive solu-
tion, and allowed a stream of nitrogen gas to pass through the fluid. A drop
of this solution was after a certain time transferred to a fresh nutritive solution,
identical in character, and this process was repeated several times. The
final culture was heated to 80° C. after Clostridium had formed its spores,
so that all admixtures of foreign organisms were killed. The result was a pure
culture of the spores of Clostridium.

How nitrogen assimilation is carried out in this case is, however, quite
unknown. We know neither the primary nor the final products of assimila-
tion ; we do not know whether ammonia is formed and made further use of
or whether a complicated nitrogenous substance, e. g. some kind of proteid,
arises at once. One of WINOGRADSKY' s researches on this subject shows
that the nitrogen occurs chiefly in an insoluble organic condition, and
only in small quantities as soluble compounds in the nutritive solutions. The
latter, which possibly becomes free only when the Clostridium cells die, serves
as a nutrient for other organisms, especially for the two concomitant Bacteria.

Clostridium pasteurianum is an anaerobe. It acts fermentatively on cane
sugar, dextrose, laevulose, and certain other carbohydrates ; but it is unable
to make use of starch, cellulose, lactose, and higher alcohols. According to
WINOGRADSKY (1902), the products of fermentation are, on the one hand,
butyric and acetic acids (about 45 per cent, of the sugar), and carbon-dioxide
and water on the other (about 55 per cent, of the sugar). The fermentation
acts as a source of energy, especially for the purpose of combining atmospheric
nitrogen, and so we cannot wonder that WINOGRADSKY succeeded in establish-
ing quite definite numerical relations between the amount of sugar used up,
and the gain in nitrogen (viz. i g. of sugar fermented for every 2-5-3 mg- °f
nitrogen combined), but he was unable to say whether the sugar was employed
only as material for fermentation, or whether it was also nutritive. There
is no evidence at present available, however, to show whether Clostridium
withdraws not merely nitrogen but also carbon from the air, or whether it
assimilates carbon-dioxide like nitro- and sulphur-bacteria.

BERTHELOT had imagined that the capacity for fixing free nitrogen was
possessed by many micro-organisms, WINOGRADSKY finds it limited to Clos-
tridium and forms related to it ; at least these forms are the only ones which
are able to commence and carry on vegetative growth without combined
nitrogen. Following on WINOGRADSKY'S researches, it has often been stated
that other organisms also are able to make use of atmospheric nitrogen. Thus
BEIJERINCK (1901) observed a bacterium (Azotobacter) of unusually large
size, which forms no spores, and which on that account could not be isolated
by WINOGRADSKY'S method ; it develops on organic nutrients in presence
of air, especially on mannite, propionic acid, &c., without any combined

NITROGEN FIXATION 235

nitrogen, since it at first makes use of the traces of combined nitrogen,
occurring as impurities in the nutritive solution, and afterwards assimilates
the nitrogen of the air. According to the same authority, there are certain
Cyanophyceae (Nostoc, Anabaena) which behave in a precisely similar way to
Azotobacter, so that BEIJERINCK has been led to establish a special physio-
logical class of organism, to which he applies the term ' oligonitrophilous ',
among which he reckons Clostridium. We must note in this relation that
there is one great difference between BEIJERINCK'S arid WINOGRADSKY'S
researches, for the latter has proved the occurrence of a nitrogenous gain by
quantitative chemical analyses, whilst the former omits all such proof. More
recently WINOGRADSKY (1902, Centrbl. Bakt. II, 9, 43) has taken the same
view of BEIJERINCK'S work, and BEIJERINCK himself (1902, Centrbl. Bakt. II,
9, 3) admits that Azotobacter does not assimilate free nitrogen. His more recent
statements as to the real nitrogen-combining Bacteria must therefore be received
with a certain amount of scepticism. [It has been shown more recently still
that Azotobacter does assimilate free nitrogen ; the essential difference between
Azotobacter and Clostridium lies in the fact that the former is aerobic (compare
the comprehensive exposition of the subject by KOCH, 1904). Both forms occur
in the sea as nitrogen combiners (KEUTNER, 1904).]

The combination of atmospheric nitrogen has been maintained for other
forms, e.g. for Aspergillus and Penicillium by PURIEWITSCH (1895), and for other
Mould-fungi by SAIDA (1901). The increase in combined nitrogen in the^cultures
is, however, very minute, and we must await confirmatory experiments before we
can pronounce definitely on the subject. CZAPEK (1902, Beitr. z. chem. Phys.
u. Path. 2, 559) was unable to observe any combination of free nitrogen in
any of his experiments with Aspergillus. The power of combining the free
nitrogen of the air is even more doubtful in the case of the endosporal bacillus
(Bacillus ellenbachensis) than in the case of the Fungi referred to ; accurate
investigations have indeed shown that such a capacity does not exist. In
spite of this, pure cultures of this bacillus are sold under the name of ' alinite '
for impoverished soils ; they are said to aid in the combination of nitrogen in
the soil when spread over the ground.

If as vigorous a combining of nitrogen took place in all soils as in KUHN'S
experiments mentioned at the beginning of this lecture, neither alinite nor
any artificial manuring with nitrogen would be necessary. Experience teaches
us the contrary, however ; generally speaking, nitrogenous manuring is in-
dispensable, and the Leguminosae only form an exception, to be considered
afterwards. The combining of nitrogen in KUHN'S fields must have been un-
usually vigorous, and for this there must have been some special reason. If we
assume that Clostridium pasteurianum was the active micro-organism concerned,
the carbohydrates necessary for its support must have been present in special
abundance, so that we are met by a problem which has not as yet been con-
sidered, viz. as to how Clostridium is able generally to procure sugar in nature.
Two sources seem possible, one from the deciduous parts and remains of culti-
vated plants, the other from the lower Algae, which always occur in the soil.
It has been clearly established that combination of nitrogen takes place re-
markably well in soils which are rich in Algae, from which one may conclude
that the Algae and Clostridium stand in intimate relations to each other, the
Algae obtaining combined nitrogen from the Clostridium, the Clostridium
receiving soluble carbohydrates from the Algae (KossowiTSCH, 1894).

As already remarked, the Leguminosae play a special part in agricultural
processes, not only because they grow in sterile sandy soils without any addition
of nitrogenous manure, but because they actually improve such soils and
make them suitable for the growth of plants which do not belong to that family.
These peculiarities of Leguminosae were known, in part at least, to the ancients

236 METABOLISM

(PLINY, Historia Naturalis, vol. 8 ; cited by JACOBITZ, 1901), but were for the
first time clearly comprehended by ScnuLTZ-LupiTZ (1881), who obtained fifteen
crops of lupins one after the other from sandy soils on his estate merely by
mineral manuring without any nitrogen, and without noticing any diminution
in their yield. He also noticed that, after a crop of lupins, the cereal harvest
was doubled or tripled in amount. SCHULTZ did not remain content with
these results ; he made an analytical estimate of the nitrogen in the soil, with
the following result : —

1. Land which had neither been manured or cultivated for fifteen years and
which had been used as sheep pasture contained 0-027 Per cent, of nitrogen in the
arable layers to 6" deep, and 0-21 per cent, in the subsoil from 6" to 24" deep.

2. The same soil after being cultivated for fifteen years with lupins and
manured with minerals only, gave to 8" deep 0-087 Per cent., and from 8" to
24" deep, 0-025 Per cent.

It will be noticed that a very apparent gain in nitrogen had been effected
in the upper soil layers, and this was confirmed later by FRANK (1888) when
he examined the same fields after twenty years of lupin culture.

Both husbandman and agricultural chemist may therefore conclude
from the statements of SCHULTZ that the Leguminosae, and especially lupins,
must have the power of combining atmospheric nitrogen. Botanists, on the
other hand, may refer to an experiment of BOUSSINGAULT'S, of the accuracy of
which there can be no doubt, which shows that peas and lupins, which germinate
in soils containing no nitrogen, and which have access to no other source of
nitrogen than that present in the air, exhibit neither gain or loss in their nitro-
genous contents after a long continued experiment. Exact though this ex-
periment may be, it in no way demonstrates the inability of Leguminosae to
combine nitrogen in general, but only under the conditions of this experiment.
BOUSSINGAULT'S experiments are in reality not antagonistic to the classical
researches of HELLRIEGEL and WILFARTH, although these latter authors have
advanced definite proof of the power of the Leguminosae to combine free nitrogen.

HELLRIEGEL and WILFARTH (1888) used as a culture medium very pure
quartz sand, which was completely sterile, save for the addition of minerals.
The Leguminosae experimented with were compared most carefully with
cereals (oats and barley), which are known to be incapable of growth in the
absence of nitrates and other sources of nitrogen present in soils, and which also
have been specially shown to be unable to make any use of the nitrogen of the air.

Certain Leguminosae (Serradella,peas and lupins) were planted in this sand,
after it had been freed from all micro-organisms by heat, i. e. sterilized, and kept
free from them, and an experiment was carried out in precisely the same way
as with cereals, when growth took place only on the addition of nitrates. This
agrees with the behaviour of the Leguminosae in BOUSSINGAULT'S experi-
ment referred to above. An important difference was noticed, however, as soon
as a small quantity of an extract of arable soil had been added to the sterile
soil free from nitrogen ; then the crop showed a remarkable gain in nitrogen,
which could only have arisen from the employment of atmospheric air. A
numerical illustration will make this clearer (HELLRIEGEL, 1888, p. 145): —

Without addition of soil extract. With addition of soil extract.

Dry weight. Gain in N. Dry weight. Gain in N.

g g ' . g g

Serradella 0-092 —0-022 16-864 -1-0-326

Lupins 0-919 —0-049 44-7J8 +1-077

Peas 0-779 —0-025 17-616 +0-449

If the experiments are carried out with oats, instead of with Leguminosae,
the addition of the soil extract produces no result.

The effect of the soil extract cannot be due to the amount of nutrient

NITROGEN FIXATION 237

which it contains, but to the activity of the micro-organisms present, since
on heating it to a temperature of 70° C., its effect is at once neutralized. That
it is not due, however, to the presence of some kind of nitrogen-combining
micro-organism in the soil is shown by the absence of any result in the case
of cereals. The organisms must be such as have special relationships with
the Leguminosae. Further, different Leguminosae require different micro-
organisms, since the extract of a soil in which beetroot has been grown in which
peas and various species of clover had been cultivated for a long time up to
the fruiting stage, but in which no Serradella or lupins had ever been grown,
furthered the growth of peas only, but not of Serradella and lupins. Hand in hand
with this stimulating influence of the soil extract goes the formation of special
nodules on the roots of Leguminosae, whose existence had been long known, but
as to whose origin and nature no explanation was forthcoming.

HELLRIEGEL and WILFARTH showed in the clearest way that the micro-
organisms present in the soil extract were also the cause of the formation of
the nodules on the roots of Leguminosae, and that these plants could assimi-
late atmospheric nitrogen only when the micro-organisms were present in
the root nodules. ' To show that the Leguminosae make use of free nitrogen
for nutritive purposes ', write HELLRIEGEL and WILFARTH, ' it is not sufficient
merely that any kind of lower organism should be present in the soil, but it
is essential that certain species of the latter should first of all enter into
symbiotic relation with the former.'

Following DE BARY (1879), we may define symbiosis as a partner-
ship of two organisms of such a nature that both receive benefit by living
together, or where at least the benefits are not all on one side. In the latter
case, one would have to speak of it as parasitism. HELLRIEGEL and WILFARTH
have not, however, explained in individual cases wherein lies the reciprocal bene-
fit to the leguminous plant and to the Bacteria which live with it, and this has
not even yet been made clear, in spite of the series of important memoirs by
BEIJERINCK (1888), PRAZMOWSKI (1890-91), FRANK (1890) and others on the
formation and significance of the nodules.

Without going into details, we may present here the essential results of
recent investigations on the leguminous nodules by a reference to Figs. 42
and 43. When certain motile rodlike Bacteria, known collectively as Bacterium
radicicola, have entered the roots of Leguminosae, they increase there to an
almost astonishing degree. Just as we have seen in the case of galls, the
Bacteria stimulate the cells of the root and produce local hypertrophy, so that
we may term these nodules ' bacterium galls '. In the majority of the cells
of the nodule one finds masses of Bacterium radicicola, which later on degene-
rate in peculiar ways, and exhibit large spherical or branched ' involution
forms ', rich in proteid. These involution forms (' bacteroids ') are then used
by the leguminous plant as proteid reserves, that is to say, they are broken
down and employed in the formation of fruit. Only part of the Bacteria
become altered into bacteroids, and are sacrificed for the good of the legu-
minous plant, the rest persist, and, after the destruction of the nodule,
remain in the soil, and serve to infect the new Leguminosae of the next year.

Bacterium radicicola has been cultivated apart from the plant in suitable
nutritive media by several investigators, and the strongest possible proof is
forthcoming that the bacterium is the cause of the nodule formation. The
conditions of life of this organism have been established in the clearest possible
way, and we have to thank MAZE (1897) especially for proof that nitrogen is
combined by its means. Similar evidence was also furnished by other authori-
ties, but they provided Bacterium radicicola with ammonia or with no combined
nitrogen at all, in the expectation of proving even more clearly that a combina-
tion of nitrogen took place. BEIJERINCK employed asparagin, and thought

METABOLISM

that he noticed a gain in nitrogen, MAZE, finally, used proteid, believing that
the Bacteria in the first instance, doubtless, obtained it from the plant. His
results were striking : —

I. II. III.

Nitrogen in the culture solution 6a-i mg. voting. 33.4 mg.

45-8 „

Nitrogen at the end of the experiment 102-9 „
Gain in nitrogen

118-2

33.4 mg.

40-8 mg. 47.5 mg.

[MAZE'S results have been questioned by HILTNER (1904).]

In addition to nitrogen in the form of proteid, Bacterium radicicola uses
up very considerable quantities of sugar, and we must conclude that both

Fig. 42. Entry of Bacteria into the roots of Legumi-
nosae. a, cortical cells of the root of the pea, containing
tabular masses of Bacteria, X 650 ; b, roothair of the
pea, on the left of which a number of Bacteria have
accumulated outside. A pi rally the Bacteria are seen
mixed with the protoplasm of the hair, whence the
infecting threads are proceeding inwards to form the
tubular bacterial aggregates, x 175. From FlSCHBR
(Vorles. u. Bakt. and ed.).

Fig. 43. Root nodules of Leguminosae, a, root
nodule of lupin, nat. size; *, longitudinal section of the
same, several times magnified (jr, vascular bundle ; w,
bacteroidal tissue); c, bacterial cells of lupin(the Bacteria
are indicated as black dots, x 600) ; d, Bacteria of
lupin ; et bacteroids of Vicia villosa \ /, bacteroids of
Luj>in us albus \ d-f x a«;oo. From FISCHER (Vorles.
ii. Bakt and ed.).

substances are provided by the leguminous plant, sugar continuously and
proteid only at the beginning, since obviously the fixing of atmospheric nitrogen
commences after a certain time.

Since, in the end, the bacterium yields nitrogen to the leguminous plant, and
the latter yields carbon in an appropriate form to the bacterium, we may speak
of the union of the two organisms as a case of symbiosis. It is true the associa-
tion has been otherwise interpreted ; FISCHER (1903), for example, considers
the leguminous plant as a parasite on the bacterium, but this does not appear
to us to be a correct view to take.

It is quite probable that in the foregoing exposition of the phenomenon
of nitrogen assimilation in Leguminosae there will be found many omissions.
Indeed the researches of HELLRIEGEL and WILFARTH prove only indirectly
that the nitrogen of the air is combined ; they show a gain in nitrogen, and
demonstrate that the air is the only source whence that element could have

NITROGEN FIXATION 239

been obtained. Such being the state of the case, it is of interest to note that
direct evidence of the fact has been advanced by SCHLOSSING and LAURENT
(1890) who have calculated how many milligrams of nitrogen a pea takes
from the air during the several months of its vegetative growth, checking
this result by estimating the increase in nitrogen in the soil and in the crop.
The following summary shows that the agreement between these calculations
is almost perfect : —

Atmospheric nitrogen introduced into the culture vessel 2681.3 ccm.
„ „ withdrawn from „ „ 2653.1 „

Amount of nitrogen assimilated 29-1 ccm.

= 36-5 ms-

Nitrogen in soil and seed

Nitrogen assimilated 40-6 mg.

Another point of interest is that demonstrated by NOBBE and HILTNER
(1899 b), that the atmospheric nitrogen is combined in the nodules and not at
all in the leaves of the nodule-bearing plant. It was previously thought that the
leguminous plant in presence of Bacterium radicicola became altered in some
way so that it acquired the power of fixing free nitrogen. NOBBE and HILTNER
infected plants of Robinia with Bacterium radicicola, growing them in a culture
fluid from which nitrogen had been excluded, and observed the formation of
nodules under water ; but so long as these remained immersed they were found
to be useless to the plant, and it was only when they were brought into the air
that they commenced to assimilate the nitrogen. These facts prove that the
nitrogen must enter the nodules themselves if it is to undergo fixation.

Finally, we may note that many authors have been successful in seeing
the evolution of the Bacteria into bacteroids in artificial nutritive solu-
tions (BEIJERINCK, 1888; HILTNER, 1900; STUTZER, 1901). Nevertheless
it has not as yet been explained why only certain and not all Bacteria are trans-
formed into bacteroids in the plant, a phenomenon which is of the utmost
importance in relation to the conservation of Bacterium radicicola in nature
(p. 240).

If we describe the association of Leguminosae with nodule bacteria as a
case of symbiosis, then, looking backwards, we may also term the relationship
of Clostridium pasteurianum to the two associated bacteria as symbiosis also,
and we may take this opportunity of drawing attention to some other cases
of the same kind. Beginning with Elaeagnus and the alder, we find that both
these trees produce on their roots nodules which remind us of those of the
Leguminosae, and which seem to carry out similar functions. HILTNER (1896)
has shown in the case of the alder that when the nodules are absent it can develop
only if nitrogenous compounds are provided, but that after the nodules have been
formed the nitrogen of the air is sufficient. [HILTNER gives a very instructive
illustration of alders, which have been grown in sand destitute of nitrogen, one
with and one without nodules (HILTNER, 1904, p. 63).] Similar conditions
obtain in Elaeagnus, but research is required as to the mode of development
of the nodule-forming organisms in both cases ; recently SHIBATA (1902) has
supplied us with an interesting account of their structure.

Seeing that the use of free nitrogen by higher plants is not confined to the
Leguminosae, we are lead to believe that FRANK'S (1890) view is the correct
one, and that this power may be possessed by all plants to a greater or lesser
degree. It will be sufficient to say on the other hand that careful experiments
made on the majority of Phanerogams, e. g. on Gramineae and Cruciferae (AEBY,
1896, PFEIFFER and FRANKE, 1897) have given only negative results. In other
cases, however, and especially in such as exhibit a symbiotic union between

240

METABOLISM

Fungi and Phanerogams, nitrogen fixation is a proved fact, or, at least,
very probable. One of the Coniferae especially, Podocarpus, is known to
possess nodules on its roots. These nodules are modified lateral roots, whose
tissues have become filled with the hyphae of a fungus. Podocarpus cannot
be cultivated in the absence of this fungus, but NOBBE and HILTNER remark
(1899 a) that they were able to grow Podocarpus with perfect success for five
years in quartz sand, from which nitrogen was entirely absent. There can
be no doubt that this plant is able to fix atmospheric nitrogen, and it is ex-
tremely probable that the fungus plays an important part in the process.
More recently, HILTNER (1899) has studied Lolium temulentum, with which,
according to VOGL and NESTLER, there is constantly to be observed associated
a fungus growth. HILTNER' s observations render it probable that here also
fixation of nitrogen takes place, and it may be concluded that many parasitic
Fungi act in the same way. HILTNER thought that the luxuriant growth of

many plants attacked by Fungi was evi-
dence in favour of his view, but BREFELD
(1902) has shown that this does not apply
to Ustilagineae, although it cannot be said
that that is the case with other Fungi.

The so-called 'mycorhiza', a symbiotic
union of a fungus with phanerogamic roots,
is of wide occurrence, and first suggests it-
self to us in this relation. There are two
forms of this union, endotrophic and ecto-
trophic. The former has been long known,
in fact since the time when SCHLEIDEN
pointed out its occurrence in Neottia nidus
avis, and its widespread distribution has
been demonstrated by FRANK (1887) and
SCHLICHT (1889), especially in Orchidaceae,
Ericaceae and Epacridaceae. These Fungi,
which have as yet been very little investi-
gated from the systematic point of view,
enter the cells of the root, and increase

M^4^^^^?^ ^nuctus0;' there without killing the cells of the host,
digestion of the fungus beginning at KI : x 210. Neottia has been carefully investigated by
£ff!3; x%oT /^TormfiS^udeus W. MAGNUS (1900). In this case the fungus
o7Tnthfnf^ enters from without, and branches at

digested by the fungus, A7, the digested fungal SOHie distance from the epidermis, SO

Af^smSTi(i^)elllllosK r&M-X24°' as to completely fill up a series of con-
centric layers of cells in the root and

rhizome. The cells in which the fungus lives do not all behave in the same
way. In certain cells the fungus grows vigorously, and on the death of the
protoplasm of the host, forms organs for the purpose of maintaining its exist-
ence over winter, and for the infection of newplants the following year. The
fungus is partly digested in other cells, on the other hand, and its abundant
proteid constituents go to the nourishment of the host, while the indigestible
portions are collected into a ball in the centre of the cell, and are there enclosed
by layers of cellulose. The same thing is seen in Psilotum triquetrum (SHIBATA,
1902), only in this case the fungal host -cells and the digestive-cells are arranged
in no definite order. In the host-cells the fungus hyphae are confined to the
periphery of the cell, and the nucleus undergoes no alteration ; in the digested
cells, on the other hand, one finds (Fig. 44) a dense ball of hyphae, which, begin-
ning at one point, is gradually disorganized (/ and // Kl.), while at the same time
the nucleus increases greatly in size, and undergoes special internal altera-

SYMBIOSIS 241

tions as well (compare Fig. 44, /// and IV). Finally, the indigestible portion
of the fungal mass aggregates into the centre of the cell (V), where it becomes
surrounded by a membrane. According to SHIBATA, there is only one slight
difference between Psilotum and Podocarpus ; the cells of the Podocarpus
nodules are densely filled with hyphae, and these are digested. Since the
assimilation of free nitrogen very probably takes place in Podocarpus, we shall
not be far wrong in interpreting the mycorhizal condition in other plants in
the same way. [TERNETZ (1904) has managed to isolate a fungus from the
roots of Ericaceae, which is in all probability the one concerned in the
formation of the mycorhiza, and which is the active agent in assimilating
the atmospheric nitrogen.] The knowledge we have gained as to the pheno-
menon in Neottia and Psilotum renders the various relations of Bacteria to
Leguminosae all the more intelligible (compare p. 239).

Accepting this interpretation of endotrophic mycorhiza, the question
comes to be whether a combination of two non-chlorophylliferous organisms,
e. g. Neottia and a fungus, can be termed a case of symbiosis, since it is not
readily comprehensible wherein the reciprocity can lie ; our knowledge of the
nutritive relations is still too inadequate, and hence we may postpone further
discussion of the question. If the Phanerogams possessing mycorhiza be
green and can therefore assimilate carbon, it may be assumed that the
activities of the two symbionts are so regulated that the fungus collects
the nitrogen and the higher plant the carbon.

At the same time there may be another explanation of this association,
other than the fixation of atmospheric nitrogen, viz. that the higher plants
are peptone or asparagin organisms, and that the duty of the fungus is to
manufacture these nitrogenous compounds out of humus. Possibly, however,
the fungus aids in the absorption of materials of the ash, and does not supply
the needs of the higher plant for nitrogen at all (SxAHL, 1900). Fungi make
very heavy demands on such materials, and since they collect these very
rapidly, they are vigorous competitors with Phanerogams which work more
slowly on soils poor in nutritive salts. Higher plants are able to grow far
better in humus which has been deprived of the Fungi naturally present. As
long as these Fungi are present the Phanerogams exhibit all the evidences of
' mineral starvation '. A mycorhizal union occurs especially in such plants as
live in humus, or for other reasons exhibit feeble inflow of minerals (e. g. weak
transpiration). Hence STAHL assumes that these plants make the Fungi contri-
bute to their wants in that respect, turning antagonistic neighbours into efficient
assistants. The part played by the higher plant so far as carbohydrate is con-
cerned, becomes intelligible on this view. On the other hand, STAHL' s hypo-
thesis appears to us to be subject to criticism, in that the fungus lives in
most cases quite in the interior of the root, and hence cannot be in a very
suitable position to aid in absorbing nutritive salts from the soil. The function
of the fungus, however, according to STAHL, consists not merely in the
absorption of the nutritive salts from the soil, but also in their transforma-
tion, so that the other member of the symbiotic union may receive the
products of assimilation ready made. STAHL comes to this conclusion from
noting that the majority of ' mycotrophic ' plants do not contain in their
tissues certain waste bodies, such as calcium oxalate, which are associated
with the assimilation of nutritive salts (comp. pp. 141 and 197).

We must now glance at ectotrophic mycorhiza. This form of mycorhiza was
first drawn attention to by KAMIENSKI (1881) in Monotropa, and soon afterwards
FRANK demonstrated its occurrence in a large number of our forest trees (Cupu-
liferae, Betulaceae, Coniferae). The Fungi — apparently members of the Agaricinae
and Tuberaceae — generally do not enter in this case into the cells of the roots,
but form a densely-woven layer covering the root, not even leaving the grow-
ing points free. Here and there single fungus cells enter in between the super-

JOST R

242

METABOLISM

ficial cells of the root, but these are always confined to the intercellular spaces.
This association is accompanied by certain differences in form and anatomical
structure in the root, which render the presence of the mycorhiza easily recogniz-
able, and which must have a physiological significance. One of these
differences is the non-formation of roothairs on roots provided with fungus
mycelium (Fig. 45), so that the absorption of all nutrients and water is possible
only through the medium of the fungus. From what has been already said
as to the small amount of nitrogen in woodland soils (p. 133), one can only
conclude that the Fungi but not the trees are restricted to ammonia; but
we are ignorant what the nitrogen compounds are which the root receives
from the fungus covering. STAHL'S hypothesis would seem at first sight to
be much more applicable to this type of mycorhiza than to the endotrophic
form. On further reflection, however, difficulties appear here also. If the
fungus actually anticipates the tree in acquiring mineral materials from the
soil, why does it not retain them ? Why does it, after assimilating them, give
them up again ? In the endotrophic mycorhiza this is explained by the fungus

being digested, but there is no
reason to believe that a similar
process takes place in the ecto-
trophic type. Several problems
await solution here, and it is
astonishing how little experi-
ment teaches us in this re-
lation. And yet systematic
experiments are not to be con-
sidered as hopeless ; we know
how NOBBE (1899) managed to
develop pines, firs, larches and
c \3£ar beeches with perfect success for
a period of twenty-five years
in pure quartz sand, free from
humus. At all events, there
..-^v/, 2 — »-»i', ", •«" «•«."•« v»-it«» appeared to be no hereditary

tactical Botany.) /- • , ,, , ,. \

fixing of the adaptation, and
hence experiments are not hopeless. According to MOLLER'S (1902) researches,
the significance of ectotrophic mycorhiza has again been called in question
[since the Brandenburg pine has all the less mycorhiza the more humus the soil
possesses]. It is to be expected, however, that these experiments will show
that among mycotrophic plants, in addition to real cases of symbiosis, there
are associations which are harmless and perhaps of no importance at all,
and others which are really cases of parasitism; it may be that the fungus
is the parasite, and again it may be that the seed-plant is the parasite. In
many endotrophic mycorhiza on green plants the fungus must be the parasite,
among the ectotrophic types on colourless plants (e. g. Monotropa) it must be
the seed-plant that is parasitic.

In speaking of symbiosis, it is impossible to ignore the case of lichens,
to whose very remarkable association of fungus and alga DE BARY (1879)
first applied the term symbiosis. The conception of a lichen as a combination
of alga and fungus types was first enunciated by SCHWENDENER (1869, Algen-
typen der Flechtengonidien. Basel). On this question DE BARY (1865, Morph.
u. Phys. d. Pilze, Flechten, &c.), and also REINKE (1894, Jahr. f. wiss. Bot. 26, 524)
should be consulted. Notwithstanding the work which has been accomplished,
we do not even yet completely understand the modus vivendi of this symbiosis.
BEIJERINCK (1890) and ARTARI (1899) have shown that certain lichen-Algae
are ' peptone plants ', and we may suppose that the fungus contributes peptone
to the combination, while it is naturally the business of the alga to assimilate

Fig. 45. /, Beech root from woodland humus (magnified) ;
fungal cords, united with soil particles at a. //, Beech root from
sterile humus (magnified) ; £, rootcap ; /*, roothairs. (From
DETMER'S Smaller Prs

SYMBIOSIS AND METABIOS1S 243

carbon-dioxide, supplying carbohydrate to the fungus. It is possible that
we have here a case of true parasitism. Not every parasite treats its host
plant so harshly as to kill it either wholly or partially ; the craftiness of the
parasite lies in this, that it keeps its attack within bounds such that the life
of the host is not imperilled, permitting in this way a longer period of useful-
ness and advantage on the part of the host. Reference should be made here
to the Uredineae and the Peronosporeae. In the case of lichens, however,
one sees not only that no damage is done to the algal cells, but that on the
contrary the fungus seems to stimulate them to further development ; the
algal cells are larger in the lichen than they are in the free condition. Still,
that fact does not negative the idea of parasitism, for in other cases we have
seen that an increase in size of the host-cells takes place owing to the stimulat-
ing effect of parasitic Fungi ; and we are able to explain the effect in some
measure by remembering that well-known poisons administered in small doses
are capable of acting as developmental stimuli.

It is not within our power to enter into the numerous cases which have
with greater or less justice received the name of symbiosis. We may, however,
conclude that the term symbiosis may be applied in a more extended sense.
Symbiosis in the most restricted sense may be applied to a single organism formed
by the coming together of two symbionts, the combination possessing certain
constant functional peculiarities, which the constituents do not exhibit or exhibit
in a less degree (lichens, Leguminosae) ; on the other hand, to a certain extent,
we may speak of symbiosis when both symbionts are united only in so far
as, e. g. Clostridium pasteurianum is with the two aerobic Bacteria, in which
case they form an amorphous zoogloea ; and we may still speak of symbiosis
when green Algae and Clostridium act in conjunction with each other in arable
soil, and assist each other with their metabolic products. If we go a step
further, we meet with organisms which appear in the same situation, but follow-
ing each other, one preparing the soil for the other ; this phenomenon we may
term metabiosis (WARD, 1899). The close relationship between symbiosis and
metabiosis is obvious. We have already drawn attention to the wide distribu-
tion of metabiosis, and we may best express our views as to plant metabolism by
arranging in tabular form a review of the metabiosis of the various organisms
of which we have spoken. We shall limit ourselves in general to the two
elements which have played the chief part in our discussion, to the study,
that is to say, of the circulation of carbon and nitrogen in the organic world.
Chemistry tells us that no substance on our planet is ever lost ; but that an
active circulation of matter takes place on the earth, a circulation which is
closely connected with metabiosis. Every organism standing by itself would
soon have caused alterations in the outer world, which would have rendered
continued existence impossible. It is only by the existence of numerous
organisms with diversified functions that the perpetual renewal of life on
the globe is possible.

The following table makes no claim to completeness ; even what it does
show is not perfect, since there are many metabolic products, which, if indi-
cated by arrows, would detract from the clearness of the diagram. The
carbon-assimilation of green plants is indicated in the middle of the table,
and for this the sunlight provides the requisite energy. On the storage of
solar energy in the carbohydrates are indirectly based all the processes which
are referred to in the summary. In the separate metabolic processes only
those products of metabolism which are of especial interest at the moment
are indicated. Chemical equations are omitted ; the outgoing and the final
products of a process are indicated by arrows which give also the direction
of the course of the reaction. If an expenditure of energy is necessary for
the reaction, the arrow is directed upwards ; a gain in energy is shown by
a downwardly directed arrow ; when the arrow is horizontally placed, it indi-

R 2

244 METABOLISM

cates a splitting without any actual change in energy. Oxygen is printed in
large Roman capitals, carbon-dioxide in small capitals, carbohydrates and
other compounds of carbon, hydrogen and oxygen in large italic capitals,
inorganic nitrogen (nitrogen, nitric acid, ammonia) in small italic capitals,
organic nitrogen in black type. The path of circulation of these materials
is indicated by the arrows at the ends of the lines.

Bibliography to Lecture XIX.

AEBY. 1896. Versuchsstationen, 46, 409.

ARTARI. 1899. Bullet, des naturalistes de Moscou, No. i.

DE BARY. 1879. Die Erscheinung der Symbiose. Strassburg.

BEIJERINCK. 1888. Bot. Ztg. 46, 725.

BEIJERINCK. 1890. Ibid. 48, 725.

BEIJERINCK. 1901. Centrbl. Bakt. II, 7, 561.

BERTHELOT. 1892. Compt. rend. 115, 569 and elsewhere.

BOUSSINGAULT. 1860. Agronomic, i, 65.

BREFELD. 1902. Centrbl. Bakt. II, 8, 24.

BUNGE. 1889. Lehrbuch d. physiol. und pathol. Chemie, 2nd ed. p. 21. Leipzig.

FISCHER, ALFR. 1903. Vorlesungen iiber Bakterien, 2nd ed. p. 163. Jena.

FRANK. 1887-88. Ber. d. bot. Gesell. 5, 395 ; 6, 248.

FRANK. 1888. Landw. Jahrb. 17, 441. (Die Ernahrung der Pflanzen mit Stick-

stoff. Berlin.)

FRANK. 1890. Landw. Jahrb. 19, 523. (Die PHzsymbiose der Leguminosen. Berlin.)
HELLRIEGEL and WILFARTH. 1888. Unters. iiber die Stickstoflnahrung der Gramineen

u. Leguminosen (Beil. Zeit. d. Vereins fur die Rubenzuckerindustrie). Berlin.
HILTNER. 1896. Versuchsstationen, 46, 153.
HILTNER. 1899. Centrbl. Bakt. II, 5, 831.
HILTNER. 1900. Ibid. II, 6, 273.

[HILTNER. 1904. LAFAR'S Handb. d. tech. Mykologie, 3, 24.]
JACOBITZ. 1901. Centrbl. Bakt. II, 7, 783.
JENSEN, H. 1898-9. Centrbl. Bakt. II, 4, 401 ; 5,716.
MENSEN, H. 1904. LAFAR'S Handb. d. tech. Mykologie, 3, 182.]
KAMIENSKI. 1881. Bot. Ztg. 39, 457.
[KEUTNER. 1904. Wiss. Meeresunters. Bd. 8.]
[KocH. 1904. LAFAR'S Handb. d. tech. Mykologie, 3, i.]
KOSSOWITSCH. 1894. Bot. Ztg. 52, 97.

KiiHN, J. 1901. Fiihling's Landw. Zeit. 1,2; reviewed in Centrbl. Bakt. II, 7, 601.
LEMMERMANN. 1901. Kritische Studien der Denitrinkationsvorgange. Jena.
MAASSEN, A. 1901. Zersetzung d. Nitrate u. Nitrite durch Bakterien (Arb. Kais.

Ges.-Amt), 18, i.

MAGNUS, W. 1900. Jahrb. f. wiss. Bot. 35, 205.
MAZE. 1897. Annales de 1'Institut Pasteur, 1 1, 44 ; 12, i.
MOLLER. 1902. Reviewed in Bot. Ztg. 60, II, Abt. 201. [Comp. also Bot. Ztg. 1903,

II, 329.]

NOBBE. 1899. Versuchsstationen, 51, 241.
NOBBE and HILTNER. 1899 a. Versuchsstationen, 51, 241.
NOBBE and HILTNER. 1899 b. Ibid. 52, 455.
PFEIFFER and FRANKE. 1897. Versuchsstationen, 48, 455.
PRAZMOWSKI. 1890-91. Landw. Versuchsstationen, 37, 161; 38, 5.
PURIEWITSCH. 1895. Ber- d. bot. Gesell. 13, 342.
SAIDA. 1901. Ber. d. bot. Gesell. 19, 107.
SCHLICHT. 1889. Landwirtschaftl. Jahrbucher, 18, 477.
SCHLOSSING and LAURENT. 1890. Compt. rend. Paris, III, 750; comp. also Annales

de I'lnstitut Pasteur, 6, 65 ; 6, 824 (Kocn, Jahresbericht, 1890-2).
SCHULZ-LUPITZ. 1 88 1. Landw. Jahrb. 10, 777.
SHIBATA. 1902. Jahrb. f. wiss. Bot. 37, 643.
STAHL. 1900. Jahrb. f. wiss. Bot. 34, 539.
STUTZER. 1901. Centrbl. Bakt. II, 7, 897.
[TERNETZ. 1904. Ber. d. bot. Gesell. 22, 267.]
WARD. 1899. Annals of Botany, 13, 52.

WINOGRADSKY. 1895. Archives des Sc. biologiques. St. Petersburg, 3.
WINOGRADSKY. 1902. Centrbl. Bakt. II, 9, 43.

PART II. METAMORPHOSIS

LECTURE XX

INTRODUCTION

IN the discussion of metabolism mention has frequently been made of the
work done by the organism as a result of respiratory and other analogous
katabolic processes. These activities were also touched on in Part I, for it
must be clearly understood that the movements of raw food material, plasta,
and products of assimilation are just as much expressions of such activities as
the movements of an entire organ, as, for example, the assumption of the erect
position by a stem laid horizontally. Having now considered chemical physio-
logy we must turn our attention more especially to the activities of the
organism, but in doing so it should be noted that we are changing not the subject-
matter but the point of view, since chemical changes are never absent when the
shape or position of a plant or plant-organ is altered. Change in material, in
energy, or in form, occur simultaneously in nature, and it is only for con-
venience of investigation or exposition that we may legitimately institute sub-
divisions in our science. Two such divisions, generally recognized, are
metabolism and transformation of energy ; we venture to add a third, viz.
change of form. Before attempting to justify the institution of this section it
may be well to inquire on what the characteristic form of an organism, and
especially of a plant, really depends, and also what manner of alterations it
undergoes.

Let us approach the subject inductively and consider a few examples.
First of all take the Myxomycetes, which we have already learned (Lecture I)
to regard as naked protoplasmic masses, or plasmodia. Plasmodia are soft,
slimy bodies which form irregular networks over their substrata, i. e. decaying
leaves, dead branches, &c., and in which continual changes in outline may be
observed with the naked eye, simultaneously with changes in position. These
movements are even more apparent under the microscope, and one may observe
a complete alteration in shape in the course of a few minutes. Phenomena such
as these are not, however, what we mean by ' change of form ', and should not
be included in the present section of our studies. Just as a viscous liquid spreads
irregularly over its substratum, so the plasmodium has no definite shape ; only
when it assumes such a shape does it come under consideration in this relation.
Under certain conditions, however, the protoplasm of a Myxomycete aggregates,
takes on a rounded form, secretes a firm external layer, and its contents divide
into a large number of spherical cells. Each of these is capable under suitable
external conditions of again becoming a naked mass of protoplasm and of
creeping about over the substratum. The slime fungi may thus exist in two
conditions, different in shape, in the vital phenomena they exhibit and in their
significance. In the 'formless' state it requires a certain amount of water,
inorganic and organic nutrients, and a certain temperature, and given these
conditions the food- materials are assimilated and the plasmodium grows.

METAMORPHOSIS

In the second state no vital phenomena are manifested, neither growth nor
absorption of food ; though completely desiccated, the protoplasm retains its
capacity for forming a plasmodium for months or even for years. We may
regard the plasmodium as the vegetative, the other we term the reproductive
stage of the fungus, or we may speak of it more definitely as the ' sporangium '
and describe the cells within it as spores. These spores preserve the character-
istics of the slime fungus at a time which is unfavourable to vegetative life —
they propagate the organism. The questions we have to ask and, as far as
possible, to answer are : — What are the factors which induce the appear-
ance of the vegetative and what of the reproductive state ? What significance
lies in the fact that the former state is destitute of definite shape and that the
latter has a constant shape ?

As our second example we will select a fungus, found in the frog's excreta,
and belonging to the Entomophthoreae, viz. Basidiobolus ranarum. It is
a heterotrophic plant, requiring organic as well as inorganic nutritive materials.
For this fungus, as for many others, peptone and sugar form an excellent
culture medium ; peptone alone, however, will serve quite well. The usual form
of a Basidiobolus is a cylindrical cell many times longer than broad. The proto-
plasm is never naked but is always surrounded by a cell-wall, and in each cell
we find a nucleus lying in the protoplasm, accompanied by one or more vacuoles.

Fig. 46. Basidiobolus ranarum.
After RACIBORSKI (1896). /, Grown
in 20 per cent, solution of glucose.
//, Cultivated in a 10 per cent, solu-
tion of sugar at a high temperature.
///, Giant cell from a similar cul-
ture, without cell-walls but with
several nuclei. IV, Palroella stage
grown in a solution of glucose and
ammonium sulphate.

Fig. 47. Basidiobolus ranarutn. After ElDAM.
/, Young branched plant (x 60). //, Young plant
with erect conidiophores. Co, Conidia (x 30).
///, Early stage in the formation of a zygote ( x 350).

IV, Formation of a zygote from two conidia(x 250).

V, Later stage of IV (x 350).

The cell grows in the culture fluid, it becomes longer, and finally forms medianly
a transverse wall by which it becomes subdivided into two cells. Previously
division occurs in the nucleus, and each daughter-nucleus finds its way into one
of the ' daughter-cells' . The division is a concomitant of growth, and although
both cells remain united, each possesses an individual vitality, and exhibits
all the features of a Basidiobolus. The cells may be isolated and each
cell is then seen to be quite independent of the other, growing and dividing
on its own account. Growth then takes place not in one direction only, the
long axis, but lateral branches appear (Fig. 47, I) which, however, do not call
for further mention here. If the composition of the culture remains unaltered
growth and cell division proceed for an unlimited time in the same way. On
the other hand, remarkable changes in form may be induced by altering the
composition of the nutritive solution (RACIBORSKI, 1896). If the concentration
be greatly increased and a 20 per cent, solution of sugar be supplied instead of
10 per cent., or if sodium chloride or another mineral be added up to 6 or 10 per
cent., growth in length is inhibited, the cells become more rounded, and the

INTRODUCTION 249

division walls are no longer strictly transverse but oblique (Fig. 46, /). Finally,
especially under high temperatures, cell division ceases entirely, though growth
and nuclear division continue and giant cells with many nuclei result (Fig. 46,
77, 777). We have now obtained forms which are no longer typical, since they
have lost their power of normal development and cannot regain their original
shape. Shapes not less remarkable, but still normal, may be obtained by
a qualitative alteration of the nutritive solution, viz. by retaining sugar as the
source of carbon but by employing ammonia or some related body (amines) in
place of peptone. The cells now become more rounded and divide irregularly
in all directions ; further, the walls surrounding the daughter-cells become
stratified in a remarkable manner (Fig. 46, IV). Each cell is surrounded not
only by its own cell-wall but by that of the mother-cell, and finally by that of
the parent cell of all. The membranes then gradually degenerate and the cells
become free, separate from each other, and take on a spherical form. This
growth form reminds one of certain lower Algae and may, as in that case, be
termed the ' palmella ' form. By keeping the nutritive solution constant,
Basidiobolus may be made to continue growing in the palmella form for an
indefinite length of time.

Since every cell is physiologically independent, multiplication results from
every division ; as is the case with many of the lower organisms, it is impossible
to separate vegetation from reproduction. Basidiobolus, however, also forms
special reproductive cells, and these have one function only — they are not at
the same time vegetative.

Conditions are complicated by the fact that two kinds of reproductive cells
are produced ; the one type is the so-called conidium (Fig. 47, 77), which arises
in the following way. A swelling appears at the end of one of the cells which
stand up erect out of the nutritive substratum; into this swelling almost all
the protoplasm, together with the nucleus, migrates. The swelling is finally
segmented off by a transverse wall and the conidium is complete ; it is after-
wards thrown off by a special mechanism. The function of the conidium is not
to increase the extent of fungus in the same substratum, but to distribute it to
another. The other type of reproductive organ — the zygospore — arises in quite
a different way. Two cells of a filament develop beak-like projections at the
limiting wall (Fig. 47, 777), which in that situation become dissolved, so
that the protoplasm of one cell, or at least most of it, can wander over into
the cavity of the other (IV, V). The protoplasts thus fuse and the product
becomes enclosed in a very thick wall, which is always characteristic of this
type of spore, and indicates that the cells concerned are able to, or must, pass
through a long resting period. The two nuclei of the zygospore fuse later.
Zygospore formation is a sexual process of a very simple type, the spore being
formed by the fusion of a male (the motile cell) with a female cell.

Thus Basidiobolus has both sexual and asexual reproductive organs, and
these are distinguished not only by their mode of origin but also by the
conditions under which they are formed and by their significance in the plant's
life-history. It has already been noted that the conidia are formed outside the
medium ; the zygospores, on the contrary, are only developed in it. Both struc-
tures, however, appear only when the nutritive solution begins to become
exhausted. The formation of either type of reproductive organ may be sup-
pressed by periodical renewal of the culture fluid.

Basidiobolus is thus an organism showing very varied shape and modes of
development, and the variations observed in it demand our special attention,
depending as they do in the most obvious manner on external conditions which
are determinable at will. This example teaches us how external factors may
influence the shape which the plant assumes.

It is unnecessary for us to describe all the intermediate stages in plant
shapes that exist between a simple fungus and the most highly developed plant,

250

METAMORPHOSIS

so that we may pass from Basidiobolus directly to our third and last example —
the flowering plant. In its earliest stage the flowering plant is nothing more
than a single cell (the ovum), similar in all respects to the cell of a fungus.
In this case also the cell grows and divides, but all the daughter-cells do not
fulfil the same function nor do they continue to have the same structure ; they
are no longer of equal value and must remain connected if they are to continue
in existence. It is not the cells of which the higher plant is composed that
attract our attention in the first instance but rather the external members of
the plant ; the cell structure of each individual part is revealed on subsequent
microscopical investigation. The external segmentation of the plant is, however,
already suggested in the embryo whilst still within the seed, exhibiting as
it does two poles, one of which we term the radicle, the other the plumule.
The distinction between them exhibits itself most clearly during germination
by the fact that the root on leaving the seed pushes its way downwards
into the soil while the plumule grows in the opposite direction. A careful
study of the conditions under which the differentiation of these two poles arises
forces us to the conclusion that we must seek its cause not in external factors,
such as play so important a part in the morphology of Basidiobolus, but in some
internal factors. The further progress of development in a flowering plant
also convinces us that internal causes play a most important part, for though
the external factors remain completely unchanged the plant itself alters in shape
from day to day. The root grows apically and increases in length in a well-
marked manner ; it forms lateral branches which resemble the principal root
in all essential respects. The plumule divides into axis and leaves and gives
origin to lateral axes, also provided with leaves. The leaves may be of diverse
shape, but usually agree in exhibiting vigorous unilateral growth, forming thin
but at the same time broad surfaces, green in colour, owing to the chlorophyll
contained in their cells. They change their shape several times in the course of
the vegetative period ; the typical foliage leaves are preceded by simple leaf -like
organs, and are followed by them also ; the branch produces alternately scale
and foliage leaves, or at the apex proceeds to form bracts, and finally flowers. In
the flowers are developed the organs of sexual reproduction, by whose agency
new plants are formed. In addition to sexual organs we find in thousands
of flowering plants organs of vegetative reproduction as well.

The flowering plant passes through a ' life cycle ' which, at least in certain
cases, begins with the germination of the seed and ends with its formation,
and which extends over one or more seasons. We found that in Basidiobolus the
various growth forms were mainly dependent on external conditions, but in the
flowering plant we have something entirely different, namely the intimate
connexion between the functions of the various members and their external
and internal structure. We have seen, in Part I, how all the parts of the
higher plant no longer fulfil the same functions, as they do in unicellular
forms. The root, we have found, is the organ for absorption of water and
soluble salts from the soil, the roothairs being specially concerned in this
absorption. There can be no doubt that these roothairs, by vastly increasing
the surface of the root, by their intimate union with the soil particles, and
by their excretion of certain substances, are specially adapted for this purpose.
The substances acquired in this way are, in part, transferred to or elaborated
in the aerial parts of the plant, and hence we find special conducting strands
developed in the interior of the root which are in intimate connexion with
similar conducting channels in the shoot. The chief of these are the tracheae,
cells which have lost their protoplasmic contents and consist of empty tubes
into which the water flows.

The function of the leaf is entirely different. It is the carrier of the chloro-
phyll by means of which carbohydrates are formed from carbon-dioxide. For
this purpose, as we have already seen, sunlight is essential. The chlorophyll has

INTRODUCTION

25'

thus to be exposed to the sunlight in a thin layer, and we may regard the
flattened form of the leaf as specially adapted to fulfil this function. SACHS
(1882, Vorlesungen iiber Pflanzenphysiologie, p. 618) has shown in a most
interesting manner how the essential structural relationships of the higher plants
are subservient to this chlorophyll-function. We can thus understand how the
leaf must have an entirely different shape from the root. The greater its surface
the greater the transpiration, and consequently the arrangement of the water-
conducting tissue in the leaf is especially adapted to carry out this function.
In order, however, that transpiration may not be so great as to act inimically to
the plant, we find numerous arrangements (to which we have already referred)
for retarding that process.

If we turn now to the stem we find it to be the medium of communication
between the root and leaf ; it has to carry the substances absorbed by the root up
to the leaf and, conversely, to transport back again the materials manufactured
in the leaf. It has also to support the entire weight of the aerial part of the plant,
its own as well as that of the lateral organs ; and when we consider, not a small
annual herbaceous plant, e. g. Draba verna, but an oak-tree several hundred
years old, we can easily appreciate how rigid the stem must be. Each cell by
itself has a certain rigidity owing to the tension of its membrane, due to the
osmotic activity of the cell-sap. This alters, however, with the amount of water
present, and on hot summer days the rigidity due to turgescence is rapidly re-
duced by increased transpiration. All land plants of large size have, therefore,
as well a special mechanical tissue system, the thick- walled sclerenchyma. We
owe to SCHWENDENER (1879) the demonstration of the fact that this sclerotic
tissue is arranged in accordance with engineering principles, so that the greatest
effect is attained with the minimum expenditure of material. Sclerotic tissue is
also present in the leaves and roots, but it is distributed differently in these
situations, since the mechanical requirements of these organs differ from those
of stems.

In this sketch we have limited ourselves to the vegetative organs and
have referred only to some of the most characteristic features exhibited by
these organs. A detailed description of the anatomical adaptations seen in
plants will be found in HABERLANDT'S (1896) Physiologische Pflanzenanatomie,
2nd ed., Leipzig ; to give such an exposition here would be foreign to the purpose
of these lectures. The general result, however, of our review is to lead us to
the conclusion that the structure of a member is adapted completely to the
function it has to perform. Doubtless, also, the cell of Basidiobolus is adapted
to the performance of its functions, although on account of the fact that all
functions are in that case carried out by one cell, the finer organs and their
adaptations are microscopic and cannot be distinguished in detail. It is not
to be wondered at that we do not know whether an elaborate differentiation,
such as occurs in the body of the higher plant, or such a purposeful division of
labour, occurs here also, or whether the single cell taken as a whole is able to
act like the complex apparatus of a higher plant, made up of millions of cells.

Division of labour in differentiated plants has a far-reaching significance,
on which a few remarks must be made here. Whether the two cells resulting
from the division of the original Basidiobolus remains united or not is quite
immaterial, but it is quite a different matter in the case of the flowering plant.
Here the individual parts, whether they be the members visible to the naked
eye or the cells seen only under the microscope, are incapable of living separately.
A leaf, for example, torn off by a gale of wind rapidly dies ; it can manufacture
organic substance, it is true, but it withers for want of water. A root also can
take water and salts from the soil after the shoot has been cut off, but it soon
ceases to grow, because no organic materials are supplied to it. An isolated
sclerenchymatous fibre or trachea removed from the organism is a dead and
useless structure. Only when these units are bound together into a concrete

252

METAMORPHOSIS

whole are they capable of performing their functions adequately, and only then
is it possible for the whole structure to grow and thrive. In this way we can
see that correlations become established among the various parts, one essential
result of division of labour. These correlations have, however, a much
greater influence on the general shape of the plant than we might conclude
from these remarks.

Numberless correlations make their appearance if members be isolated, if
leaves, branches, or roots of plants be separated off and prevented from rapid
decay by appropriate artificial means. The capacity for regeneration then
makes itself apparent, that is to say, the capacity of a member to reconstruct
the whole body by budding out the missing organs. The root can give rise to
shoots, the shoot to roots, the leaf to both shoot and root. The normal plant
gives us no hint of this power, and yet that power must have been latent in it ;
the inter-relationships of the members only must have prevented the individual
organs from exhibiting all the capacities which they possess. Owing to this cor-
relation the growth and formative power of the different members are regulated
and made subservient to the whole in such a way that the structural evolution
which we are accustomed to see in the plant proceeds harmoniously. What is
true of the root and shoot as a whole is true also of the individual cells. Num-
berless myriads of parenchymatous cells die off at last when they have reached
a certain stage in development and after they have lived a long time in that
stage. They can all be induced to form every possible kind of cell by inhibiting
correlations, and hence may be made to continue alive for indefinite periods.
If this subordination of cells did not exist in the multicellular plant each cell
would endeavour to develop as much as it could, and then we should have not
an organism but such a chaos run wild as to make existence impossible.

The flowering plants are perpetually altering their shape, and the organs
to which they give rise are not only so far adapted that they perform the
specific functions which enables them to carry out their structure, but also in
so far that they do not exhibit all the activities of which they are capable.
The entire life-cycle of the plant from the germination of the seed to the
formation of seed takes place under constant external conditions, so that we are
unable to refer these changes in shape to these factors with the same degree of
accuracy that we did in the case of Basidiobolus. Nevertheless, this life-cycle
is affected by external influences, and that, too, in a double sense.

Whenever a seed gives rise to a seedling the first thing it requires is a certain
amount of water ; that is self-evident, since we have seen that water is an
absolutely essential constituent of the living organism. Since the seed in a state
of rest is quite air-dry, the addition of water is necessary to awaken its activities.
If a member of a plant has accumulated a store of water during its resting
period it is able to start developing without any such addition. But not only
must water be absorbed, it must also be immediately available if growth is
to be effected, and the same is true for all other substances needed for this
purpose. They must be absorbed, or have been absorbed previously, and
in that respect every growth phenomenon in a plant is dependent on the
external world ; this, however, requires no further elucidation — it is self-evident.
The influence of temperature on development is not so obvious. Yet a glance
at re-awakening nature in spring time indicates to us what part temperature
plays in vegetative phenomena. Experiments confirm this and show that the
carrying out of every individual function in the plant is dependent on the
existence of a certain amount of heat. Beans first show evidence of growth
when the temperature reached 9° C. ; growth increases as the temperature rises
(up to 34° C.), and finally ceases when 46° C. is reached. Three cardinal points
of temperature, a minimum, optimum, and maximum, may be established
for all organisms, and the very diverse positions of these cardinal points indicate
the varied requirements of organisms as regards temperature, and at the same

INTRODUCTION

253

time demonstrate the great importance of this factor in determining the
geographical distribution of the plant. Since certain Algae have their optimum
about o° C., and certain Bacteria at from 60° to 70°, it is manifest that they
must necessarily be inhabitants of very different regions. This example shows
us that external factors are indispensable conditions of plant development,
and that, too, not in the case of the higher plants only but in the case of all
organisms. We term these * formal ' conditions of development to distinguish
them from special formative external influences which on closer examination
affect the higher plants also.

On examining plants of different families which live together under one or
other extreme, we find that they have one capacity in common, viz. that they
can adapt themselves to these extremes (compare VOLKENS, 1887, GOEBEL,
1889-93, i, 25 ; i, 149 ; 2, 3 ; KEENER, 1891, &c.). Thus desert plants, which have
difficulty in satisfying their requirements so far as water is concerned, exhibit
numerous adaptations for retarding transpiration ; their surface is limited in
extent by reduction in the size of the leaves, the function of assimilation being
undertaken by the stem (Cactaceae and Euphorbiaceae) ; further their cuticle
is thickened, their stomata are deeply seated and they cover themselves with
layers of wax or hairs. On the other hand, we find in them many arrangements
for bringing about a maximum absorption of water if such be available ; they
exhibit an especially extensive and deeply penetrating root-system.

In marked contrast to these ' xerophytes ' are the ' hydrophytes ' or
aquatic plants, especially the submerged types which we shall now consider
(compare ASKENASY, 1870 ; SCHENCK, 1886 ; GOEBEL, 1893, 2, 215). These
forms are capable of absorbing water by their entire surface and have nothing to
fear from loss by transpiration. Accordingly, the root in these plants is quite in
abeyance as a water-absorbing organ; the water-conducting tissue is feebly
developed ; the cuticle is thin and easily permeable and the mechanical tissue
is often entirely wanting. Submerged plants experience, however, difficulties
in gaseous exchange. They can obtain gases only from the surrounding water,
and thus, doubtless, we may account for the enormously increased leaf-surface
produced by the formation of numerous delicate projections. Roots and
rhizomes especially which are imbedded in mud must be supplied with the
necessary oxygen from the parts above, and hence may be explained the extra-
ordinary development of intercellular spaces which is a feature of all hydro-
phytes. Stomata, on the other hand, the normal apertures for gaseous exchange
in land plants, are wanting entirely in submerged forms.

These brief notes may suffice to show that plants adapt themselves to their
surroundings. We should be incorrect, however, in assuming that the special
shape of the hydrophyte was in any sense induced by the medium in which
it lives. We are acquainted with forms, the so-called amphibious plants, which
are capable of living both on the land and in the water. We may note especially,
in illustration, Polygonum amphibium, whose land and water forms differ remark-
ably from each other. When in water, the rhizome is long and obliquely ascending,
bearing several leaves with long petioles and heart-shaped, broadly lanceolate,
leathery blades floating on the water. The entire plant is smooth and glabrous.
When grown on land the stem is erect, the leaves are narrow lanceolate, quite
sessile, wrinkled, and partly hairy. The aquatic and terrestrial forms may exist,
however, concurrently as two branches of the same rhizome. The aquatic form
of Ranunculus aquatilis possesses extremely finely divided leaves and long
internodes, the land form has short internodes and broader leaf apices. The
anatomical differences between these leaves are especially striking ; those of
the land form are rigid, bear stomata, and have their assimilatory tissue dorsi-
ventrally arranged ; while those of the aquatic type are soft, have no stomata,
and have radially-arranged assimilatory tissue. Still in this plant also it is
possible by cultivating the terrestrial form in water to turn it into the aquatic type.

254 METAMORPHOSIS

Since it has been shown that here water itself has a formative influence on
the plant and induces adaptations directly, we are bound to conclude that in
other cases, as in aquatics which no longer have land forms, and in xerophytes
which do not alter their habit although in the presence of abundant water,
the direct effect of the medium does not exhibit itself in the life-cycle of the
individual, but has developed during the evolution of the species and has now
become permanent. We involuntarily reach the conclusion that species are
variable, but that many of their characteristics are hereditary adaptations.

Although the formative effect of the external world has been abundantly
proved we must not suppose that the plant reacts to all factors with purposeful
adaptations. We need advance only one example of such a reaction due to
external influences which does not appear to be of any service to the plant,
namely, the palmella form in Basidiobolus. It is often by no means easy to
determine whether a change in shape is to be considered as an adaptation or
as ' a product of a fortuitive mechanism ' (BERTHOLD, 1898) ; for, in keeping
with the views they hold on certain general questions, some botanists are inclined
to look for adaptations everywhere and to find them, whilst others are content
to discover merely the operation of a purposeless mechanism. In the latter
case they follow in the footsteps of physicists and chemists, and recognize in
the organism the selfsame forces which operate in the inorganic world. If we
look on all these changes as ' adaptations ', we have still the all-important
problem to solve : why does the plant react in a purposeful way ? It reminds
one of an organism possessed of intelligence, and the problem appears to be
beyond our power to solve.

We come now to a most important question, viz. whether results springing
from the operation of these forces in the organic world obey the same laws
which they do in the inorganic, or whether we have here to deal with quite
special relations. Before" attempting to discriminate between these two
alternatives let us glance at what we have learnt in this connexion from our
experimental treatment of the problem of plant formation. Wherever we look we
are forced to the conclusion that every change in an organism is a complex
process, which is due never to one solitary cause but to a large number of co-
operating factors. The phenomena are thus remarkably complicated, and when
we compare them with those of other sciences the probability of being able to
arrive at a mathematical-physical explanation of them is very slight. As every
one knows, Astronomy can calculate with the greatest exactness the path by
which a body in obedience to the law of gravity moves towards another ; if a
third body makes its appearance influencing the path of the first, the course
may still be determined empirically, though no longer strictly mathematically.
Glancing now at meteorological phenomena no one doubts that they obey simple
physical laws ; in principle, these are quite intelligible, but an explanation of
an individual case or an exact prediction of a meteorological condition is not
possible. If then in any science, only that may be considered as explained which
can be expressed in terms of mechanics, how may we dare to hope ever to arrive at
a physical explanation of life ? Still, as in the science of Meteorology, we may
succeed in reaching, at least, a knowledge of principles. In inanimate nature
alone there are plenty of phenomena which mock our attempts to refer them
to mechanical causes, e. g. the inherent characters of bodies. The peculiar
characteristics of an element are incomprehensible and inexplicable ; even more
inexplicable is the fact that compounds of these elements assume new charac-
ters not to be referred to combinations of those of the elements themselves.
It is impossible to affirm that the characteristics of living bodies are distinct in
principle from those of non-living ; all we can say is that we are equally de-
barred from a knowledge of those of either. Generally speaking, a mechanical
explanation of life is out of the question ; at most a physico-chemical
explanation is all we can hope for (ALBRECHT, 1901).

INTRODUCTION 255

Many of the phenomena which we have become acquainted with suggest
comparisons not only between organisms and complex conditions in the non-
living world, but also in another direction. We can distinguish internal and
external causes in vegetable phenomena ; only when these work in harmony is
development or any other activity possible (C. BERNARD, 1878). Take the
bean, for example. Germination takes place only if certain external condi-
tions be fulfilled ; there must be certain materials present in the medium in
which the development takes place, and also water and oxygen ; again, there
must be a certain temperature, and in the later stages, at least, sufficient
illumination. That co-operation of internal factors on the other hand is essential
is shown by the fact that identical external conditions can induce no develop-
ment in seeds which have died after prolonged keeping, but which are otherwise
unaltered, and further that bean plants always arise from bean seeds, while
from peas an entirely different type of plant arises. It would be an arbitrary
proceeding to assume that any one of these many causes is the chief factor in
the phenomenon concerned.

The activity of a piece of machinery is also dependent on the interaction
of internal and external factors. Its specific activity depends on the arrange-
ment of its constituent parts, and it is only when these parts are co-ordinated
in a systematic manner that it can perform its functions properly. But if the
machine is to do work the external conditions must also be fulfilled— in a steam-
engine, for example, when a certain pressure of steam acts on the piston. Hence
it is frequently the custom to compare an organism with a piece of mechanism,
and this comparison may be carried further when the significance of each in-
dividual factor in the performance of work is taken into account. In the plant,
as in the machine, we may distinguish certain factors which directly provide it
with energy to carry out the work, and others which may be looked upon as
merely liberating energies. The opening of the stop-cock which permits the
steam to enter the piston-box is a liberator of this type ; so is the pulling of the
trigger of a rifle. In neither case does the necessary pressure of the finger bear
any direct relation to the work done by the machine ; it only releases a pre-
existing energy and allows it to perform work. The work in the one case is
done by the expansion of steam, in the other, in the first instance, by the spring
of the rifle, and then by the explosive force of the powder. In the plant only
a few cases are known in which an external factor supplies directly the energy
required for the production of the result, e. g. the action of sunlight in carbon
assimilation, or the sugar in the nourishment of heterotrophic plants ; in the
large majority of cases the external factors merely liberate energy, that is to
say, act as ' stimuli ' (PFEFFER, 1893) — and the work is done by energies already
stored in the plant. It is very frequently the case in the plant that one released
movement releases another and so on, so that a whole series of reactions may
occur between the obvious primary release and the obvious final result, just
as in the case of the rifle between the pulling of the trigger and the impact
of the bullet on the target. The plant is, in a sense, ' loaded ' and ready to
transform its potential energy into kinetic whenever the necessary stimulus is
applied.

Another important similarity between an organism and a machine lies in
its power of self -regulation. Just as in a steam-engine an excessive speed is
reduced automatically by that very increase, so in an organism similar self-
regulating mechanisms occur ; compare in this relation what has been said as
to the production of diastase (p. 183).

Differences between organisms and machines are, however, not wanting.
We must take into account, in the first place, the much more complicated nature
of the organism, in which respect naturally there is no fundamental difficulty in
making a comparison. Although we have compared the organism with a loaded
rifle, still we must note that this comparison gives us but a feeble idea of the

256 METAMORPHOSIS

mode of life of an organism. We have only one act of releasing and one reaction
in a rifle, whilst in an organism we have numberless liberations of energy and
all sorts of reactions. We have a further more important difference to note,
viz. that one of the chief activities of an organic machine lies in its special
structure, its development and its reproduction, while we have still to find a machine
which can grow and reproduce. Finally, we know that the machine works to
achieve a certain end, being constructed for that very purpose, but we cannot
do more than form the vaguest guesses as to the purpose for which the organism
works.

To sum up ; the causes of life are as yet entirely unsolved, we are ignorant
both as to the materials and the forces which give to living things their charac-
teristics ; just as little are we able to prove that other materials and other forces
operate in the organisms than in the non-living world. Our position with
regard to biological research may be permitted to rest with this expression of
our ignorance, since the enunciation of hypotheses on questions so general as
these would only too easily do injury to Natural Science. He who believes that
the organic world is nothing more than a collection of complicated chemical and
physical processes can only do so by shutting his eyes to such phenomena as
cannot be fitted into his theory ; he who, on the contrary, once admits that
the vital characteristics of the organism begin where physics and chemistry end,
is content to abandon altogether the toilsome path of exact investigation, and
aim merely at collecting the most readily accessible results of speculation at
his study table. As to the possibility of reaching an explanation of vital
phenomena the following works dealing with the subject may be consulted : —
ALBRECHT (1901), BUTSCHLI (1901), CLAUSSEN (1901), DRIESCH (1901, [1905]),
HERTWIG (1897-8 [and 1905]), NAGELI (1860), REINKE (1901), WOLFF (1902).
[There can be no doubt, however, that physico-chemical experimental investi-
gation and not philosophical speculation has been the chief means of advancing
the science of plant physiology.]

From the examples considered above we can readily appreciate the kind
of questions we have to study in the lectures yet to follow. In the present
lecture we need only attempt to justify the allotment of a complete section
of physiology to the discussion of the ' form ' of the plant and to show that this
question of ' form ' may be contrasted in a certain sense with ' material ' and
with ' energy'. If we study the introduction to SACHS'S famous memoir, ' Ueber
Stoff und Form ' ( 1880), it would appear as though this statement were to a certain
extent subject to criticism. SACHS says, ' Plant morphology often suffers the
misfortune of being considered from the point of view of form without any
reference to its material characteristics '. * A survey of the material character-
istics of organs ' is, however, absolutely necessary ' since it is in these only that
the causes of their form are to be sought for\ ' Just as the form of a drop of water
or of a crystal is the result of the action of certain forces which bring the
material in question under the influence of its surroundings, so also the organic
form can only be the outward result of forces which transport those materials which
make themselves apparent in the plant substance.'

Valuable as are the opinions which SACHS has expressed in this treatise on
the subject of ' causal morphology ', we are nevertheless unable to agree entirely
with the sentiments expressed in the sentences quoted. We cannot find that
SACHS, or indeed any other author, has succeeded in referring the form of an
organ to its material characteristics, and, keeping before our eyes the phenomena
of non-living nature, we must confess that it is not probable that anything of
this kind is ever likely to be established. Numerous chemical compounds have
characteristic crystalline forms, and often these forms serve for the diagnosis
of different bodies. Still the same form may be possessed by different materials
and it would be in the highest degree dangerous to refer the leaf form, for
example, to one special but as yet unknown material, and still more dangerous

INTRODUCTION 257

to attribute the various types of form seen in leaves to differences in these
materials. But even if that were possible, we must still, as in mineralogy, treat
of the form of a plant by itself ; and even though we may be able to prove that
a definite form results from the presence of a definite material constitution still
we do not know why it is so ; why, for example, calcium oxalate crystallizes in
a tetragonal form when three molecules of water are present, and in the mono-
symmetrical form when there is only one, Nowadays, when an explanation of
form as due to chemical constitution is still quite impossible, it appears to us
that a special section dealing with ' metamorphosis ' is essential.

It is, perhaps, not possible to sharply separate off change of form
either from the second department of physiology, the transformation of energy
— more especially the phenomena of movement — or from morphology. How-
ever, the delimitation is of service from the practical point of view, since a large
series of vital phenomena, especially those connected with reproduction, heredity,
evolution of species, &c., may be most naturally classed along with other forma-
tive processes, while they cannot be treated of in a binary classification of the
subject nor be dealt with in an appendix. This, however, is of no consequence
so far as the plant is concerned.

In the following pages we have to investigate first of all how growth and
formation is carried out under constant external conditions (Lectures XXI-
XXIII) ; we shall thus learn to appreciate the mode of action of internal causes
of growth, although we shall not gain thereby any closer insight into their
nature ; at the same time we shall endeavour to form a conception of what
growth and formation really are. Thereafter we will devote ourselves to the
study of the influence of the most important external factors on growth
(Lectures XXIV-XXVI). In conclusion we will devote ourselves to the study
of the developmental life- cycle itself, which results from the simultaneous opera-
tion of both internal and external factors.

Bibliography to Lecture XX.

ALBRECHT. 1901 Biolog. Centrbl. 21, 97.

ASKENASY. 1870. Bot. Ztg. 28, 193.

BERNARD, CL. 1878. Lecons sur les phenomenes de la vie etc. Paris.

BERTHOLD. 1 898. Unters. zur Physiologic der pflanzl. Organisation, i . Leipzig.

BUTSCHLI. 1901. Mechanismus u. Vitalismus. Leipzig.

CLAUSSEN, J. 1901. Jahrb. d. hamburg. wiss. Anstalten, 18.

DRIESCH. 1901. Die organischen Regulationen. Leipzig.

[DRIESCH. 1905. Der Vitalismus als Geschichte und als Lehre. Leipzig.]

GOEBEL. 1889-93. Pflanzenbiolog. Schild. Marburg.

HERTWIG. 1897. Mechanik u. Biologic. Jena.

HERTWIG. 1898. Die Zelle u. die Gewebe, Vol. 2, Chap. 5. Jena.

[HERTWIG. 1906. Allg. Biologic. Jena. (Cited by Wolff, 2nd ed. 1905.)]

KERNER. 1891. Pflanzenleben. Leipzig and Vienna.

NAGELI. 1860. Beitr. z. wiss. Botanik, 2, 46.

PFEFFER. 1893. Die Reizbarkeit der Pflanzen. (Verhandl. d. Gesell. deut.

Naturf. u. Aerzte.)
RACIBORSKI. 1896. Flora, 82, 107.

REINKE. 1901. Einl. in die theoretische Biologic. Berlin.
SACHS. 1880. StofiE u. Form der Pflanzenorgane. Arb. Wiirzburger Instit. 3, 452.

(Ges. Abh. 2, 1159.)

SCHENCK, H. 1886. Biologic d. Wassergewachse. Bonn.
SCHWENDENER. 1874. Das mechanische Prinzip im anat. Bau d. Monocotylen.

Leipzig.

VOLKENS. 1887. Flora d. agypt.-arab. Wiiste. Berlin.
WOLFF, G. 1902. Mechanismus u. Vitalismus. Leipzig.

JOST S

258

LECTURE XXI
THE GROWTH OF THE CELL

THE most simply organized plants are unicellular, and the microscope
shows us also that in the most complicated forms cells and their derivatives
constitute the units out of which these plants are entirely built. The cell is
thus to be considered as the fundamental constructive unit of the plant (and
incidentally it may be mentioned of the animal also). Any attempt to in-
vestigate the growth and formation of the plant will thus naturally commence
with a study of cell-structure. We will assume that all the conditions necessary
for the development of the plant have been fulfilled and that the determining
external factors remain constant.

What do we understand by the terms ' growth ' and * formation ' ? It is
obvious in the first place that the growing cell increases in size, but not every
increase in size is a case of growth. If we place some seeds in water we at once
observe a rapid increase in size owing to the enlargement of the individual cells ;
but it is due merely to the introduction of water into the organic substance,
to a process which we have termed imbibition. If we place the swollen seeds
hi the air, the water evaporates and the seeds regain their original dimensions.
If we place the cell of an alga which has been plasmolysed by cane sugar (Lecture
II) in water, the cell increases in size owing to the absorption of water, but the
mode of absorption in this case differs from imbibition. Excess of water is taken
into the vacuole, but it induces no separation of the individual particles of the
wall and of the protoplasm. Generally speaking, this increase in size, owing
to turgor or imbibition, produces changes which are temporary, while in true
growth the changes are permanent. ' Growth ' is for the most part accompanied
by an increase in volume, but there are cases in which increase in one direction
is accompanied by a decrease in another. In the latter case, elongation can take
place without any change in volume ; still we may also speak of it as growth if
the change be permanent.

We might speak of ' formation ' equally well when the organism forms
cells and when the cell has a specific shape in relation to the life -conditions.
But so interpreted ' formation ' would be no subject for science. The
causes of cell-formation we cannot indeed ascertain because we have never
seen non-cellular organisms (SACHS uses the term * non-cellular ' in quite
another sense from this. ' Lectures on Plant Physiology ') — only on theoretical
grounds we suppose their pre-existence (or also present existence) and regard
them as the simpler precursors of the cell. But if in speaking of ' formation '
we think of changing of shape then we acquire a notion of some fertility.
A changing of shape may happen alike in swelling and in osmotic enlargement
as well as in growth. But all these processes might go on also without
a changing of shape, if, for instance, the body simply became larger without
any alteration in its proportions ; in this case we would not speak of a change
of shape.

We have already discussed the most important constituents of the cell
and their interrelationships elsewhere (Lecture I). Two of these only are of
interest to us at present, viz. the protoplasm and the cell-wall, and we will
attempt now to deal with their growth and form. We will begin with
the protoplasm, the essential living substance, and by far the most important so
far as our problem is concerned. Unfortunately, our knowledge of the growth of
protoplasm is very limited; we know little more than the mere fact that protoplasm
grows. It is possible to observe in many cells the actual increase of protoplasm

THE GROWTH OF THE CELL

259

under the microscope, doubling its amount in some circumstances in twenty
to thirty minutes, but we are quite ignorant how the protoplasm is formed
from the nutritive substances supplied to it ; we can only say that
we are dealing with a process of assimilation, or, more accurately, the
process of assimilation (compare DRIESCH, 1901). What we have hitherto
spoken of as ' assimilation ' has been the comparatively simple synthesis of
organic substances. The chloroplast synthesizes carbohydrates, but the carbo-
hydrates are no more like the chloroplast than the carbon-dioxide is ; they want
life, an essential characteristic of the chloroplast. We can only speak of assimila-
tion in the real sense of the term when the raw materials are transformed into a
living state, and this is what takes place when protoplasm grows, when, in other
words, new protoplasm is formed. The characteristic feature of the organism
is, more than any other, the way in which protoplasmic growth is accomplished.
When a crystal grows, it finds the same material already dissolved in the matrix;
the protoplasm, on the other hand, constructs itself out of substances unlike itself
but always as an addition to protoplasm there already. We are quite unable, at
present, to say how the process is carried out, since we are
quite ignorant what protoplasm itself really is.

The newly-formed protoplasm must in some way or
other be incorporated in that already present ; in a word it
must beadded to it or interpolated betweenits constituent parts.
To the question ' where does the protoplasm grow ? ' we
can give no definite answer, for it leads us at once to another
as yet unsolved problem, viz. the ultimate structure of the
protoplasm. The position we take up as to the various
theories of protoplasmic structure will determine what view
we take as to the mode of growth of protoplasm. It is
needless for us to follow out the various explanations which
have been given, since none of them have been generally
accepted ; nor do we by accepting any of them gain any
deeper insight into the nature of protoplasm. What applies
to protoplasm in general applies also to its members, the
nucleus and the chromatophores ; we see that they grow,
but we do not know where nor how.

We may omit any discussion of the formation of proto-
plasm since, for the most part, it has no definite form. It
is a viscous fluid, whose form is determined frequently by
external forces. It is only when its external layer has attained a firm con-
sistence that we may speak of it as having actually morphological formation.
The cases in which that condition is reached, however, lead back to others
where the protoplasm has no such characteristic.

We are much better, though still imperfectly, acquainted with the mode
of growth of the cell-wall. The chief difference between the protoplasm and the
cell-wall cannot be more clearly expressed than by saying that formation of
new protoplasm takes place only by addition to that already existing, whilst
a cell-wall may be formed where no cell-wall existed previously ; the forma-
tion of a cell-wall necessitates the presence of protoplasm but not of another
cell- wall ; the protoplasm makes itself, the cell- wall is made by the protoplasm.
This dependence of the cell-wall on the protoplasm is the first thing that strikes
us in a study of its earliest beginnings, and we may start with a consideration
of the initiation of the cell-wall (STRASBURGER, 1898). Swarmspore formation
takes place in many Algae and Fungi. In the simplest case (Fig. 48) the contents
of the cell retreat from the wall, and finally escape through a crack in the wall into
the surrounding water in which the free mass moves about in the water as a
nake d 'swarmspore '. After a certain time the movement ceases, the swarmspore

S 2

Fig. 48. Oedogonium.
A^ two cells whose
contents are in process
of transformation into
swarmspores ; JSt free
swarmspore ( x 350).
After PKINGSHEIM, from
the Bonn Textbook.

260

METAMORPHOSIS

fixes itself and forms a new cell-wall, in the form of an excretion on the outside
of the protoplasm. In other cases, also, the cell-wall arises by excretion, but
this mode of formation is not universal ; for it has been definitely shown that
in some cases the wall arises by transformation of protoplasm, i. e. by a direct
alteration of protoplasmic strands. When this method is adopted, carbohydrates
must be split off from the protoplasm and ttye nitrogenous remainder be with-
drawn, because the wall so formed exhibits the same characters as that formed
by excretion. Cell-walls which arise by this solidifying of the protoplasm must
have .an extremely complicated composition, and CORRENS (1898) has shown
that this is indeed the case. Cell-wall formation by excretion is much more
common than by transformation of the protoplasm.

Cell-wall formation may also be artificially induced. A new wall may
appear under certain conditions on the surface of the protoplasts of plasmolysed
cells, and, further, isolated protoplasmic particles (e. g. in the Siphoneae) may
be induced by mechanical means to form cell-walls.

The majority of cell- walls have, apart altogether from their mode of origin,
the power of growth ; they increase both superficially and in thickness. Surface-
growth takes place first, growth in thickness follows later and continues long
after surface-growth has ceased. Although these two processes in many cases

Fig. 40.. Diagram of apical
growth in a fungus hypha.
After REINHARDT (1802).

Fig. 50. Stellate parenchyma of Thalia
dealoata \ f, in the young state ; //, more fully
developed. After ZIMMERMANN (1893).

take place at the same time and affect each other, we will as far as possible treat
of them separately. Let us begin with superficial growth, which is of impor-
tance from the point of view of the configuration of the organism, since the shape
of the adult cell is determined by the superficial growth of its membrane. We
have already made ourselves acquainted with the temporary alterations in form
of cells produced by turgor only (stomata, p. 39), and we will recur to these
changes elsewhere.

Let us endeavour, in the first place, to gain some idea of the different
types of superficial growth. We know of only a few cases where superficial
growth takes place equally on all sides, where an increase takes place without
any alteration in form ; as examples we may cite approximately tetrahedral
pollen-grains and spores (compare Fig. 54, p. 264) and the cylindrical cells of
Hydrodictyon. Generally, only certain parts of the cell-wall grow superficially
and these parts may be distributed in various ways on or between parts which
show no such growth. Examples of such localized surface growth are to be
seen in the simple spherical cells of some Algae (Pleurococcaceae) which are
hemispherical in the young state and are hence limited on one side by a flattened
cell-membrane. This flat surface alone grows and transforms the hemisphere into
a complete sphere.' In many cylindrical cells also, e. g. in the Conjugatae, surface
growth is localized, for the cylindrical wall alone grows while the disk-shaped trans-
verse walls retain their original dimensions ; the cell-wall increases in extent
but not in thickness. In both the examples we have selected the greater part of
the cell -wall undergoes surface growth, but there are many cases known where

THE GROWTH OF THE CELL 261

growth is confined to one small part of the wall, and this part may be either
terminal or, indeed, in any other definite region of the cell. In the first case
we speak of the growth as apical, and then the growth is distinctly unilateral in
relation to the full grown part of the wall ; the other case we term intercalary
where interpolation of a new region takes place between two zones which are
already fully developed. Examples of apical growth are to be found in roothairs,
pollen-tubes, fungal hyphae, &c. (HABERLANDT, 1889, REINHARDT, 1892). Accord-
ing to REINHARDT growth is limited to the convex apex of the cell, to the minute
projecting region of the cylinder; further, growth is greatest at the extreme apex
and decreases gradually backwards. In Fig. 49 two stages in the growth of an apex
of a hypha are shown, and corresponding regions are indicated by dotted lines.
We see how much the surface cd has extended (to c'd'), and how little difference
there is between the surfaces ab and a'V. The best example of intercalary growth
is furnished by Oedogonium, where the intercalated region is marked 08 from
the older parts in the clearest possible manner. This example will be dealt
with in a greater detail later ; meanwhile reference may be made to Fig 51.

Another case of intercalary growth is illustrated at Fig. 50, which shows
two stages in the development of stellate parenchyma. The originally closely
applied walls of two cells separate from each other at several points, and inter-
cellular spaces, M, appear between them. We may observe thereafter that the
cell- wall exhibits further growth practically only opposite the intercellular spaces,
while the regions where the two cells are in contact (in Fig. 50, 7) scarcely grow
at all (Fig. 50, //).

Surface growth of the cell-wall, like its primitive formation, takes place
only in the presence of protoplasm and nucleus, and, as a rule, growth in the wall
occurs only where the protoplasm is closely applied to it. This close application
of the protoplasm is firmly maintained by osmotic pressure, while at the same
time, and due to the same cause, the membrane is kept tense. Owing to the
fact that this osmotically-induced tension was observed in the majority of
growing cell-walls, and because a certain relation had been noted between
pressure and intensity of growth, it was for long considered that osmotic pressure
played a mechanical part in surface growth, and the phenomena which occur in
artificially formed cells were compared with those in a state of nature. Artificial
cells (TRAUBE, 1867) may be readily produced by taking a little gelatine to
which sugar has been added and allowing it to exude from the end of a glass
pipette submerged in a weak solution of tannin. A precipitation membrane
makes its appearance at once on the surface of the drop, a membrane whose
characters we have already studied (Lecture II). It is very permeable to
water, but quite impermeable both to tannin and gelatine. Under these con-
ditions an osmotic pressure develops inside this membrane which stretches it.
One of two things may now happen, either a simple extension of the
particles of the membrane by the intercalation of the gelatine solution between
those of the membrane, or fine cracks may appear permitting the exposure of
the gelatine solution, which, as soon as it comes in contact with the tannin,
develops at once a new precipitation membrane of gelatine tannate. Obviously
since the formation of new parts takes place quite regularly between the old parts
of the wall, the artificial cell will assume the form of a sphere of considerable size.

Is there any likeness between the growth of the precipitation membrane
and that of the natural cell ? This question cannot be answered offhand. Of
course it is obvious that the membrane in the plant cell is not due to the pre-
cipitation of some product of the reaction of two fluids ; still osmotic pres-
sure might, for all that, play a mechanical part in surface growth. This has
indeed been accepted as true, and on this basis two theories have been advanced
to account for surface growth. According to one view, the cell-wall is simply
stretched mechanically by osmotic pressure and that too far beyond the limit

262

METAMORPHOSIS

of elasticity (' plastic growth '). In proportion as the surface increases the
thickness must decrease, and since in nature no such decrease in thickness can
as a rule be observed it must be assumed that simultaneously with this super-
ficial extension a deposition of new layers, or an increase in thickness from
within, takes place. This conception, when carefully studied, denies real sur-
face growth altogether, and recognizes only a passive stretching and deposition
on the membrane. In sharp contrast to this theory is the second, which
explains surface growth by assuming the intercalation of new materials between
those already existent ; osmotic pressure on this view acts mechanically by
pulling apart the finest particles of the cell-wall and thus favouring the inter-
polation of new particles between them.

These two opposing views have long been known as the theories of ' appo-
sition ' and ' intussusception ' respectively, and it is only recently that it has
come to be believed that both methods in all probability occur in nature. We will
illustrate this by reference to a few examples.

In a cell of Oedogonium which is about to divide we observe, near one end
and closely applied to the inside, a thickening of the wall in the form of a ring-like

pad, manifestly composed of two different chemi-
cal substances as seen in section (Fig. 51, 7). The
central mucilaginous region of the pad is the first
to be laid down (Schl), and is covered later by a
cellulose layer (C) which has the same characters
as the remainder of the cell-wall. Apparently,
owing to swelling of the mucilaginous core the old
cell-wall is torn asunder by a circular crack,
and the whole cell is in this way greatly
increased in length. The material forming
the pad becomes extended so as to form a
cylindrical uniting zone between the two older
regions of the cell-wall, in such a way that the
mucilaginous region forms the external and the
cellulose part the internal layer of the interca-

•••c

pad.

horisianum

I; c, the cellulose covering o? the lated piece. Contemporaneously with the elonga-
tion of this interpolated cylinder (Fig. 51, //, ///)
there is an obvious decrease in the thickness of the wall, and the whole process
gives one the impression that the stretching is purely passive and results from
the action of osmotic pressure. On this account Oedogonium has been usually
regarded as an example of 'plastic growth'. Closer investigation shows us, how-
ever, that the case is not quite so simple as it seems. The new membrane at
first is stretched in the longitudinal direction only, and the transverse diameter
of the cell remains unaltered or even decreases. Again, it is worthy of note that
the growth ceases when the intercalated portion has become quite thin.
If the entire growth of this portion depended only on plastic extension, one
would be led to expect that the stretching must continue, and all the more easily
the thinner the membrane became. If stretching be the factor in the case at all,
then changes in the elasticity of the membrane must occur during that process,
calculated to render the membrane gradually less extensible. Even of this
simple and apparently purely mechanical growth phenomenon we are far
from having arrived at a satisfactory explanation. It would seem to us as if
the physiological aspect of the case has not as yet received the attention it
deserves.

BERTHOLD has observed intercalary growth in other Algae also, which in
many respects suggests that seen in Oedogonium, e. g. in a member of the Con-
fervaceae. The cell structure of this alga is illustrated schematically at Fig. 52.
Each cell consists of two pieces, zr-shaped in longitudinal section, each becom-

THE GROWTH OF THE CELL

263

Fig. 52. Structure of the
cell-wall of Microspora
amoena. After KNUT BOHLIN
(1897, PI. I, Fig. 18). x 300.

ing thinner towards the middle of the cell and having these thinner regions
overlapping. The protoplasm next deposits a median band of membrane, thick
in the centre and thinning off towards either end. Union of the two consti-
tuents renders the wall of the cell of equal thickness throughout. When the cell
starts growing the overlapping borders of the external membrane separate
and the inner membrane comes up more and more to the surface of the cell-
filament. After a transverse wall has been formed this inner wall takes on the
x-form and new inner membranes appear in each daughter-cell. Similar
observations on other Algae have been made by many other investigators
(compare BERTHOLD, 1886, and KNUT BOHLIN, 1897). The difference between
this case and Oedogonium lies chiefly in the fact that the deposition layer does
not grow so suddenly in the present case, and hence the idea of purely me-
chanical stretching is excluded.

Phenomena similar in principle have been demonstrated as taking place
in apical as well as in intercalary growth. Thus SCHMITZ
(1880) and STRASBURGER (1882) have observed at the
growing apex of Bornetia secundiflora, one of the Flori-
deae, a special kind of lamination which is figured at Fig.
53. The growing point is made up of successive lamellae
of limited thickness which increase in superficial extent
and burst through similar older lamellae, which thus
become bevelled off at a certain distance from the
apex. NOLL (1887) successfully stained the cell-walls of Derbesia, Caulerpa
and other marine Algae with prussian-blue and thus differentiated these layers
from those succeeding them. These experiments make it certain that deposition
of new lamellae takes place at the growing point, by whose superficial growth
the older lamellae are burst. We may certainly assume from
a study of such observations that the separate lamellae then
exhibit no further growth where they have been torn through,
or do not, at least, grow as vigorously as the newly-
formed lamellae. The older lamellae are, doubtless, passively
stretched, but whether the young lamellae grow pas-
sively also cannot be decided from the experiment. If in
this and other similar cases the young cell-wall also suffers
passive stretching we should naturally attribute it to osmotic
activity ; but it is of importance to note that this osmotic
pressure is insufficient of itself to stretch the membrane plasti-
cally, since it is known that much greater pressures are unable
to extend the cell-wall beyond its limits of elasticity (PFEFFER,
1892, 241). Again, we never find membranes in the living cells which are
stretched beyond the limits of their elasticity. We may assume, however,
with NOLL (1895) that plastic stretching is possible without overstepping the
bounds of elasticity.. It should be remembered that a bent wooden bow
gradually unbends, a fact which can be due only to an internal concentric
layering. In this unbent condition, however, the bow at any moment can be
again elastically bent. Similarly, in the cell-wall stretched by turgor pressure,
an unbending and even a plastic extension might arise without the wall be-
coming thereby inelastic. It is often assumed that the protoplasm has an
effect on the elasticity of the membrane, but we have no grounds for such an
assumption.

It was for long believed that surface growth in the cell-wall arose only by
deposition of lamellae and plastic stretching of these by turgor pressure, but more
recently many arguments have been advanced against this view ; moreover,
there are individual cases which have been thoroughly examined, and to which
this explanation cannot be rightly applied. There is first of all the apical growth

Fig. 53. Lamination
in the apical cell of
Bornetia secundifloray
X 7<?. After SrRAS-
BURGER (1882, PI. IV,

264 METAMORPHOSIS

of fungalhyphae androothairs, studiedby HABERLANDT (1889), ZACHARIAS (1891),
and REINHARDT (1899). So long as these structures are allowed to grow. without
any interference, no deposition of new lamellae and bursting of older layers can
be observed in them. Such a negative result taken by itself certainly does not help
us much, but we may obtain an explanation of the mode of growth, such as has
been demonstrated in Bornetia and Derbesia, if the cessation of growth be studied.
Thus ZACHARIAS has shown that the roothairs of Chara, when placed in water
in which Chara has never been grown, at once cease to grow. When, after
a certain interval, growth begins again, one sees that the parts of the cell-wall
which previously had shown power of growth, have now lost that power and
are burst asunder by newly-deposited layers. REINHARDT observed the same
phenomenon in roothairs which had been plasmolysed. Although REINHARDT
perhaps goes too far in assuming that in Bornetia the bursting of a layer always
follows a previous interruption in growth, one is forced to admit that in the
cases studied by REINHARDT and ZACHARIAS, growth may be normally carried
on without successive deposition and bursting of lamellae. The burstings must
take place in these examples with quite remarkable frequency ; according to
REINHARDT at least once or twice a minute in the case of fungus cells. It is

scarcely possible that the mode of growth
can in this case be other than by intussus-
ception, i.e. by the interpolation of new
wall substance between the older mole-
cules. An observation of ZACHARIAS, fol-
lowing directly on NOLL'S experiments,
supports this view. He succeeded in intro-
ducing congo-red, under certain condi-
tions, into the membrane of the roothair
without occasioning an interruption in
growth, and was able to determine clearly

Fig. 54. Development of the megaspore of Sela- that the Coloured apCX gradually lost its

«£V£1* M? »™e JgffiAS colour duri"g surface growth while older

stages, similarly magnified (x 180). ex, exosporium ; portions of the Same Wall retained it. It

K^SS?^^1SS^Si^S!!^m was very apparent that in this case there

was no bursting of the coloured lamellae

as in NOLL'S experiments, but rather an interpolation of materials between
the minute older particles. This result admits of more than one interpretation,
and hence we cannot conclude with certainty that the mode of growth in
Caulerpa in NOLL'S experiments is conditioned only by the colouration with
prussian-blue and by artificial interruption of growth.

Further important evidence in favour of surface growth by intussusception
has been brought forward by ASKENASY (1890) and STRASBURGER (1889). It
is impossible to quote all these proofs at present, and we will, therefore, note
only a special case which has been recently drawn attention to by FITTING
(1900). The young spore of Selaginella possesses two chemically different mem-
branes, the exosporium and the mesosporium (Fig. 54, ex and mes) ; they are
separated from each other by a fluid containing very little solid material. As
the spore increases in size (Fig. 54, 1-III) these lamellae retain their relative
positions but increase correspondingly in surface growth and in thickness. In
an ordinary cell-wall the innermost lamella at all events can increase in surface
as a result of plastic stretching and simultaneous deposition of new layers from
the protoplasm ; the outer layers, however, must become thinner as a result
of plastic extension. In the spores of Selaginella, however, the inner lamellae are
quite debarred from any possible deposition of new layers, since the protoplasm
is contracted into a sphere within the mesosporium (Fig. 54, //), and touches
the membrane at most at one spot only. Between the protoplasm and the

THE GROWTH OF THE CELL 265

mesosporium there is a fluid from which a coagulable substance may be pre-
cipitated by alcohol (///, ger). The growing membrane is in contact with fluid
on both sides, from which it obtains the necessary nutrients and thus grows in
extent without being stretched by osmotic pressure.

We are doubtless dealing here with an extreme case, since in general the
membrane grows superficially only so long as it remains in contact with proto-
plasm ; when the protoplasm is withdrawn from the wall by plasmolysis it
tends to excrete a new membrane on its outer surface. Why the sporogenesis
in Selaginella and Isoetes shows deviations from the normal has not as yet been
explained, but that this difference exists cannot be doubted after FITTING'S
researches. The behaviour of the spore-walls in relation to osmotic pressure
is not so extreme, since it has been observed in other cases that a vigorous
surface growth can take place without an increase in turgidity, and that
growth in general may go on although turgor be reduced. Thus PFEFFER has
shown (1893) that the growth of the walls of the root-cells continues after osmotic
pressure has been more and more reduced by appropriate means, the cell-wall
finally becoming entirely, or almost entirely, contracted. In PFEFFER' s ex-
periments this retraction was effected by a sheath of plaster of Paris, but
KOLKWITZ (1896) showed that similar conditions may also occur in nature ; the
cells of the medulla of Helianthus, for example, exhibited growth at their ends
after they had been squeezed by the expansion of the vascular bundles.

Such observations render it probable that the part played by turgor in
these and other cases is not merely mechanical. To effect plastic stretching
an expansive force is certainly necessary, and this force must be that of turgor ;
turgor is, however, quite unnecessary for growth by intussusception. This
pressure is indeed quite insignificant in comparison with molecular forces, such
as that of crystallization for example. Thus crystals of calcium oxalate may
originate and grow in the walls of certain cells (PFEFFER, 1892, 250 ; MULLER,
1890), and these must have overcome the cohesion of the particles of the cell-
wall. Intussusception may therefore be best compared with phenomena of
this kind.

One would think there ought to be no difficulty in settling experimentally
the part played by turgor in this process. If we place a turgid cell in an osmoti-
cally active solution the expansion of the wall is thereby reduced ; the stoppage
of growth which follows is, however, by no means a purely physical result of
the reduction of tension, but is due to a complicated stimulus action. Further,
we may increase the tension of the wall by reducing the osmotic value of the
external medium (e. g. in marine Algae), but in this case also the alteration acts
as a stimulus and growth ceases in consequence. In both cases, growth after
a time recommences, but in the interval a readjustment of osmotic pressure takes
place ; in short, it is a matter of great difficulty to determine experimentally
the relations between growth and turgidity. Theoretically we may say that
turgor, though it does not act directly by supplying the energy necessary for
growth, is nevertheless necessary for it. It may be considered ' as a condition
of growth, just as in an analogous manner a certain degree of heat may be so
regarded ' (PFEFFER, 1892, 219). Growth, however, does not appear to be
proportional to turgor pressure ; in many cases turgor appears to be regulated
by growth rather than growth by turgor (COPELAND, 1896).

Concomitantly with or after surface growth of the membrane growth in
thickness takes place. Very frequently this may be readily shown to be due to
the deposition of new lamellae, in other words, to apposition. If it continues
after surface growth ceases this apposition results in a lessening, and, finally, in
partial occlusion of the lumen of the cell ; hence this type of growth takes place in
opposition to the pressure of the cell contents. We are acquainted, however,
with other well-established observations which prove that a marked increase in

266 METAMORPHOSIS

volume takes place in lamellae laid down by apposition, and also in cases where
these lamellae are separated from protoplasm by other layers. Growth in
thickness can thus be produced by intussusception also. We will content our-
selves with one example, viz. the cell -wall of Gloecapsa alpina, which has been
carefully investigated by NAGELI (1858) and CORRENS (1889). When the cells
of this alga divide each daughter-cell forms a new cell-wall of its own. The
original wall, however, now separated from the protoplasm by these new walls,
increases also both in thickness and in extent as shown in Fig. 55, //. CORRENS' s
measurements and arguments show that this growth is explicable only on the
assumption of an interpolation of organic material.

Not infrequently the greatly thickened membrane remains completely
homogeneous, but most usually we can make out a concentric lamination in it
which may be due to different causes (CORRENS, 1891). The successive layers
may be differentiated chemically, or only by the amount of water they contain. In
the latter case the lamination is due to the same cause as in starch grains ;
but the causal connexion is no more clearly explained in the one case than in
the other. We need not, however, enter in further detail here as to the struc-
ture and growth of starch grains, although these bodies have taken a foremost
place in all discussions on growth, and will always have a historic interest as
being the basis of NAGELI' s (1858) theory of intus-
susception. Thanks to the investigations of MEYER (1881
and 1895) and of SCHIMPER (1881) we now know that the
growth of a starch grain is effected by the external apposi-
tion of new material from without, and that the increase in
size of the grain takes place in the same way as in-
crease m s^ze °f a crystal or a spherocrystal. In
a certain sense starch grains are rather subjects for
crystallographical than physiological study. We say ' in
a certain sense ' intentionally, since although growth by
apposition has been definitely proved to occur in them
thick- the occurrence of intussusception also is not completely

excluded.

One point of similarity between the starch grain and the cell-wall must be
noted. Many cases in which we were in the past accustomed to assume growth
in the cell-wall by intussusception have now been shown to be due to apposition ;
similarly cases of increase occur due to supplementary intussusception, and
hence it was customary in any generalization to give ' apposition ' or ' intus-
susception ' as alternatives. At the time when there was a reaction against the
NAGELI-HOFMEISTER theory of intussusception one had become accustomed to
regard the cell-wall as a non-living structure, and to compare it with the shell of
a mollusc. HOFMEISTER (1867), on the other hand, held that the membrane was
alive and ascribed to it all the characters we now associate with protoplasm only.
When cases became known which militated against the universality of apposition,
and which rather suggested the activity of a living structure, attempts were
made to explain the situation by assuming an infiltration of the protoplasm
into the cell-wall, and so to account for its apparent vitality (WIESNER, 1886 ;
STRASBURGER, 1889). The existence of this hypothetical plasma in the wall
has never as yet been demonstrated (CORRENS, 1894), and so perhaps we may
come back once more to HOFMEISTER' s conception and regard the membrane
itself as living, although certainly not independent life as we understand it in
protoplasm, since there is no doubt whatever that protoplasm may grow without
a cell-wall, but not a cell-wall without protoplasm. But the membrane is
perhaps alive in so far as it can exhibit the power of ' assimilation in the real
sense of the word ' (p. 259), being able itself to form new wall material out of
definite but as yet unknown substances provided by the protoplasm. If this be

THE GROWTH OF THE CELL 267

so we should have a living substance which contained no nitrogen. It is un-
necessary for us to trace any further the consequences of the acceptance of this
hypothesis ; it will be preferable to bring forward an example where we may most
readily consider such an active formative power in the cell-wall, viz. the bands,
spines, and other sculpturings formed on the surface of spores and pollen-grains
(WiLLE, 1886 ; STRASBURGER, 1889). Here we have to deal not only with a
simple thickening of a previously existent cell-wall, but with differential thicken-
ing at special places, resulting in a configuration fixed by heredity. We meet with
such a case in the exosporium of Selaginella galeotti (FITTING, 1900), after the
exosporium has been separated first from the mesosporium and later on from
the protoplasm. The extent to which the protoplasm has retracted renders the
hypothesis of migration of protoplasm into the wall, in this case, especially unlikely.

We have dwelt too long already on the question of the growth of the cell-
wall, although we have left unmentioned many questions which call for in-
vestigation, but we may refer, before leaving the subject, to one further point
only, the cessation of growth. If we limit ourselves to surface growth, we may
distinguish cells which theoretically grow on for ever, unless destroyed, from
those which continue to grow only for a definite time (p. 273). The question as
to what brings about this cessation of growth has been very variously answered.
Looking only at the great thickness and varying chemical characters of the
fully-developed cell-wall attempts have been made to give a purely mechanical
explanation of how such a wall has no further power of growth. As a matter of
fact, however, in the first beginnings of lateral branches, and also after
wounds, completely adult walls may start growth afresh, and we often see
very thick- walled cells still capable of growth (KRABBE, 1887), while thin-walled
cells do not show that power. The directing influence inducing or stopping growth
must arise from the protoplasm, for all growth phenomena are regulated by the
living organism. Such regulating processes meet us at every turn, wherever
growth and formation in plants are closely examined. It is impossible to say
with certainty whether definite organs in the cell, the nucleus more especially,
play any special part in this phenomenon. We know for certain (TOWNSEND,
1897) that the protoplasmic complex can manufacture a membrane only when it
is provided with a nucleus, or when it is connected by protoplasmic strands —
however delicate — with protoplasm which contains one. It does not follow,
however, that cell-wall formation is a special function of the nucleus, for the
nucleus is no more able to form a cell-wall without protoplasm than protoplasm
without a nucleus. HABERLANDT (1887) has pointed out that the nucleus
frequently approaches the region of the wall which is actively growing, and he
concludes from that that the nucleus has some special duties to perform in
relation to the formation of cellulose. At the same time cases are known in
which the nucleus obviously occupies other situations, and these militate against
the correctness of this conception.

Reviewing the whole position we may say that the growth of the cell-wall
takes place in a variety of ways. All theories which involve the acceptance of
only one mode of growth must be rejected ; for even in those cases where growth
appears purely mechanical we cannot do without the complicated and invisible
influence of protoplasm. Protoplasm administers the stimulus which induces
both construction and growth of the cell-wall and it determines finally when they
shall cease. Protoplasm determines the degree of osmotic pressure, which must
decrease with every increase in the size of the cell unless the osmotically active
material be renewed, and thus also influences the capacity for extension of the
cell- wall. Further, although we may speak of the cell- wall in individual cases, or
in general, as possessing life, still that does not alter its dependence on protoplasm.

Growth of the cell is associated frequently but not always with cell division.
There are plants, such as the Siphoneae and Mucorinae, which attain to quite

268

METAMORPHOSIS

considerable dimensions and complicated form, and which still possess the
characters of a single cell ; but the great majority of cells divide after reaching
a certain size. In a typical case this takes place in the following manner. The
first alteration to be observed is in the nucleus. In consequence of complex
internal processes, shown in Fig. 56, part of the chromatin first of all forms a much
twisted thread (2), which afterwards breaks up into a definite number of seg-
ments, the so-called chromosomes (_?, 4). In plants there are usually several such
chromosomes, which after taking on a U-shape, aggregate at the equator of the
nucleus. Each chromosome then splits longitudinally (y, 6), and the halves so
formed separate from each other in opposite directions (8, 9). Each group then
exhibits a fusing of the chromosomes into a network similar to that present in
the original nucleus (10-12). Thus there arise two daughter-nuclei separated

Fig- 56. Mitosis and cell division in embryonic tissue. Somewhat diagrammatic. «, nucleolus ; />, polar
plates ; w, nuclear wall ; c/tt chromosomes ; j, spindle fibres (x 600). From the Bonn Textbook.

from each other by an interval. The stages 1-6 in the figure are collectively
spoken of as the prophase, 7-9 as the metaphase, 10-12 as the anaphase. In the
stage represented at Fig. 56,4 we observe a system of fine threads stretching from
one ' pole ' of the nucleus to the other. They arise either from the ground sub-
stance of the nucleus (ZACH ARIAS, 1888) or, according to many authors, they
enter into the nucleus from the surrounding cytoplasm (STRASBURGER, 1888).
Certain of these spindle threads (' contractile threads '), by their contraction,
pull the chromosomes towards the poles (Fig. 57), the remainder persist as
connecting links between the two daughter-nuclei and form a basis for the
formation of the cell-wall. In the equatorial plane of the division figure (Fig.
56, /o, //) there arise thickenings on the spindle threads, so that the individual
filaments approximate laterally at this point and forma protoplasmic wall (12),
the so-called cell or equatorial plate, which cuts the spindle medianly in two.
The cell-plate splits afterwards into two lamellae between which the cell-wall is
excreted (STRASBURGER, 1898).

THE GROWTH OF THE CELL

269

If now, as is often observable, the nuclear spindle broadens, so that it
occupies the whole diameter of the cell, the new cell-wall placed at right angles
to the old membrane divides the cell at once into two halves. A ' simultaneous '
cell-wall formation of this sort occurs especially in narrow cells. It is not con-
fined to such however, but occurs also in broad cells, when the nuclear spindle
extends across the whole cavity of the cell (Fig. 56, 12).
In addition to the simultaneous type we also not infre-
quently meet with a gradual or ' succedaneous ' forma-
tion of the cell-wall. The new wall, as in Spirogyra.,
may extend inwards from the mother cell -wall in the
form of a plate or ridge, with a central aperture which
gradually becomes contracted (Fig. 58), or the forma-
tion of the wall may commence at a definite spot on
the wall of the mother cell and slowly grow across
it (Fig. 59). In this latter case the nuclear spindle
moves forward as the new cell-wall gradually crosses
the mother-cell. After the wall has been formed, the
remainder of the nuclear spindle left over in the two
daughter-cells disappears. In a few cases, such as
that of Oedogonium, the complete wall is formed
within the cell and unites later with the older cell-wall
(compare HIRN, 1900) ; as a rule, however, the wall is TexTbwk.')
built out from the spot where it originates.

The division of the mother-cell takes place in a quite definite manner, and
the laws governing cell division were first, at least in part, elucidated by HOF-
MEISTER (1867) and SACHS (1878-9), but the sequences were worked out more
accurately by ERRERA (1886) and BERTHOLD (1886). Both authors found that

Fig. 57. Diagram to illustrate
the separation of the chromosomes.
2", spindle threads ; a and 6, the
two longitudinal halves of the
chromosome. (From the Bonn

Fig. 58. Division of a cell of Spirogyra. «, daugliter-
nucleus; w, partition wall ; ch^ chloroplast (x 230).
From the Bonn Textbook.

Fig. 59. Cells of Epipactis palustris in three
successive stages of division. After TREUB, from
the Bonn Textbook (x 365).

in the majority of cases the new walls were placed across the old ones exactly
as would be the case with a soap film. The laws of arrangement of liquid lamellae
have been closely studied and we know that these lamellae always show what
are termed ' minimum surfaces '. If a soap film be stretched diagonally across
a cube (Fig. 60, /) it will shift its position until it has attained a minimum area,
that is to say, in this case, until it has divided the cube into two parallelopids
(2). If we extend a partition in the cube quite close to one of the walls and
parallel to it, it shifts itself and bends until it has cut off a segment of the cube
(Fig. 60,^). Similarly in the plant-cell partition walls appear, sometimes straight,
sometimes curved, but to study the details of these would take us too far. As we
have already said, they exhibit in the majority of cases minimum surfaces, save

270

METAMORPHOSIS

that exceptions are known, as, for example, in cells which divide longitudinally
when one would expect transverse division (e. g. cambium cells).

The occurrence of cell-walls as surfaces of * minimum area ' might be ex-
plained most simply by assuming that the freshly formed wall is in a liquid con-
dition. ERRERA (1886) is responsible for this view, though observation would
lead us to an opposite conclusion. More recently, WILDEMAN (1893) has suggested
that it is sufficient if the cell-wall be in a liquid condition for one moment only and
then becomes rigid. Such a hypothesis cannot be denied, at least in the case of
simultaneous cell-wall formation, but it does not lend itself to the explanation
of the succedaneous type. There, undoubtedly, one part of the wall already existed

Fig. 60. After BERTHOLD (Protoplasma mechanik).

in the solid state, while the rest is not even deposited. What is it, then, that makes
the cell- wall grow on in a definite direction ? WILDEMAN has shown that even a
liquid lamella may increase by succedaneous accretions. If one places a soap film
within a rectangle made of iron wire (Fig. 61, /) into which runs a silk thread
attached at a and b, the ends of which pass through a straw suspended in the
soap film, and if the lamella be pierced between a and b and the threads, one
obtains the condition represented at Fig. 61, 2. If now the ends of the thread
be pulled (as indicated by the arrows in the figure) the soap film gradually grows
from right to left till it fills the whole rectangle. There are
great difficulties, however, in applying this experiment to
dividing cells. What corresponds in the cell to the silk
threads and to the frame ? No answer can be given to this
question ; but without the threads and the framework no
succedaneous growth of the lamella is possible.

Although at first sight one is tempted to refer cell
division to the laws governing the equilibrium of liquid
membranes, still it is not possible for us to accept this
explanation. We must content ourselves with pointing
out the great similarity between these two sets of phe-
DE WILDE- nomena and confessing that the reasons for this comparison

are still far from apparent.
After the sketch we have now given of cell division it will be obvious that
we must attribute very important functions in that process to the nucleus, and
especially to the phenomena of mitosis. We must not, however, overrate their
importance, for we must remember that quite normal cell division may take
place without mitosis, and indeed without segmentation of the nucleus at all.
NATHANSOHN'S (1900) and WASILIEWSKI'S (1902) researches show that by em-
ploying certain anaesthetics (ether, chloral hydrate) the normal nuclear division
(mitosis) taking place in the cells of Spirogyra and of the root apex of Vicia faba
may be transformed into the so-called direct nuclear division, where the
nucleus divides into two parts without any formation of chromosomes or nuclear
spindle and between these two nuclei a partition wall is formed. [WissELiNGH
(1903, Bot. Ztg., 61, 201) has raised objections to this interpretation of NATHAN-
SOHN'S discoveries. The controversy between these authors (Bot. Ztg., 1904, 62,
II, 17) suggests the necessity for a re-investigation of the whole question. Doubt

Fig. 61.
MAN'S experiment.

THE GROWTH OF THE CELL 271

has also been thrown on WASILIEWSKI'S work by the experiments of N&MEC
(1904, Jahrb. f. wiss. Bot. 39, 645). At present it cannot be affirmed with
certainty whether ' amitosis ' does take place under the conditions mentioned
above; still less certain are we of the results contingent on it.] The daughter-
cells, which are produced in this way, behave normally and divide mitotically
later on.

Cell division also occurs without any visible bi-partition of the nucleus.
This is the case, for instance, in the multi-nucleate cells of Cladophora ; division
takes place as in the case of Spirogyra, but the partition wall is not formed on
a nuclear spindle. The cells of Cyanophyceae and Bacteria also exhibit normal
division, yet they appear generally to contain no nuclei. Further, GERAS-
SIMOFF (1899) succeeded in so altering the division process in Spirogyra that one
of the daughter-cells contained two nuclei, the other none ; and although in this
case also the transverse wall could not be formed on the spindle still it showed
no difference in appearance to the normal wall. GERASSIMOFF (1901) also suc-
ceeded in some experiments in so influencing the cells during division that the
division of the nucleus was suppressed while cell division still went on ; one
daughter-cell thus had one nucleus while the other had none. The cell contain-
ing the nucleus and its descendants were distinguished for a long time by their
remarkable size, and attained a great length before proceeding to divide afresh.
The nucleus has also a certain influence on the specific amount of division,
which depends also on many internal and external factors. We cannot go
into these here and will content ourselves with noting that one internal factor,
viz. the function of the cell, influences in a remarkable way the amount of division
it undergoes. The varied size of different cells in the complex plant depends
on this fact ; but the same phenomenon may be observed in quite simple
organisms also. Compare, for example, the difference in size between the
ordinary vegetative cells and the sexual (male) cells in Oedogonium. In spite
of its variability the amount of division in cells is in general a character of
the species. Thus cells as small as those which occur in Bacteria are unknown
in Phanerogams ; cells as large as those of Caulerpa, are found in flowering
plants only in isolated positions and adapted to carrying out special functions
(laticiferous tubes).

When a cell has reached a certain size it divides, and so attains once more
its normal size, and division into two as a rule leads to this result. Often, how-
ever, a formation of many cells takes place, four or even more cells being formed
from one. To give illustrations of this phenomenon and to describe the arrange-
ment of the walls in such cases would lead us too far, and so we may merely
refer to detailed descriptions of cell morphology.

Although in general a certain amount of growth precedes cell division still
this is not a universal condition of that division. Certain embryonic cells divide
into numerous small cells without exhibiting any actual growth ; thus the ovum
of Fucus, the spores of many Hepaticae and Fungi, the segments of the apical
cell of Stypocaulon are illustrations in point, but of these we shall speak in our
next lecture.

Bibliography to Lecture XXI.

ASKENASY. 1890. Ber. d. bot. Gesell. 8, 61.

BERTHOLD. 1886. Studien iiber Protoplasmamechanik. Leipzig.

BOHLIN, K. 1897. Bihang svenska Vet. Akad. Handl. 23.

COPELAND. 1896. Einfl. d. Temperatur und des Lichtes auf den Turgor. Diss.

Halle.

CORRENS. 1889. Flora, 72, 298.
CORRENS. 1891. Jahrb. f. wiss. Bot. 23, 254.
CORRENS. 1894. Ibid., 26, 587.
CORRENS. 1898. Bot. Ztg. 56, II. Abt. Sp. 221.

272 METAMORPHOSIS

DRIESCH. 1901. Die organischen Regulationen. Leipzig.

ERRERA. 1886. Bot. Centrbl. 34, 395.

FITTING. 1900. Bot. Ztg. 58, 107.

GERASSIMOFF. 1899. Bullet. Soc. d. Naturalist, de Moscou, 1899, 220.

GERASSIMOFF. 1901. Ibid. 1901, 185.

HABERLANDT. 1887. Funktion u. Lage des Zellkernes. Jena.

HABERLANDT. 1889. Oestr. Bot. Ztg. (Nr. 3).

HIRN. 1900. Monographic u. Ikonographie der Oedogoniaceen. Acta Soc. scient.

Fenn. 27.

HOFMEISTER. 1867. Die Lehre v. d. Pflanzenzelle. Leipzig.
KOLKWITZ. 1896. Fiinfstuck's Beitr. z. wiss. Bot. i, 246.
KRABBE. 1887. Jahrb. f. wiss. Bot. 18, 346.
MEYER, ARTHUR. 1881. Bot. Ztg. 39, 841.

MEYER, ARTHUR. 1895. Untersuchungen iiber die Starkekorner. Jena.
MULLER. 1890. Entstehung von Kalkoxalatkristallen in Zellmembranen. Diss.

Leipzig.

NAGELI. 1858. Pflanzenphys. Unters. 2. Die Starkekorner, p. 281.
NATHANSOHN. 1900. Jahrb. f. wiss. Bot. 35, 48.
NOLL. 1887. Abh. d. Senkenbergschen Gesell. 15, 101.
NOLL. 1895. Flora 81, 65.

PFEFFER. 1892. Abh. math.-phys. Cl. K. Sachs. Gesell. d. Wiss. 18.
PFEFFER. 1893. Druck u. Arbeitsleistung (Abh. math.-phys. Cl. K. Sachs. Gesell.

d. Wiss. 20).

REINHARDT. 1892. Jahrb. f. wiss. Bot. 23, 479.
REINHARDT. 1899. Festschr. fur Schwendener. Berlin.
SACHS. 1878-9. Arb. bot. Instit. Wiirzburg, 2, 46 ; 2, 185.
SCHIMPER. 1 88 1. Bot. Ztg. 39, 185.

SCHMITZ. 1880. Verhandl. naturw. Verein d. Rheinlande, 36.
STRASBURGER. 1888. Kern- u. Zellteilung im Pflanzenreich. Jena.
STRASBURGER. 1882. Ueber den Bau u. d. Wachstum der Zellhaute, p. 189. Jena.
STRASBURGER. 1889. Histolog. Beitr. Heft 2. Jena.
STRASBURGER. 1898. Jahrb. f. wiss. Bot. 31, 511.
TOWNSEND. 1897. Jahrb. f. wiss. Bot. 30, 484.
TRAUBE. 1 867. Archiv f . Anat. u. Phys. p. 87.
WASILIEWSKI. 1902. Jahrb. f. wiss. Bot. 38, 377.
WIESNER. 1886. Sitzungsber. Wiener Akad. math.-nat. Klasse, 93.
WIESNER. 1 892. Die Elementarstruktur u. das Wachstum d. leb. Substanz. Wien.
WILDEMAN. 1893. Mem. couron. par 1'acad. Belg., 4to, 53.
WILLE. 1886. Ueber d. Entw. der Pollenkorner d. Angiospermen. Christiania.
ZACHARIAS. 1888. Bot. Ztg. 46, 33.
ZACHARIAS. 1891. Flora, 74, 466.
ZIMMERMANN. 1893. Beitr. z. Morph. d. Pflanzenzelle, i, 209.

LECTURE XXII
THE GROWING POINT

THE daughter-cells which arise by cell division may either separate from
each other or they may remain connected. If they separate, the plant in
question is unicellular in the truest sense of the word, in the other case cell
aggregates arise in the form of cell filaments, cell plates and cell bodies according
as growth takes place in one, two or three dimensions of space. If all the cells
of such an aggregate are like each other, and each cell is physiologically quite
independent, the distinction between those which take the form of such aggre-
gates (colonies) and those which are strictly unicellular is not well marked,
and numerous transitions between these conditions are to be found ; indeed
one and the same plant may, according to external conditions, exhibit a uni-
cellular form or a colonial form.

The two sister-cells are not, however, always morphologically and physio-

THE GROWING POINT

273

H'N'I-)

Fig. 62. Diagram of a cell filament with
an apical cell, s.

logically alike ; on the contrary, we meet with greater and greater differences
between the individual cells as we ascend in the organic scale, and these differ-
ences are indicative of their physiological activities. Every cell has no longer
the same function ; division of labour has appeared, and," as a consequence,
the individual cells have lost their physiological individuality and have
become mutually dependent on each other ; in a word, they are no longer
capable of individual and isolated existence and can carry out their functions
only when united into a ' differentiated ' com-
plex. The first of these differentiations is the
separation of a series of cells capable of con-
tinued growth and division from cells which,
having reached the adult state, are in a resting
condition, and since these merismatic cells are
not distributed irregularly among the resting
cells, but occur in the simplest case at one end of the plant body, it comes about
that this primary differentiation results in the formation in the plant of two
poles, a base and an apex. Let us study, as a simple example, a cell filament,
composed of a row of cylindrical cells, a, b, c, &c., all fully developed. The
apex is occupied by the only cell capable of further growth (the apical cell s),
differing in form from the rest. When the apical cell has reached a certain length
it divides, and the daughter - cell
d, lying next to c, becomes a resting
cell while the other, s', remains as an
apical cell, and so the process goes
on indefinitely. We may term the
apical cells embryonic cells, and the
cells derived from them somatic
cells. Hence we may describe the
higher plants which exhibit this dif-
ferentiation as somatophytes, whilst
the lower forms, which possess no
fully developed region, i.e. no
'soma', may be termed asomato-
phytes (PFEFFER, Phys. II, § 2).
Although the transition from em-
bryonic to somatic, or fully deve-
loped, cells takes place gradually,
still the contrast between them is
sufficiently distinct. It will be better
to speak in this case not of cells but
of 'substance', embryonic substance
and somatic substance, for the em-
bryonic substance need not be con-
fined to one cell, as in the example
we have taken, but may exist in
many cells or may be limited to
a part of one cell. The regions where
the embryonic substance is found in a plant may be styled, following the older
nomenclature, the 'growing points'. The growing point, however, does not
always lie at the free apex of a plant body ; it may occur at the base or between
two somatic regions. We have, therefore, to distinguish terminal, basal, and
intercalary growing points.

In the simplest case the entire activity of the growing point is devoted
to the elongation of a previously existing body, but when the plant is branched
then the growing point has also to provide for the production of these outgrowths.

Fig. 63. Dichotomy in the growing point of Dictypta.
a, apical cell (x 500). After DE WILDEMAN in the Bonn
Textbook.

JOST

274

METAMORPHOSIS

The formation of branches at the growing point may take place in one or
other of two ways. In many plants two branches are formed at the same time ;
the previous direction of growth is abandoned, and instead of the prolongation
of the plant body in a straight line a forking or dichotomy appears. The alga
Dictyota may be cited as an example of this type. Here the dichotomy is
effected by the longitudinal segmentation of the apical cell (a in Fig. 63).
Similar cases of dichotomy are found in many liverworts ; in higher plants,
on the other hand, this method is seldom met with (shoots and roots of Lyco-
pqdium and Selaginella). In the highest plants generally, and also very fre-
quently in the lower plants, another type, viz. lateral branching occurs, that is
to say, in addition to the elongation of the body in a straight line, there are
formed also lateral projections on the growing point, so that we may distinguish
an axis and its lateral branches. As a rule, lateral branches are formed not once
only or one at a time on the growing point, but they occur for the most part
in great numbers continuously or at periodic intervals, and their developmental
succession is quite definite, in fact, progressive. The youngest are nearest to
the growing point, and the further we get from it the older do we find the
lateral branches to be. According as the growing point is situated at the fixed

Fi{*. 64. Arthrocladia vfllosa, showing a basal
growing point v ; i, 2, 3, &c., indicate the basipetal
development of the lateral branches. After FALKEN-
BERG (1882, Art. Algae in SCHENK'S Handbuch der
Botanik).

Fig. 65. Dasycladiisclavaeformis.
Growing point with three whorls of
branches. After CRAMER (1887)
(x 40).

base or the free apex of the axis, the progressive development is basipetal or
acropetal. Fig. 64 shows an example of basipetal growth, while the more
ordinary acropetal development is illustrated at Figs. 65 and 66.

Definite relations of symmetry, purely geometrical in character, exist in
the arrangement of the lateral organs at the growing point, which we must
study more carefully. We are able to distinguish radial, bilateral, and dorsi-
ventral growing points. In radial symmetry we find three or more longitudinal
regions exactly like each other, in bilateral symmetry there are only two such, in
dorsiventral symmetry only one plane.

To take an example, Fig. 65 shows us a radially symmetrical growing point
of Dasycladus, a member of the Siphonaceae. This seaweed consists of one
long cell fixed at the base and elongating at its apex. The lateral appendages
are arranged in regular whorls on cask-like swellings of the axis. Immediately
below the apex of the growing point arises a whorl of fourteen branches, and
at a certain distance behind are two successively older whorls each composed
of the same number of branches, whose bases only are shown in the figure.
The successive whorls alternate (NoLL, 1896), that is to say, each of the branches
of the whorl above stands over the gap between each pair of branches of the
whorl next below. We find this alternation of branches to be the general rule
in whorled arrangements of lateral organs, whether we are dealing with simple
projections from a single cell, as in Dasycladus, or with complex organs such as

THE GROWING POINT

275

the leaves of higher plants. Interruptions in the alternation are (as in Dasy-
cladus), however, always found if the number of the branches in the whorl be
altered and an increase takes place in proportion to the vigour of the plant.
The whorls in Dasycladus are especially worthy of notice in this relation because
they are laid down at a considerable distance from each other. The case figured
at Fig. 66 is much commoner ; here also the whorls alternate, but they are crowded
so closely together that the lateral branches in successive whorls are in actual
contact with each other.

Another type of arrangement is met with when we pass from the 'whorled '
to the ' spiral ' distribution of lateral members. The characteristic feature of
this type is that at any definite level on the axis only one lateral branch is
developed, not two or more. We will select a very simple case and represent
it diagrammatically. In Fig. 67, /, the bases of the leaves are represented in the
usual way, so as to show their relation to each other and to the central axis.
The lowest leaf is indicated by the figure /, the highest by 6. It is easily seen
that the leaves lie in five longitudinal planes (' orthostichies '), and that, by pro-
ceeding from leaf / to the next, one passes round two-fifths of the circumference
of the axis. Looked at sideways, as shown in Fig. 67, 2, one sees that the leaves
may be united by a spiral line (the so-called ' fundamental spiral ') and that

Fig. 66. Growing point of Hippuris vul-
garis. After SACHS (Vorlesungen iiber
Pflanzenphysiologie, ist ed., Fig. 307).

Fig. 67. Diagram of a two-fifths spiral,
plan ; 2, seen laterally.

in ground

leaf 6 stands exactly above /, 7 above 2, and soon. This very common arrange-
ment is described as a ' two-fifths spiral ' and from this description it will be
readily understood what is meant by a one-third, three-eighths, or a five-thir-
teenths spiral respectively. In simple arrangements, such as the one-third or
two-fifths spiral, the numbers indicate for the most part the order of development
of the members. It is possible even in such complicated cases as, for example,
the succession of the flowers in the inflorescence of the sunflower, to number the
lateral members in accordance with certain geometrical rules, and from the
numbers to construct a ' fundamental spiral.' In the latter case, however, this
has no significance, since we are not entitled to assume that the development of
the branches takes place in the order in which the numbers follow each other.
The formation of the lateral members frequently takes place more rapidly on
one side of the growing point than on the other, and the successive appearance
of the young branches is not indicated at all by the position of the member
preceding it in the numerical series but arising far from it. The position of
the new member depends, for the most part, far more on its immediate neigh-
bours, and hence ' oblique lines ' (parastichies) are determinable, of which
there are often in Helianthus fifty-five in one direction and eighty-nine in
the other, so long as growth is regular. Their regularity disappears, however,
if the space relationships at the growing point alter, and if the relation between
the diameter of the lateral axes and that of the growing point varies (com-
pare SCHWENDENER, 1878 ; HOFMEISTER, l868).

It is not possible for us to go further into the question of the arrangement

T 2

276

METAMORPHOSIS

of lateral branches ; on that subject we must refer to the special literature
concerned (BRAVAIS, 1837-9 ; A. BRAUN, 1831 ; HOFMEISTER, 1868 ; SCHWEN-
DENER, 1878 ; GOEBEL, 1898), and merely point out that the general distri-
bution of the lateral members in the examples selected above is uniform on all
sides, so that we have before us a radially arranged growing point. As an
example of a bilateral growing point we may take that of Caulerpa holmesiana,
a unicellular alga (Fig. 68). The lateral buds arise in this case on two sides
only, 180° apart, while the sides above and below, parallel with the plane of the
drawing, show no outgrowths. If we turn the body round through an angle of
90° we obtain an appearance like that shown at Fig. 68, //, where only the
bases of the lateral members are indicated. If we turn a radial growing point

Fig. 68. Growing point of Caulerpa holmesiana. 7, lool
on face ; 77, looked at laterally. After REINKE (1899).

looked

Fig. 69. Growing point
of a frond of Caulerpa ob-
scura. After REINKE (18991.

through 90°, or, in fact, through any angle, the appearance remains unaltered.
In Fig. 68 the lateral axes are arranged in pairs, the members of each pair
arising at the same level, and we term such an arrangement ' pinnate ' ; in so
far it corresponds to the whorled type of radial growing point. If, on the other
hand, the lateral branches be arranged in two alternate rows as in Fig. 69, we
have to do with the spiral type and may term it a one-half spiral. Looked at from
this point of view bilateral symmetry is merely a special case of radial. Dor si-

ventral symmetry is fundamentally distinct from
either of the preceding types (compare GOEBEL,
1880). As an example we may select the in-
florescence of Vicia cracca, where the whole of the
flowers arise on one side only (Fig. 70, 7), while the
other side has no outgrowths at all (Fig. 70, 2). We
do not distinguish in this case two flanks but a front
and back. The arrangement of the lateral organs
individually is of the greatest interest. They are
arranged in regular parastichies as on a radial
growing point and we could number the organs in
this case also were it not that they early assume a unilateral arrangement.
We see, however, that unilateral development does not preclude an essentially
spiral taxis, confirming the conclusion already come to that the ' fundamental
spiral ' has no significance of any importance.

We must now consider the form of the growing point. In the examples
hitherto given it takes the form of a slender paraboloid, or, as it is generally
termed, though less accurately, a cone. Very frequently, however, it has the
shape of a flattened cone or disc. We meet with such forms not infrequently^in

Fig. 70. Inflorescence of Vicia
cracca. /, Front view ; 2, back
view. After GOEBEL (1880, PI. X,
Figs. 1 9 and 20).

THE GROWING POINT

277

the floral regions of the higher plants, and transitions may be obtained between
such forms and the depressed growing point, where somewhat older parts grow
round the real ' apex ' so as to form a kind of crater, while the younger organs
develop progressively downwards on its inner wall (Fig. 71). It may be noted in
this relation that this form of growing point may occur not only owing to de-
pression during its development from the rudiment up to its definitely completed
state (e.g. many flowers), but that the depression maybe induced by supplemen-

f-

Fig. 71. Longitudinal section through
the capitulum of a sunflower. The central,
somewhat depressed, growing point shows
as yet no rudiments of flowers. Slightly
enlarged.

Fig. 72. Longitudinal section of the bud of
Phanerogam, v, growing p '
ments ; jrt axillary bud ( X 10)

a Phanerogam, v, growing point ; ft leaf jrudi-
ments ; g^ axillary bud ( X 10). F
Textbook.

From the Bonn

tary modifications. In the latter case, the depressed situation of the growing
point protects this very delicate part from possible injuries from the environ-
ment. Such protective arrangements are attained, however, in other ways. In
the formation of buds, for example, the lateral members grow more rapidly than
the apex of the growing point, curving over in such a way as to enclose
it. Fig. 72 will give an idea of the appearance of such a bud in longi-
tudinal section. In other ways also, e. g. by inr oiling
of lateral members, a protective arrangement is not
infrequently acquired by dorsiventral growing points

(Fig. 73).

We know now that the duty of the growing point
is, while continually elongating its own axis, to give
rise also to lateral axes. The same relations in general
obtain on these lateral branches as on the chief axis.
They have each at least one growing point, originally a
process from the chief growing point ; it elongates into
an axis of secondary rank and eventually gives rise in
turn to lateral axes. The lateral branches of the first
order, however, do not all behave alike, and even in many Algae one can
distinguish two types : those which behave in all respects like the main axis,
long shoots, and those whose growing point after a short time loses its activity,
the short shoots. The distinction is not, however, of much importance and
intermediate conditions are not infrequent.

There are quite special relationships in the highest plants, and especially
among the Phanerogams, which demand our attention. The shoot bears on
its axial region, the stem, special lateral appendages which we term leaves

73- Herposiphonia re-
After

278 METAMORPHOSIS

One might well conceive of these organs as derived phylogenetically from
short shoots, but in reality they exhibit many important points of difference
from them. The point which is of the greatest interest to us at present is
the difference in the mode of growth. The leaves have in general a very
limited capacity for growth, for in a short time they become fully developed,
their growing points as such disappear, that is to say, become transformed
into permanent tissue. Similarly growth in short shoots is, as we have learned,
very limited, but the growing points are often still retained, and can, if the
appropriate stimulus be applied, give origin once more to new members. A short
shoot may very often change into a long shoot, whilst the alteration of a leaf
into a shoot, whether long or short, is impossible. This is certainly the rule,
but the rule may have exceptions. Thus, as a matter of fact, we have been for
long acquainted with cases among ferns and other plants as well where apical
growth continues in the leaves for many years. We know again of many long
shoots in which growth is arrested owing to the complete transformation of
the growing point. As an example may be taken the case of a shoot ending
in a thorn, or flowers where the growing point is generally transformed into
an ovary. The difference quoted is not enough to serve as a distinction between
a short shoot and a leaf, but there are others, only one of which need be men-
tioned here, viz. their relative positions on the axis.

There are quite a number of plants which form only one axis on which
no lateral members save leaves are produced, e. g. Isoetes, many ferns and
palms. In a second series may be placed the numerous Coniferae which develop
lateral branches only. The majority of the higher plants, however, form both
lateral buds and leaves, and between these members perfectly definite space
relations subsist. The bud is formed in the upper angle of the leaf insertion, in
the so-called leaf axil (Fig. 72, g), and is termed in consequence an axillary bud.
Its point of origin is sometimes more towards the base of the leaf, sometimes
more towards the side of the stem ; it appears sometimes at the same moment
as the leaf rudiment, sometimes considerably later.

These recognized relations between the leaf and the axillary bud may
serve to distinguish these organs from each other, but they are applicable only
to radial (and bilateral) shoots. If the growing point be dorsiventral, on the
other hand, the leaves arise on the upper side, while the lateral branches arise
on the flanks and the roots on the under surface, at some distance, it is true,
from the growing point.

The lateral branches in turn may be again branched. The shoots of secon-
dary, tertiary, &c., rank maintain the same relations we have described as
characteristic of those of primary rank. It is otherwise in the case of leaves.
The leaf-blade, the part of special interest to us, is very frequently flat and
dorsiventral, i.e. with clearly marked upper and under surfaces Such a surface
may be simple or it may be branched. Branching in many cases (Palmaceae,
many Aroidaceae) may result from rupture of an originally simple leaf, but
usually the lateral branches are predetermined from the beginning. Occa-
sionally dichotomous branching occurs (ferns and many Dicotyledons, e. g.
Utricwaria, Ceratophyllum, and many species of Drosera) ; for the most part,
however, the branching is lateral and commences, in the simplest cases (the
only ones we are considering at present), either from the base towards the apex
(Umbelliferae, Leguminosae) or in the reverse direction from the apex towards
the base (Rosa, Potentilla), or, finally, in the middle, progressing towards either
end (Achillea mille folium). Each branch arising in this way may again branch
in a similar manner. All branches arise on the flanks of the mother axis and
the individual leaflets lie in this way more or less in one plane. The fact that
the leaves develop for the most part in a bud, that is to say, in a confined space
surrounded by older leaves, necessitates many deviations from the flattened

THE GROWING POINT

279

form. This flattened appearance is generally developed after the opening of
the bud and the bud usually exhibits an exceedingly complicated arrangement
of its individual parts (prefoliation).

A consideration of the branching of the stem and leaf should naturally
be followed by that of the branching of the root. There are, however, reasons
why a discussion of this subject
should be preceded by a study of
the cellular structure of the grow-
ing point. The varied methods
of formation of members which
we have hitherto studied occur
as we have already pointed out
(compare p. 273) not only in the
higher multicellular plants but
also in unicellular forms. This
is especially the case in the poly-
morphous genus Caulerpa (Siphon-
aceae), which in its habit resem-
bles one of the creeping forms of
the higher plants. Its dorsiventral
growing point produces a hori-
zontally growing stem, giving off
leaves from its upper side and
roots from below, and occasionally
forming lateral branches on the
sides, and yet the entire plant,
many centimetres or even deci-
metres in length, consists of a
single cell. The complete simi-

^& (>\. Caulerpa prolif era. a growing point ; «, leaves ;
r, roots. From the Boon Textbook.

between the PTOwinp- nnint
UClween me growing pOJ

of this unicellular plant and that

of a multicellular type, proves most clearly that cell division cannot have the

importance which has for so long been ascribed to it. Nevertheless the subject

is naturally of interest in itself and we must devote a sentence or two to its

consideration. We shall put on one side the simple growing

points which go to form an unbranched plant-body (cell

filament, cell surface, or cell body) and glance only at the

branched forms, confining our attention further to those

which exhibit lateral branching.

In the simplest case the growing point consists of
a single terminal cell, the apical cell. This cell (s in Fig. 75)
determines, for example, in Stypocaulon, the extension of the
chief axis, on which arise alternately to right and left the
lateral processes which are the rudiments of lateral branches.
They are scarcely formed before they are cut off by a con-
cavo-convex wall (/, Fig. 75) while the apex goes on growing.
When they have reached a considerable length they divide
into two cells by a transverse wall. The distal one retains the
characters of an apical cell, and will after a time develop
a lateral branch to the left, while the lower cell divides by successive walls into
a row of cells and thus becomes a cell body.

This is an extreme case. Generally, the apical cell itself does not give
rise to lateral branches directly, but only indirectly, from the segments cut off
from it. The growing point is further not limited to the apical cell but includes
in addition a number of cells, and posteriorly merges gradually into cells which

Stypocaulon
. s, the a

5.

'uift.

cell: /, 2) ?, 4, succes-
sively older
branches.

apical
ucces-
lateral

280

METAMORPHOSIS

are fully developed. The form of the apical cell is, however, still identical
with, or at least very like, that of Stypocaulon. At the ends of the stems of
mosses and ferns we meet with another form of apical cell, distinguished by
being two- or three-sided. In the latter case it has the form of a tetrahedron
or a three-sided pyramid with its curved base facing outwards. Cell divisions
take place parallel to the three surfaces which face inwards and follow each other
in definite order, so that the individual segments which are cut off from the
three-sided pyramid are arranged in three rows behind the apical cell. Very soon,
however, further division-planes divide these up into a large number of cells.

The stem-apex of the Phanerogam is more complicated still. Here we find
no predominant apical cell to which all the other cells may be referred ; the
apex consists, on the other hand, of a group of many cells. Fig. 76 shows the
conical growing point of an aquatic plant as seen in longitudinal section. An
axial cellular strand (pi) stretches backwards and is surrounded by four envelop-
ing layers of cells (pr), whose outlines almost converge into five confocal para-
bolas. These parabolas are traversed by a number of parabolas cutting them
at right angles, which have the same focus and the same axis, but which are

f Fig. 76. Longitudinal section through the
growing point of Hippuris^ vulgaris. ./j leaf

pnnu

From the Bonn Textbook.

Fig. 77. Diagram of the arrangement of cell-
layers in the growing point. After SACHS (1878).

laid out in the opposite direction. The orthogonal trajectories of the first-
mentioned parabolas are not so apparent because they are often interrupted ;
still they are seen in SACHS'S schematic figure (Fig. 77) as complete curves. We
will confine any further remarks we have to make to this figure, thereby defi-
nitely affirming that the small differences between the diagram and what occurs
in nature are of no consequence so far as the correctness of our exposition is
concerned. In the diagrammatic longitudinal section we note rows of cells
which appear like bent cell filaments ; those which run almost parallel with
the outer surface, and indicated in the figure by the Roman numerals 1 -VI
we term periclines, those crossing them, 1-11, we term anticlines (SACHS, 1878).
In both periclines and anticlines the same phenomenon occurs, viz. that the
series with lower numbers are flatter, while those with higher numbers are always
more bent, until finally the highest numbers of all are curved round the apex and
the two arches touch each other in the line of the axis. In order to give some
conception of the probable mode of growth of a growing point of this kind, Fig. 78
shows on the left of the median line the upper portion only of Fig. 77 so far as the
anticlines 7-77 are concerned, while to the right the same five anticlines are repre-
sented at a later stage of development. It will be seen from this diagram that

THE GROWING POINT

281

each individual anticlinal cell row has become twice as broad as it was, and has
become divided by new anticlinal walls into two layers of cells. The periclinal
rows have elongated considerably, but have become obviously broader at their
bases only, where further divisions have also taken place. This result has been
arrived at on the special assumption that all the anticlines, 7-11, have been
growing at about the same rate, and this assumption may be true in nature of
a certain part of the growing point. It cannot, however, be generally true ;
for a maximum or a minimum growth may equally well occur at the apex while
towards the base a gradual change ensues. Since, unfortunately, the anticlinal
rows are not so obvious in nature as in the diagram, we are on the whole ignorant
of the exact nature of the growth divisions taking place in them. We know
more about the periclinal divisions ; their gradually increasing transverse exten-
sion may be deduced off-hand from Fig. 76. In the illustration the growth
division has not as yet proceeded so far as to result in periclinal division. If
the growing point be less conical, periclinal divisions soon appear beneath the
apex. It is only on the outermost layer, the future epidermis (d, Fig. 76),
that these periclinal divisions cease in all cases. In the growing points of
mosses and ferns the distribution of intensity of growth can be made out much

Fig. 78. Diagram of a growing
point in longitudinal section. The
right hand half represents a later
stage in the development of the left
hand half.

K

Fig. 79. Diagram of the growing point A^
with successively older leaf rudiments, B^ C, D.
After SACHS (1879).

more accurately than in Phanerogams owing to the very regular divisions
resulting from the activity of the apical cell (compare WESTERMAIER, 1881).

The rule followed at the growing point as to the direction of the new cell-
walls is that which we have already become acquainted with. The new walls
appear throughout as ' minimum areas ', and in very many cases the new walls
arise at right angles to those already in existence. We cannot follow out this
subject further however.

Leaf formation always occurs below the apex of the growing point. In
the mosses this is seen especially clearly because in that group each segment
cell gives rise to a leaf. The leaves are thus laid down in a one-third spiral, but
the segment cells of the axis which give rise to them at once undergo peculiar
changes in form (CoRRENS, 1899; SECKT, 1901), which bring about torsions and
so lead to great complexity in the leaf arrangement. The outer wall of the seg-
ment cell forming a leaf rudiment bulges outwards, and from this bulging is
next developed the two-sided apical cell destined to give rise to the leaf. In
the ferns relationships as intimate as these between the stem segments and the
leaves can no longer be distinguished, and the origin of the latter takes place
as in the Phanerogams, save that in the ferns the apical cell of a leaf may still
be distinguished, while in the Phanerogams it cannot. The leaf formation of the
Phanerogams may be made out from Fig. 76, but more clearly, perhaps, from
SACHS'S diagram (Fig. 79). In the diagram three successively older leaves B,
C, D, are represented beneath the apex A. It shows that the leaf arises from

282

METAMORPHOSIS

one or more adjacent anticlinal cell rows which by their active growth create
a prominent swelling on the outer surface of the growing point. The cells of
the periclinal row, IV, take scarcely any part in the formation of this swelling,
while on the other hand, pericline HI shows extreme activity both in growth
and in cell formation, and the swelling so formed is covered by the periclinal
layers // and 7. Layer 77 has become double, but the outermost layer (/)
remains, as elsewhere, unilamellar. The leaf rudiment grows progressively
along with the growing point of the stem from which it arose ; later on,

Fig. 80. Median longitudinal section of the root apex of Hordeum vulgarc. k, initial cells of the root-cap ;
rt cells of the cap about to be thrown off; ^/, central cylinder ; en, endodermis; *, intercellular spaces;
d, epidermis ; c, thickened external wall of the epidermis. From STRASBURGER (Das botanische Prakticum, 4th
ed., Jena, 1902).

when growth becomes more marked perpendicularly to the plane of the
drawing, the flattened dorsi ventral shape of the typical leaf is assumed.
The lateral buds arise in the same way as the primary growing point of the leaf.
The manner in which the young leaf is attached to the growing point of the
stem is worthy of notice. At first the leaf rudiment has approximately the
form of a hemisphere, whose surface in contact with the stem is nearly
circular in outline (elliptical or rhomboidal). Frequently, however, this pro-
jection proceeds to grow in two directions, encircling the growing point, so
that finally it forms a ring-like wall, whose greatest height lies medianly or oppo-

THE GROWING POINT 283

site the point at which it first arose. Thus the stem comes to be enclosed by
successive leaf sheaths.

The lateral branches arise practically in the same way as the leaves, so that we
need not discuss them further. The root, however, from its special peculiarities
claims our attention. The growing point of the root is always intercalary and
forms new and different structures in two directions. The apex of the root is
always covered by a rootcap which protects the delicate growing region. The
rootcap, so far as its function is concerned, may be compared with the other
protective adaptations to which we drew attention in speaking of the growing
point of the stem. It consists of simple parenchymatous tissue which is especi-
ally well developed at the extreme apex of the root, but also encloses the sides
of the root for some distance. Its cells are short lived, but they are constantly
renewed by the growing point itself. In spite of this renewal the rootcap does
not increase in size because the older cells die off in front and are cast off as
new ones are formed behind. On the other hand, the root itself exhibits con-
tinued increase in length at the apex owing to the activity of this same growing
point. In detail there are important differences in structure among root apices,
but all that concerns us is the general principle on which they are formed, so
that we may confine ourselves to one example, e. g. the longitudinal section of
the root-apex of Hordeum shown at Fig. 80. We see that the growing point
is constructed in a manner similar to that of the stem, especially as regards
the regularity of the periclinal divisions. The formative layers of the rootcap
are seen at k. (As regards the remaining features see the explanation beneath
the figure.) If, as in ferns, an apical cell occurs, the cutting off of segment cells
follows entirely the rule which governs segmentation of the apical cell of the
stem, save that segments are cut off on the fourth or outer side of the pyramid
also, and from these the cap is formed.

The difference between root and stem comes out especially in the branching.
The root produces no leaves and no buds but lateral roots only, which are
quite like the main root. Hence branching in the root is extremely uniform.
The point of origin of the lateral root branches is, however, quite unique.
' Root buds', comparable with stem buds, do not exist. The terminal part of
the root is quite free from lateral roots for a considerable distance, and when
they do appear, some way back from the growing point, they burst out from
within, through tissues already full grown ; in other words, roots arise endo-
genously, while leaves and buds arise exogenously. At first sight it might appear
as though the lateral roots could not be derived from the growing point of the
main root, as though in short we had here an exception to the rule which we
have established for the stem, viz. that every new growing point is a portion of
a previously existing one. When, however, we examine a transverse section of
a fully-developed root we notice a layer of cells lying within the parenchymatous
cortex separated off by the ' endodermis ', and which, under the name of ' peri-
cycle', surrounds the central vascular system. This pericycle may be traced
right up to the growing point, and its cells have the peculiarity of retaining for
long the characters of embryonic tissue. When the whole of the surrounding
cells have been transformed into permanent tissue they remain still capable of
division. The pericycle is in short a residuum of the growing point and it is
from it that the lateral roots arise. Their origin from the pericycle is illus-
trated in Fig. 8 1.

After the young root has been laid down it grows out to the exterior by
dissolving and mechanically destroying the cortical tissue before it. We shall
see in our next lecture wherein lies the biological significance of the late forma-
tion of lateral roots ; at present it is sufficient to note the fact that in spite of
their late formation we can still refer their origin to the growing point of the
chief root, and that the rule enunciated above, which we may, with SACHS, term

284

METAMORPHOSIS

briefly the ' continuity of embryonic substance ' applies to the root as well as to
the stem. [As to this embryonic substance, compare NOLL, 1903.] Every
growing point is a portion of an older one ; all arise in the long run from the
growing points of the embryo, and these in turn come from the egg-cell. The
egg-cell, however, is itself formed from the growing point of the parent plant.

We must not forget to note that in addition to this normal development
of new growing points there is also another method. One example of this
phenomenon may be referred to. The much-cultivated Begonias are multiplied
by laying isolated leaves on wet sand. New buds then develop from certain
epidermal cells (HANSEN, 1881), which in turn speedily form roots and become
independent. As we have seen, the leaf generally ceases to grow at a very
early stage and the epidermal cell of a fully-developed leaf is a type of a per-
manent tissue element. So long as the conditions are normal, such a cell will
not show any visible signs of capacity for growth. Since, notwithstanding,
this cell becomes, under certain conditions, a growing point it proves that the
capacity for becoming such was latent only, and became actual on the applica-
tion of a stimulus. Such cases of
adventitious origin of growing points are
remarkably widespread. They teach
us that the difference between fully-
developed and embryonic cells is a
quantitative and not a qualitative one,
and the correctness of this view will be-
come more evident when we study the
subject of cambium (Lecture XXIII).
Turning back once more to simple
plants and plant parts, we see that in
many cases the most vigorous growth
occurs at the growing point, and that
just behind it fully-developed regions
are met with. We have already (p. 261)
studied such cases in considering the
apical growth of fungus cells and root-
hairs, but similar phenomena are to
be seen in more complicated growing
points which form lateral branches,
e. g. in Caulerpa, where also growth
rapidly ceases behind the apex (compare REINKE, 1899, p. 61). The same
appearance is shown by multicellular Algae. In Stypocaulon, for example,
the whole growth takes place in the apical cell, and cell division follows in
completely developed segments of the apical cell. Doubtless, detailed research
would establish the same results in many other plants. Nor is it remarkable
that it should be so, for there are plenty of plants in which nothing else is
possible than the localization of growth in embryonic cells since they consist of
such cells only.

In contrast to this case stands another type represented especially by
the higher plants. Growth at the apex of the growing point is very restricted,
and the plants attain their proper length only by extension of cells which are
situated at a greater or less distance from the apex of the growing point. Thus
we must follow SACHS in distinguishing a primary growth period, when the
members are laid down, a secondary when they are elongated, and usually also
a tertiary when their internal differentiation is completed. [BERTHOLD (1904)
attempts to distinguish six growth regions : (i) the initial region (the apex of
the growing point) ; (2) the region of morphological subdivision ; (3) the region
of anatomical differentiation ; (4) the region of elongation ; (5) the region of

Fig. 81. Portion of a longitudinal section through
the main root of Reseda, r, vessels; p, pericycle; <?,
endodermis ; r, cortex. After VAN TlEGHKM (Ann.
sc. nat. 1898, ser. 7, vol. 8.)

THE GROWING POINT 285

extension ; (6) the region of completion. The first two regions only correspond
to our ' growing point', the remaining regions will be dealt with in the next
lecture.] Obviously, the growth periods pass gradually into each other, still
they differ sufficiently to justify a special treatment of the second and third
in the next lecture, just as we have dealt with the more important phenomena
presented by the first in the present lecture. A detailed discussion of the
growing point will be found in SACHS (1882), and GOEBEL (1884).

Bibliography to Lecture XXII.

[BERTHOLD. 1904. Untersuchungen z. Physiologic d. pflanzlichen Organisation.

Leipzig, 2, 185.]

BRAUN, A. 1831. Nova acta acad. Leop. 15, I, 199.
BRAVAIS. 1837. Annal. Sc. nat. II, 7, 42.
BRAVAIS. 1839. Ibid., II, 12, 5.

CORRENS. 1899. Festschrift f. Schwendener. Berlin.
GOEBEL. 1880. Arb. d. bot. Inst. Wiirzburg, 2, 353.
GOEBEL. 1884. Vgl. Entwicklungsgesch. der Pflanzenorgane (Schenk'sHandbuch

der Botanik, 3). Breslau.
GOEBEL. 1898. Organographie, i, 61. Jena.
HANSEN. 1881. Abh. d. Senkenbergschen Gesell. 12.
HOFMEISTER. 1 868. Allgem. Morphologic d. Gewachse. Leipzig.
NOLL. 1896. Sitzungsber. Niederrhein. Gesell. f. Natur- u. Heilkunde, 3. Febr.

1896.

[NOLL. 1903. Biol. Centrbl. 23, 281.]

REINKE. 1899. Ueber Caulerpa (Wiss. Meeresunters. Kiel, N. F. 5, i).
SACHS, J. 1878 and 1879. Arb. d. bot. Inst. Wurzburg, 2, 46 and 185. Gesam.

Abhdl. 2, 1067, &c.

SACHS, J. 1882. Vorles. iiber Pflanzenphysiologie, p. 939. Leipzig.
SCHWENDENER. 1878. Mechan. Theorie d. Blattstellungen. Leipzig.
SECKT. 1901. Bot. Centrbl. Beihefte, 10.
WESTERMAIER. 1881. Jahrb. f. wiss. Bot. 12, 439.

LECTURE XXIII

ELONGATION AND INTERNAL DIFFERENTIATION

WE have learned to recognize in the continued activity of the growing
point an essential difference between the plant and the animal. In the latter
the primordia of all the organs are laid down in the embryo, and growth con-
tinues for long afterwards, often for many years, during which the enlargement
of these embryonic primordia unto the fully adult condition takes place, but
a fresh formation of new organs from a persistent embryonic substance occurs
only in plants. We might say, indeed, that a normal plant is never full grown,
but consists of fully grown parts coupled with parts having a capacity for further
development. This difference between an animal and a plant is, however, not
so thoroughgoing as one might at first sight suppose.

Growth, as we have seen, may be restricted to the growing point, so that
the axis attains its prescribed length and thickness at quite a short distance
behind the active apex, and the rudiments of the lateral organs are placed at
the same intervals apart that they maintain later on. In other cases apical
growth is only feebly developed, and the organs which arise from the growing
point frequently exhibit, at a certain distance from it, conspicuous increase
both in length and in thickness. It is with these phenomena of elongation that
we have to deal in this lecture.

286

METAMORPHOSIS

We will commence by studying a member of the family of liverworts
(AsKENASY, 1874). In Pellia epiphylla the fertilized egg-cell gives rise after
several months to a sporogonium, of which only the median region, the seta,
is of interest to us at present. The young seta consists of numerous embryonic
cells, but even after several months growth is only 2 mm. long. When the
spores begin to ripen, however, it begins to grow rapidly, reaching a length
of 80 mm. in three or four days, thus elevating the
capsule into the air. The cells of the seta are at first
filled with protoplasm and contain abundant starch,
but after this great elongation has occurred, the
starch is found to have been completely used up (having
been employed in the manufacture of new cell- walls),
and the protoplasm now forms merely a thin layer in
the interior of each cell, which is occupied chiefly by
large vacuoles. It is especially interesting to note
that no cell division occurs during the elongation.
This elongation does not take place equally and uni-
formly throughout the entire length of the seta, but by
differentiation of a zone of maximum growth, which,
however, does not remain constantly in one place.

Similar growth elongations are known to occur
among other lowly- organized plants such as certain of
the larger Fungi and many Algae, such as the unicellular
Siphonaceae. These appearances are met with, how-
ever, especially among the higher plants, and to these
we may now direct our attention. The chief features
of this elongation are as follows : —

1. Growth at the very beginning is accompanied
by cell division, but later on this division ceases, and
the cells elongate considerably.

2. The increase (Fig. 82) in the volume of the cells
is by no means equalled by the increase in the amount
of the protoplasm, hence the vacuoles become larger
and larger till they reach a quite remarkable size,
apparently by absorption of water. In this way
growth may be effected rapidly and at little cost
of material.

3. Growth is not uniform, but varies in amount
in a manner still to be described.

Various methods are employed for measuring
growth ; we will describe those first which aim at
determining the total growth in length of an organ.
For rough measurements an ordinary scale is suffi-
cient ; for more accurate measurements, especially
> Fig. s*. Elongation of ceils when these have to be taken successively at short
^companied by increase in the intervals, we employ a reading microscope. In dealing
From°the Bon^Textbook. soc with vertically growing structures such as roots and
shoots the tube of the microscope must be placed

horizontally, and for this purpose PFEFFER'S horizontal microscope will be
found very appropriate. In addition to the optical method of magnifying the
amount of growth we may employ mechanical means, e. g. a lever. A very
simple form of ' auxanometer ' is that used by SACHS, which consists of arc
and pointer (Fig. 83). A light wire frame carries a small pulley (r) and a long
pointer (z) playing on a scale (q). If a fine silk thread be attached to the apex
of the plant and then carried over the pulley and kept stretched by a small

ELONGATION AND INTERNAL DIFFERENTIATION

287

weight, every elongation exhibited by the plant will cause a movement of the
pulley, which will be exaggerated by the pointer. This instrument does very
well if or lecture demonstration. The more elaborate auxanometers employed
in scientific research are constructed on the same principle, but are arranged
so as to record the amount of elongation automatically. In Fig. 84 we "have
again a pulley, serving the same purpose as in SACHS'S apparatus; it is, however,
connected with a larger pulley by means of which the movement is still further
exaggerated. Over the larger pulley runs a silk thread bearing at one end
a writing style, which registers the growth movements of the plant on a
blackened revolving cylinder. Registering auxanometers of this sort have
been designed by WIESNER (1876), BARANETZKY (1879), and PFEFFER
(1887).

) PFEFFER 's auxanometer, as manu-
y Al.BRECHT of Tubingen.

Fig. 83. Simple auxanometer. After DET-
MER (Smaller Practical Botany, Jena, 1903).

If we have to deal, not with the total
growth of the plant, but with the distri-
bution of growth in it, and the amount
of growth in different zones, we must
measure, macro- or micro-scopically, seg-
ments mapped out by natural or artificial
marks (usually made with Indian ink), and
observe the distances these marks are apart at successive intervals of
time.

We will now endeavour to make ourselves acquainted with the characteris-
tic features of growth in the root and shoot ; but we must take care to see
that in growth calculations all external factors, more especially temperature,
are kept as nearly as possible constant. We will begin with a study of growth
in the root, and in order to do this most conveniently we will cultivate the plant
in water. Should we desire to study its growth in natural surroundings we
employ boxes of sheet zinc filled with soil, but with one side replaced by a sloping
plate of glass. We allow the root to grow backwards along this plate and
observe it from without. SACHS (1873) marked off a zone just behind the
growing point on the main root of a seedling of Vicia faba by two fine lines of

288 METAMORPHOSIS

Indian ink I mm. apart, and found that in the following days this zone increased
in length in accordance with the following numbers : —

Days ....i 2 3 4 5 6 78

Increase in mm. 1-8 3-7 17.5 16-5 17-0 14-5 7.0 o

The rate of growth at first is slow and then rapid ; the maximum rate is
maintained for a certain time, later on it decreases and finally ceases altogether.
This phenomenon, which has been observed in all cases where growth has been
measured, has been termed by SACHS (1872) the ' grand period of growth '.

We will now mark out with Indian ink on a root not one transverse zone
only, but several, beginning at the growing point, and passing backwards, each
being I mm. apart. If we calculate the increase in relative length of these
zones on the following day we shall find that the increase is different in each
zone, but that the differences are undoubtedly subject to certain laws. A few
examples will make this clear. In the following table the several zones marked
off from the growing points are indicated by the numerals I, II, III, &c., and
the growth-increase during 22-24 hours is indicated in mm. (means of several
measurements) : —

XII. XI. X. IX. VIII. VII. VI. V. IV. III. II. I. Total.

Viciafaba (SACHS, 1873) o o 0-2 0-6 0-7 0-8 2-0 3-5 6.5 8-0 2-5 i-o 25-8

Viciafaba (Popovici, 1900) 0-25 0-35 0-5 i-o 1-25 1.5 2-5 4-0 6-0 12-0 7.0 i-o 37-35

Phaseolus (Popovici, 1900) o 0-25 0-25 0-35 0-6 i-o 1-5 3-0 5-0 7-0 16-0 i-o 35-95

Peas (SACHS, 1873) 000000-3 0-5 1-5 3-0 5-5 4-5 0-5 15-8

It will be seen from these different examples that only a few of the zones
marked out on the root are growing in length, and that the grand period begins
at I and ends at XII. In order to obtain a better conception of the periodic
changes in rate of growth in the individual zones we will attempt to give
a graphic representation of the example (Vicia faba) first cited. We will repre-
sent the time in hours by abscissae, and the lengths of the zones at the beginning
of the experiment and after twenty- two hours by ordinates, and endeavour to
draw curves indicating roughly the successive increments of growth (Fig. 85),
assuming that growth as a whole is uniform, and that the length of the growing
zone remains 10 mm. in length. This mode of representation shows very clearly
that the upper zones, after a very brief interval, have completed their growth,
while the lower ones, usually after several hours, begin to elongate. We see that
a certain zone, the third in the figure, has attained the greatest length, and we
also see that the position of maximum growth must shift as time goes on,
always approaching the apex. To make this clearer still we will express Fig. 85
by a series of measurements as in the following table. The zones, each I mm.
long, have increased by lengths corresponding to the successive numbers
indicated (in mm.) : —

Hours : o 3 6 9 12 15 18 21

X. i-o 1-2 growth completed

IX. i-o 1-5 „ ,,

VIII. 1.0 1.8 „

VII. i-o i-8 2-0 growth completed

VI. 1.0 1-6 2-8 „ „

V. i-o 1-2 2.8 4-2 4-6 growth completed

IV. i-o i- 1 1-4 3-2 j'O 6'4 growth completed

III. i-o i-o 1-2 1-4 2-2 4-4 6'8 8-6

II. i-o i-o i-o i-o 1-2 1.2 1.8 3-0

I. i-o i-o i-o i-o i-o i-o 1-2 1-6

We see from this that the maximum growth is reacned after three hours in
the zones VIII and VII, after six hours in VI and V, and steadily advances
thereafter until, after eighteen to twenty-one hours, it occurs in zone III. If
we were to allow more time to elapse between each two measurements we should

ELONGATION AND INTERNAL DIFFERENTIATION

289

find the maximum elongation finally taking place in zone I. That the
graphic representation figured at Fig. 85 agrees in this important point of
the shifting of the zone of maximum growth will be seen by comparing it with
SACHS'S measurements (1873), some of which we may quote : —

Vidafaba.

Increase in the first 6 hours .
,, in the next 17 hours
„ after 24 hours . .
,, after 48 hours . .
,, after 72 hours . .

X.

o-i

o.i

o

o

o

IX.

O-I

0*2
o
o
o

VIII.

Increase in mm.
VII. VI. V.

IV. III.

I'O

I-O

o-5

0.4

o-3

0

o

o-5

1-5

2-5

4-1

3-7

2-0

1.0

0-4

o-5

j.6

4-5

1.8

0-4

o-5

1<5

3'°

6.6

15.0

5'°

0.4

o-5

*-5

3-o

6-6

17-0

23-0

Hours

It is obvious, however, that this forward march of the zone of maximum
growth towards the apex is apparent only ; for our curves show in the clearest
possible way that the maximum zone is always situated at approximately the
same distance from the apex, and if we carry out our observations at short
intervals and always mark afresh each time, we shall find that this will be clearly
proved by the measurements also. We . thus
see that to allow longer time to elapse between
each pair of observations in determining the
region of maximum growth would lead to serious
error.

By way of summary we may say : The grow-
ing region of the root is limited to a few millimetres
behind the growing point. In the course of this
growing region each individual transverse zone
passes through a grand period, those zones which
are nearest to the apex are at the beginning, those
farthest away from it at the end of their grand
periods.

The shortness of the growing region in the
root is a matter of great importance to it. The
root in its passage into the soil has to overcome
great opposition, and we may compare it (SACHS,
1873, 424), to a nail driven into a plank of
wood. As in the case of the nail so in the root
there is a danger of bending : the shorter the
growing point the more safely it enters the soil.

Fig. 85. For explanation see Text.

If we contemplate this entry of the root into the soil, the pointed form of the
growing point covered by its cap is explained, and we further appreciate the
reason why the lateral roots develop first at some distance behind the apex, and
from parts which have long before attained their maximum growth and have
now become quiescent. If the new roots arose from the growing point itself
they would add to the difficulties the root has in entering the soil, or they
would have to form a kind of bud — just as, in fact, we see stem buds, e. g. of
seedlings, boring through the soil.

Roots which do not live in soil, especially the long aerial roots of lianes
and epiphytes, exhibit a much longer growth zone, as SACHS (1873) long ago
pointed out, and as WENT (1895) has more recently confirmed. Thus WENT
found a growing zone 40 mm. long in Philodendron. These aerial are, in fact,
comparable with shoots, growth in which we will study by and by.

Let us now inquire what the total growth resulting from the addition of
the several increases of the individual zones amounts to, as evidenced by the
forward development of the root apex in space. We assumed above that growth
was uniform, and, acting on that assumption, we have represented the undermost
of our curves (Fig. 85) by a straight line. ASKENASY has, indeed, shown that

290 METAMORPHOSIS

the roots of maize grow at a relatively uniform rate, for with approximately
half-hourly observations he obtained the following increases in growth calcu-
lated up to one hour : —

Growth increments, calculated by micrometer (i = ^T mm.).

Hours i 23456789

Root No. i 34-0 27.0 30-0 29.5 36-0 35-0 38-0 31.0 33.5
Root No. 2 32.5 34.5 37.9 34.5 33-0 33-6 33-0

These growth increments may be, therefore, considered as relatively uniform,
while in other bodies for the most part much greater variations have been ob-
served, variations the causes of which are not known, and which we may describe
as 'spasmodic growth variations'. If the observation of a root be carried on
not merely for several hours but for days or weeks, we find that the total growth
also shows a ' grand period ' (PEDERSEN, 1874).

The growing point of the shoot is, as we have seen, enclosed by leaves
growing more rapidly than the apex itself and thus forming a 'bud'. In many
annuals and perennials, and also in some trees, we find at the growing point,
during the whole summer, the rudiments of new leaves and the parts of the axis
relating to them ; they at once become transformed from the rudimentary con-
dition into the adult by elongation. This is not the case with the majority of
trees. In these cases during summer and autumn all the parts of the bud within
the leaves, which act as bud-scales, undergo slow embryonic growth, and these
parts become unfolded in the following year. In this case embryonic growth
and growth in length are sharply separated. Thus in many Coniferae one sees
in autumn, after removal of the bud-scales, a green cone several millimetres
long covered with small spirally arranged outgrowths. This is the rudiment
of an entire shoot which will elongate in the following year in the course of a few
weeks. In other trees the same features are seen, but the buds for the most
part are not so easily examined as in the case of the spruce. The elongation,
however, may take place in a few days (beech).

The cases in which growth consists merely in the elongation of parts laid
down in the previous year are the simplest to understand, and we will begin
with them. Two types have to be distinguished (RoxHERT, 1894). The whole
bud may behave uniformly and grow approximately equally in all its parts, or
exhibit a distinction into nodes which grow but little and internodes which grow
vigorously. The unsegmented bud axis of the spruce, which may serve as a type
of the unsegmented shoot, becomes uniformly elongated in springtime through-
out its entire length, and may become five times as long as it was during the
winter, attaining in this way about one- tenth of its ultimate length. In the
course of further extension a zone of maximum growth appears, which lies at
first at the base of the shoot, but passes gradually nearer and nearer to the
apex. Exact measurements demonstrate the fact that each individual zone
of a spruce-shoot passes, during its elongation, through a grand period.

Let us now compare the bud of Fritillaria with that of the spruce. This bud
is divided into nodes and internodes, but one only of the many internodes
elongates actively, and a grand period may be demonstrated in its case with
the greatest readiness. The following table gives relative elongations of this
internode (SACHS, 1872, 129) : —

Day 123456 7 8 9 10 ii 12 13 14 15 16 17 18 19
mm. 2.0 5.2 6-i 6-8 9.3 13.4 12-2 8.5 10-6 10.3 6-3 4.7 5-8 4-4 3-8 2-0 1-2 0-7 o

If, finally, all the internodes of a bud become elongated (as, for example,
in the case of the horse-chestnut) we obtain just as many growth zones as there
are internodes, separated by nodes which grow slightly or not at all. Each

ELONGATION AND INTERNAL DIFFERENTIATION 291

individual internode obviously goes through its own grand period, although we
know but little as to the distribution of growth in it.

The case becomes more complicated if the shoot exhibits not merely growth
extensions of parts already laid down, but if it continually adds new parts to
those already present in the growing point, and if these also begin to elongate.
If the shoot be undivided into nodes and internodes as, for example, in Aspara-
gus, Linum, &c., growth may in general be considered as equivalent to that
seen in the root ; a single growth zone only is developed and in it a single
maximum. The only difference between such a case and the root is that the
growth zone is much longer. We are acquainted with shoots which have
growth zones 10 cm. or even 40-50 cm. in length, but in these cases the region
of maximum growth lies much further back from the apex than it does in
the root.

As an example of a shoot with obvious segmentation and with a continu-
ously growing apex, we may select Chara or Nitella. These highly-organized
Algae increase by means of a terminal apical cell. Each segment of this apical
cell divides into two cells ; the upper cell is biconcave, and after several sub-
divisions becomes a node, the lower biconvex cell remains undivided and be-
comes an internode. The nodes retain approximately their original length, in
Nitella about 0-02 mm. (ASKENASY, 1878); the internode, on the contrary,
becomes stretched often as much as 2,000 times its original length. If we
correlate the length relationships of successive internodes on an actively grow-
ing shoot we obtain the following values (ASKENASY, 1878) : —

Internode 12345 678

Length in mm. 0-02 0-07 0-16 0-45 3-33 14-0 33.5 35.0

If we make the not improbable assumption that one internode undergoes
similar extensions in similar intervals of time, as shown above in the case of
successive internodes, then each internode will exhibit the following grand
period, where the increments in similar periods of time are indicated in mm. : —

0-05 0-09 0-29 2-88 10-77 r9-5 i-5

In fact each individual internode in a segmented shoot passes through a
grand period independently. In each, also, a zone of maximum growth may be
demonstrated at some definite place, and possibly this shifts from the base
towards the apex (or in the reverse direction from the apex to the base), in the
same way as we have seen it do in the shoot of Picea. It not infrequently
happens that the region of the stem, where the zone of maximum growth occurs
last of all, exhibits not a simple stretching of the cells merely, but both cell
formation and cell elongation, lasting for a long time. In every individual
internode a portion of the primitive growing point remains, and this goes on
acting as an intercalary growing zone. In fact there is no line of demarcation
between localized extension and an intercalary growing zone.

The question now arises, what is the total amount of growth resulting
from the activity of several independent growth zones? It is known that
often 3-4 or, in other cases, as many as fifty internodes are elongating at the
same time. The sum of their activities may give a single uniform curve of
growth not differing from that of the single internode of Fritillaria given above,
or it may be entirely different (ROTHERT, 1894). If only a few internodes be
concerned in the elongation, it might come about that a younger internode might
start growing after the older one had entirely or nearly ceased to grow, and thus
we should have a periodic rise and fall of the growth curve, that is to say
' spasmodic variations ' such as we have previously drawn attention to. Such
variations are, generally speaking, to be found almost everywhere ; they owe
their origin, however, doubtless not to the cause just mentioned only. The

u 2

292

METAMORPHOSIS

spasmodic variations in the growth of Bambusa are very remarkable (KRAUS,
1895) as may be seen from the following curve (Fig. 86).

We can distinguish, finally, in the leaf during its embryonic growth, as
a rule, two regions, a proximal and a distal. From the distal region is developed
the blade, from the proximal part arises either a leaf -sheath or merely a flattened
point of attachment to the stem, which may become enlarged into a well-dif-
ferentiated structure, 'the pulvinus'. In accordance with the area of the leaf
attachment to the stem, in so far must it elongate with the stem, and thus we
see the growing stems of Conif erae densely covered with pulvini, as is specially
evident in the spruce itself. In all cases where the leaves are thus developed
close together, the leaf-bases must accompany the stem in its elongation as in
the Coniferae. It is true one very often sees nothing of them externally, and
distinct pulvini may be entirely absent. All the same a careful investigation
will show generally the existence of such where a free stem-surface between the
leaf rudiments does not exist from the outset at the growing point.

Between the leaf -base and the blade there appears frequently in the growing

Fig. 86. Daily growth increments (in cm.) in the stem of Bambusa^ measured at Buitenzorg from Nov. 13,
1893, to Jan- K>, 1894. After KRAUS (1895, PI. 20).

leaf a very distinct region, the leaf-stalk, which arises generally by intercalary
growth in a zone of tissue of minimum extent between the leaf -base and the
blade, but only after the blade has progressed considerably in its development.
We learned previously that at the beginning the blade also shows apical
growth. Only in a few cases does this apical growth continue for long, generally
it ceases long before the rudiments of all the parts are complete, or at least
before their elongation commences. Among ferns, Gleichenia and Lygodium
are known to possess leaves with apical growing points which remain active for
years, and even among our ordinary ferns it may happen that new pinnae
are developed at the growing point when the basal ones have already opened
out. Similar cases occur in Phanerogams, as we learn from the researches of
RACIBORSKI (1900), who found that in certain of the Meliaceae (Guarea, Chiso-
cheton), the power of producing new pinnae at the apex of the leaf was long
retained. According to SONNTAG (1887), the leaf of Guarea possesses only a brief
apical growth, during which a limited number of pinnae are laid down, which
expand partly in the first, partly in the second vegetative period. The supposed
likeness to ferns thus breaks down, or to speak more accurately, it is limited to
a slow and purely acropetal expansion, which occurs also elsewhere.

Frequently we meet with another type of leaf-expansion where the apex

ELONGATION AND INTERNAL DIFFERENTIATION 293

at once passes into a state of rest. This is the case in many lianes, where speci-
ally formed apices fulfilling particular functions (' forerunner-tips,' RACIBORSKI,
1900) are produced long before the rest of the lamina is completed. The elon-
gation is basipetal also in the long leaves of Monocotyledons, owing to the
development of an intercalary growing zone at their bases. The distribution
of growth in this case is illustrated by the following numbers, which represent
the fortnightly increments in zones, each 2.5 mm. long, marked off on the
leaf of the onion (STEBLER, 1878) : —

Leaf-sheath. Leaf-base. Leaf-apex.

Zone I. II. III. IV. V. VI. VII. VIII. IX.

Increase 7-9 26-4 25-1 48-1 30-1 19-0 16-7 10.4 1-4

Intercalary growth zones are of frequent occurrence in leaves ; but it is
impossible for us to enter into a discussion of the effect such intercalary growth
.zones have on the formation of the leaf ; reference must be made to the morpho-
logical literature, and more especially to GOEBEL (1898-1901).

It was noted earlier that leaves during their embryonic growth assumed
si

protects its growing point by bending
curvature must be again compensated for during elongation by increased growth
on the upper side. Unequal growth in length of this kind is to be found not
only in cases where it is necessary to compensate for previous curvatures or
foldings, but it occurs also by no means infrequently in uncurved rudiments,
transforming them from a straight embryonic form to a permanently curved
adult form. The physiologist sees, more often, indeed, than he desires, how
the root-apex or the tip of the shoot pushes itself forward, not in a straight line
but in a curve, for such curvatures, known as nutations, resulting from small
irregularities in growth of the different sides, frequently interrupt experimental
work to a serious degree. We shall take another opportunity of referring to
such cases.

Only a few examples of specially rapid growth need be quoted, for the
measurements which have been noted vary extremely. The following table
gives the maximum rate of growth per minute for a few plants : —

Dictyophora (HOLLER, 1895) 5 mm.

Stamens of Gramineae (ASKENASY, 1879) 1-8 ,,
Bambusa (KRAUS, 1895) 0-4 „

Coprinus (BREFELD, 1877) 0-225 ,,

Botrytis (REINHARDT, 1892) 0-034 ,,

There are cases known where it is possible to watch the organ actually
growing without employing a microscope. These observations have, however,
no scientific value, since the actual rate of growth, i. e. the increase in unit of
length per unit of time cannot be expressed. The growing region in Bambusa
is very long (several centimetres) ; in Botrytis it is only 0-02 mm. in length,
for although the former shows ten times as great an increase as the latter per
minute, still its rate of growth is much less. To work out the rate of growth
we must use percentages. The following table gives the increase per cent, of
the growing zone per minute (BiJCHNER, 1901) : —

Pollen-tube of Impatiens hawkeri 220 per cent.

,, „ balsamina 100

Hypha of Mucor stolonifer 1 18

Botrytis 83

Stamens of Gramineae 60
Shoot of Bambusa 1-27

,, Bryonia 0-58

294 METAMORPHOSIS

We may also calculate the time necessary for the attainment of a certain
increase, e. g. for doubling the length of the organ : —

Botrytis i rain.

Bacteria 20-30 ,,

Grass stamen 2-3 „

Root of Viciafaba about 1 80 ,,

If we know the duration of growth we may from the rate of growth and the
extent of the growing zone calculate the definite amount of elongation a plant
part undergoes. According to the variations in these factors the size of the
plant is determined, and it is, as every one knows, dependent also on external
factors in manifold ways, and yet in each case it is specifically different. Draba
verna in the course of its vegetative period attains the dimensions of a few centi-
metres, Ricinus or Helianthus must be measured in metres ; Calluna vulgaris,
after ten years, still remains a small shrub, but a Eucalyptus tree reaches the
height of Strassburg Cathedral (compare p. 62). A definite size is as much
a specific characteristic of an organism as leaf -arrangement is ; the entire
organization of a plant is co-ordinated with the attainment of a certain size.
This is a point which SACHS (1893) has demonstrated most clearly, showing
what an impossible monstrosity would result were a Marchantia enlarged fifty-
fold, or diminished to a like extent.

We have as yet limited ourselves to a consideration of the longitudinal
extension of the parts mapped out at the growing point. [BERTHOLD (1904) has,
as already remarked on p. 284, advanced ' stretching ' as a characteristic phase of
growth in addition to elongation. He understands by that term the ' inflation '
which many parenchymatous cells of the leaf, the root, or the stem-cortex under-
go after the whole organ has reached its maximum growth in length. Whether
this ' stretching ' is identical or not with the primary increase in thickness about
to be described it is impossible for us to judge.] Every microscopic investiga-
tion, however, demonstrates the fact that increase in thickness also takes place.
The diameter of the fully-formed root or stem is greater, and often markedly
so, than that of the region just behind the growing point. This is demonstrated
by Fig. 76, which shows an increase in the diameter downwards in the periclinal
cell rows. Growth in thickness has been much less carefully studied than
growth in length, still all the essential features which we have learned to recog-
nize in growth in length are also found here. In the first place we can establish
the existence of a 'grand period'. On account of anatomical relations we
distinguish * primary ' and ' secondary ' growth in thickness. Primary growth
in thickness is universally distributed, and consists in the increase in size of
all cells, which at first divide, but which cease to do so later on. Not infre-
quently primary growth in thickness begins vigorously just after growth in
length ceases, and FRANK (1892) has established the fact that an internode of
the sunflower which has reached its greatest length may increase in diameter
until it has become nearly five times its original size. Many plant-organs which
exhibit considerable dimensions in the transverse direction, e. g. fruits, tubers,
&c., must attain their permanent form by such primary growth. In certain
groups, especially in Dicotyledons and Gymnosperms, we find another mode of
increasing in thickness which we may term secondary, and by which a con-
tinuous growth is effected in stem and root for many, even hundreds, of years.
The difference between primary and secondary increase in thickness does not
lie in duration however, for in palms (and also tree-ferns) primary increase
in length goes on for many years; the characteristic feature of secondary
increase in thickness is the existence of a special growing layer, an inter-
calary zone known as cambium. Cambium, in so far as it lies within the
vascular bundle, is formed from a tissue which remains over, after the forma-
tion of the bundle, between the xylem and the phloem regions, which does

ELONGATION AND INTERNAL DIFFERENTIATION 295

not become transformed into permanent tissue, but retains its embryonic
characters. This cambium is a remnant of the primary growing point. If it
alone be present then the bundle only increases in thickness ; but for the most
part there arises also an interfascicular cambium, i. e. certain cells of the medul-
lary rays, having all the characters of permanent tissue elements, revert to the
embryonic condition and form cambium arcs uniting themselves to the already
existing cambiums on either side. In this way arises in a cylindrical organ,
a complete circular (in transverse section) intercalary growing layer which
produces new tissue actively and continuously. Fascicular and interfascicular
cambiums possess the same capacities, so that it is immaterial whether the
origin is directly or indirectly from the growing point. Ordinary parenchyma-
tous-cells are simply intermediate stages between embryonic and permanent
tissue-elements, which retain for a long time, frequently all their lives, to
a certain degree, a capacity for renewal of growth, although they do not always
exercise it. Although we may make a sharp distinction between primary and
secondary growth in thickness, we do not do so with regard to growth in length ;
more careful consideration shows, however, that the intercalary growing regions,
such as those at the bases of leaves of Monocotyledons or in many internodes,
have as much right to be termed secondary growth areas as cambium has. We
may, therefore, speak perfectly legitimately of secondary growth in length.

Before we leave the consideration of growth in length wje must study
certain peculiar phenomena concerned with the relationships of growth in length
and in thickness. If longitudinal growth be rapid, a diminution in diameter may
take place, and vice versa. A diminution in diameter, although quite insignificant,
occurs, according to ASKENASY (1879), in the stamens of Gramineae, which in
a quarter of an hour may increase their length fourfold by absorption of water.
The converse process occurs much more frequently ; it was found by BERTHOLD
(1882) in Antiihamnion, and is of common occurrence in roots. In the latter
(DE VRIES, 1880 ; RIMBACH, 1897), immediately after vigorous elongation, an
increase in thickness occurs which causes a reduction in length of as much as
10-70 per cent. This reduction is due to a certain alteration in form taking place
in some but not all cells. Owing to the activity of these cells, an activity not
as yet sufficiently well understood, other tissues, such as those of the cortex
and of the vascular system, which are unable to contract, are thrown into folds.
This contraction in the root is of very great importance. Its effect is to draw
down more and more towards the soil the leafy regions of many ' rosette plants ',
in spite of the continued elongation of the axis by growth ; it causes and regu-
lates the entry of many tubers and bulbs into the soil to a definite depth, and
finally strengthens the hold the plant has on the soil, because tense roots render
the plant as a whole more stable than slack ones.

Following SACHS, we have distinguished three periods of growth ; during
the first of these the different members are laid down on a certain plan, during
the second their absolute and relative size is determined, during the third
(which we have now to deal with) the internal anatomy is developed. These
three periods, as already noted, cannot be sharply marked off from each other,
more especially in the case of the third, which often begins before the first has
come to an end.

When the growing point is multicellular, its cells are generally full of
protoplasm ; each possesses a large nucleus and shows no vacuoles. The growing
point in such plants as the Siphonaceae or Mucorinae, which consists merely of
a part of one cell, also shows, for the most part, a dense aggregation of proto-
plasm in that region. We might, therefore, conclude that abundance of
protoplasm is one of the most general characteristics of an embryonic cell and
closely bound up with its specific function. A number of facts, however, tend
to show that this conception is incorrect. Thus NOLL (1902) found that the

296 METAMORPHOSIS

activity of the growing point of the Siphonaceae was quite independent of the
presence there of a large amount of protoplasm. [Further, NOLL (1903) has also
shown that the protoplasm in the apex of the growing point does not differ
from the remainder, that it overflows into it and is supplied from it.] He also
showed that in numberless lower plants every cell is in an ' embryonic ' condi-
tion, and yet that the protoplasm is present there in only moderate quantity.
The ordinary condition of the cells found at the growing point must be explained
otherwise. PFEFFER (Phys. II, p. 7) points out that the abundance of proto-
plasm may be accounted for by assuming that it is intended to permit of rapid
growth in length by the taking in of water, without any further construction
of protoplasm.

From the cells of the growing point are derived all the permanent tissue-
elements in the higher plant, no matter how varied they be in appearance and
in function. Their differentiation takes place at different times in the several
organs. While, for example, the definite structure may be mapped out, though
not completed, in an internode still undergoing elongation, anatomical dif-
ferentiation takes place in a root at a later stage, often long after elongation has
ceased. Certain elements, such, for example, as sclerotic cells, which have no
longer the power of growth, may expand after the completion of their legitimate
length, while, on the other hand, vessels generally push ahead of all other ele-
ments. Their early appearance is obviously necessary because the demand
made on water by the growing points can only be supplied by a continuous
water channel.

To trace the transition of embryonic cells into permanent tissue-elements
in detail would necessitate a study of the fundamentals of plant anatomy ; we
must limit ourselves, therefore, to a consideration of principles only and
refer for details to the special literature on the subject, especially HABERLANDT

Let us first look at the alterations in the general outline of the cells. As
they are approximately isodiametric at the growing point, they must become
pulled out during longitudinal growth unless their original length is reproduced
by continuous transverse division. The relative length, that is to say, the
relation of length to diameter, may be increased by several longitudinal divisions.
Very frequently there is a tendency on the part of cells to round themselves
off ; in this way walls meeting each other at an angle of 90° get displaced in
such a way that now three walls meet together at one point at an angle of 120°.
At the corners or angles also, owing to increased stretching, a splitting of the
middle lamella often takes place, in consequence of which intercellular air-
spaces appear, communicating with each other ; these spaces are of extreme
importance in relation to gaseous exchange. Possibly all these rounding-off
processes may be explained in the first instance by osmotic pressure in the
interior of the cell, in regard to which the cell-wall behaves passively. But
active local growth of the membrane is also an important factor in form dif-
ferentiation. Just as from epidermal, so occasionally from internal cells border-
ing on intercellular spaces outgrowths (hairs) may arise. The vessels may become
filled with ' thyloses ', ingrowths from neighbouring cells which become pressed
against each other, and so the vessels may become blocked up by a luxuriant
cellular growth entering through clefts formed in the course of development
(e. g. between separated sclerotic rings). Local surface growth may appear
also in individual cells in compact tissues, and these cells may force their way
between neighbouring elements, splitting their middle lamellae and sliding over
their cell-walls. This kind of growth has been termed ' sliding growth ', and
appears to be of much more general occurrence than was at first imagined. When
no individual cell grows more rapidly than its neighbour, but the whole tissue
shows equally vigorous surface growth, the resulting form is attained without

ELONGATION AND INTERNAL DIFFERENTIATION 297

breaking the continuity, and so long as this continuity is maintained, the differ-
ence in growth may be accounted for by mere tension.

These tensions have been termed tissue tensions, and they deserve a word
of explanation at this point. If we extract, by means of a cork-borer, the medulla
of a young internode of the sunflower, we may easily observe that the isolated
medulla increases in length by a certain percentage, while the peripheral region
shortens also to a certain extent. In the uninjured stem the cortical region
is in a state of extension while the medulla is in a state of compression. If we
split the stem longitudinally the two halves bend outwards like bows, so that
the cortex and the medulla assume their proper lengths, the former contracting,
the latter expanding. If we strip off a ring of cortex from a branch and sever
it at one point and then attempt to put it back again over the wood, we shall
find that the ring is too short; it has contracted tangentially. Transverse
tension, therefore, exists as well as longitudinal. These phenomena of tissue-
tension have been studied with great care, because it was expected that con-
clusions as to various physiological phenomena might be obtained from them.
These expectations have not been fulfilled, and hence the brevity of our refer-
ence to the subject. When we come to the consideration of the phenomena of
movement we will take an opportunity of again referring to the matter.

After this digression let us return to the subject of tissue-differentiation.
In addition to the form of the cell, the peculiarities of its membrane are of
importance. Just as from a chemical point of view we may distinguish a whole
series of alterations in the cell-wall, so from the physical standpoint the cell-
membrane may exhibit varied modifications. Lastly, there are the morphologi-
cal differences occasioned by the degree to which the membrane becomes
generally or partially thickened.

Thirdly, there are all the cell-contents to be taken into account. The
presence or absence of vacuoles, nuclei, chloroplasts, leucoplasts, starch-grains,
and other constituents, are all characters of the several types of cell. In many
cells contents are absent altogether, that is, the living contents are replaced by
water and air ; nevertheless these constituent cells may also carry out im-
portant functions in the plant.

Lastly, we must note that certain elements, when fully formed, may unite
with other neighbouring elements of similar form by complete or partial absorp-
tion of their transverse walls. Such cell- fusions stand in marked contrast to
single cells, but this contrast is neutralized by the fact that the independence
of individual cells, notwithstanding the sliding growth previously referred to,
is by no means complete. For all living cells are connected to each other
by delicate strands of protoplasm which pierce the cell-membranes, and on the
basis of this discovery we may affirm that in the complex plant, whose body is
broken up by numberless cell-walls, only one protoplasmic body is to be found,
just as in Caulerpa or Mucor.

Bibliography to Lecture XXIII.

ASKENASY. 1874. Bot. Ztg. 32, 237.

ASKENASY. 1878. Verb, naturw. Verein Heidelberg, 2, i.
ASKENASY. 1879. Ibid. 2.
ASKENASY. 1890. Ber. d. bot. Gesell. 8, 61.
BARANETZKY. 1879. Mem. Acad. Petersbourg, VII, 27, No. 2.
BERTHOLD. 1882. Jahrb. f. wiss. Bot. 13, 607.
[BERTHOLD. 1904. Phys. d. pflz. Organisation. Leipzig. 2, 146.]
BREFELD. 1877. Unters. aus d. Gesamtgebiete d. Mycologie, 3, 61.
BUCHNER. 1901. Zuwachsgrosse u. Wachstumsgeschwindigkeiten bei Pflanzen.
Leipzig.

298 METAMORPHOSIS

FRANK. 1892. Lehrbuch d. Botanik. Leipzig, i, 376.

GOEBEL. 1898-1901. Organographie d. Pflanzen. Jena.

HABERLANDT. 1896. Physiol. Pflanzenanatomie, 2nd ed. Leipzig.

KRAUS, Gr. 1895. Annales Jardin Buitenzorg, 12, 196.

MOLLER, A. 1895. Schimper's bot. Mitt, aus den Tropen, Jena, 7, 119.

NOLL. 1902. Sitzungsber. niederrhein. Gesell. f. Natur- u. Heilkunde.

[NoLL. 1903. Biol. Centrbl. 23, 281.]

PEDERSEN. 1874. Arb. d. bot. Inst. Wurzburg, i, 569.

PFEFFER. 1887. Bot. Ztg. 45, 29.

POPOVICI. 1900. Bot. Centrbl. 81, 35.

RACIBORSKI. 1900. Flora, 87, 17.

REINHARDT. 1892. Jahrb. f. wiss. Bot. 23, 479.

RIMBACH. 1897. Fiinfstuck's Beitr. z. wiss. Bot. 2, i.

ROTHERT. 1894. Cohn's Beitr. z. Biologie, 7, i.

SACHS. 1872. Arb. d. bot. Inst. Wurzburg, i, 99.

SACHS. 1873. Ibid, i, 385.

SACHS. 1893. Flora, 77, 49.

SONNTAG. 1887. Jahrb. f. wiss. Bot. 18, 236.

STEBLER. 1878. Jahrb. f. wiss. Bot. n, 47.

DE VRIES. 1880. Landw. Jahrb. 9, 37.

WENT. 1895. Annales Jardin Buitenzorg, 12, i.

WIESNER. 1876. Flora, 59, 467.

LECTURE XXIV
EXTERNAL CAUSES OF GROWTH AND FORMATION. I

THE form of the plant is determined by a large number of factors, which
we may divide into two groups ; internal factors, i. e. those originating within
the organism itself, and external factors, i. e. those emanating from the outer
world. In nature, external and internal factors always co-operate, and none
of the plant activities can be manifested under the influence of one set of factors
only. Still, for the purpose of investigation and description, we must, as far
as possible, keep the two series distinct. Among external factors — the only ones
we will consider at present — we may distinguish the purely chemico-physical
influence of the inorganic surroundings from the complex influences resulting
from association with other organisms. We have already dealt with the depen-
dence of certain functions (e.g. respiration, assimilation, &c.) on external fac-
tors, and have now to study the influence of these factors on growth ; we will
not limit ourselves, however, to this phenomenon only, but inquire into the
action of the outer world on life as a whole.

It is not our intention to give a complete catalogue of all the known effects
produced by every individual factor ; we must content ourselves with a few
examples without going into every influence which affects the whole period, of
growth, for sometimes embryonic growth, sometimes elongation or internal
differentiation, will be found to claim our chief attention.

We may note in general that all external factors (PFEFFER, Phys. II, 85)
operate either directly or indirectly. Direct effects, where the external factor
provides the energy for the resulting phenomenon, are remarkably rare, while
indirect influences may be recognized almost everywhere. The external world
acts as a stimulus on the plant, and in co-operation with the special capacities
possessed by the plant, it brings about certain results. We may recognize
amongst stimuli a first series of factors which we shall term formal conditions ;
they are absolutely essential, if any development is to take place, they are the
essential conditions of vitality. Then again we have also stimuli which are non-
essential, but which, all the same, produce conspicuous results when applied to
the plant. All stimuli may be regarded first as either accelerating or retarding

EXTERNAL CAUSES OF GROWTH AND FORMATION. I 299

the rate of growth, and in this respect their influence is ephemeral ; or, secondly,
they guide growth and form into definite channels (formative stimuli), in so far
as they influence either only the number and size, or also the position, symmetry,
direction, or polarity of organs. The results produced are in individual cases
either quantitative only or, in more complex cases, qualitative as well.

Let us begin with the consideration of the influence of ether vibrations, and
of these we need consider here only heat and light, for electricity plays no part in
determining growth and form in plants. At the very outset we become conscious
of the fact that growth, and indeed all the vital processes in the plant, take place
only within certain limits of temperature, and the phenomena of everyday life
prove to us that these limits are most varied amongst different plants — a fact
which is confirmed by scientific research. Just as in other functions we discover
that there are three cardinal points in temperature for growth, a minimum,
a maximum, and an intermediate optimum, the data with regard to which we
may take from PFEFFER (Phys. II). Arctic Algae appear to be able to grow
at temperatures under o° C., but the minimum temperature for most fresh-
water Algae lies about o° C., or slightly higher. Among higher plants seeds
of Triticum vulgar e and Sinapis begin to germinate just above o°, while
Phaseolus commences its development at 9°, Cucumis sativus at about 16°,
the bacillus of tuberculosis at 30°, and the thermophilous Bacteria at
temperatures even higher still. The maximum temperature for many marine
Algae is especially low; there are, however, no data on record as to the
maxima which Algae inhabiting arctic seas can tolerate. The very low
maximum of 16° is recorded for Hydrurus (a fresh-water alga), while that
of the majority of land plants lies somewhat between 30° and 45° ; it is
only in the case of succulents that growth may take place at a temperature of
from 50° to 52° C. (compare p. 44). On the other hand, thermophilous Bac-
teria can flourish in media which, owing to fermentative processes, exhibit
a maximum temperature of 75°, and certain Algae can endure temperatures
even higher than that, e. g. in the neighbourhood of natural hot springs. L6 WEN-
STEIN (1903, Ber. d. bot. Gesell. 21, 317) finds, however, that the Algae in the
Karlsbad springs cannot endure a temperature above 52° C. In general terms
it may be said that in the case of plants inhabiting cold regions both cardinal
points of temperature are low, while in the case of those accustomed to warm
surroundings, and also of parasites in warm-blooded animals, these points stand
high ; the range of temperature between these maxima is very considerable,
ranging as it does from 16° in the case of Hydrurus, between 30° to 40° for the
majority of plants, up to 50° or even more in many Cactaceae. As may
easily be understood, the requirements of the plant as regards heat, indicated
by the position of the minimum and maximum cardinal points, is a factor of
fundamental importance in determining the distribution of plants on the earth's
surface. The position of these points is by no means fixed, since they may be
altered both in higher plants and also, and more especially so, in Bacteria. Thus
DIEUDONNE (cited by PFEFFER, Phys. II, 91) found that he was able, by gradual
acclimatization, to get Bacillus anthracis to endure a minimum of 10° instead
of I2°-I4°, and Bacillus fluorescens to withstand a maximum of 41-5°, in place
of the normal 35°. It has also been shown that the position of the cardinal points
often depends on other factors, such as food, oxygen, light, &c. (compare PFEFFER,
Phys. II, 91). In addition to specific differences there are also differences in the
case of individual organs and various developmental stages. For example, the
flowers of many of our spring plants develop at a much lower temperature than
the vegetative organs, which begin to appear after the flowering period is over
(Tussilago, Crocus, cherry, &c.). Germination of spores in Penicillium takes
place between 1-5° and 43° C., the further growth of the mycelium between 2-5°
and 40° C., and the formation of spores only between 3° and 40° C. (WIESNER,

300 METAMORPHOSIS

1873). A high temperature appears to be required for the formation of roots
in cuttings.

Within these limits, however, the different temperatures are by no means of
equal value to the plant. In studying the rate of growth we discover that as the
temperature increases growth at first becomes more active, but that later on a
higher temperature retards it. If we make a graphic representation of the
amount of growth per unit of time in relation to temperature we obtain a curve
which at first rises and then falls ; the highest point of the curve is known as
the growth optimum, and this lies sometimes medianly between the minimum
and maximum, or sometimes nearer to one than the other. The curve, more-
over, has generally not one principal apex only, but several secondary ones as
well, and these occur, for unknown reasons, very irregularly. A glance at the
following estimates, which we owe to KOPPEN (1870), will show this.

Lengths of the hypocotyl of Pisum in 48 hours (average of several measurements).

Temp. 10-4° 14.4° 17-0° 18-0° 21-4° 23.5° 24-2° 25-1° 26-6° 28-4°

Length 5.5 5.0 5-3 8-3 25.5 30-0 45-8 27-8 53.9 23-0

Temp. 28-5° 29-0° 29.9° 30-2° 30-6° 30-9° 3n° 33.5° 33-6° 36-5°

Length 40-4 24.5 34-6 38-5 40-8 28-6 38-9 23-0 8-0 8-7

If the maximum be exceeded growth gradually ceases, but the life of the
plant is at first not necessarily in danger ; the organism comes to be in a state
known as 'heat rigor ' ; the condition of being capable of growth within the tempera-
tures bounded by the maximum and the minimum we term ' thermotonus '. A
temperature i°-2° in excess of the maximum acts injuriously in a very short
time, and is fatal if exposure to it be continued for long. While Penicillium can
tolerate for many days a temperature i° C. above the maximum, many Phanero-
gams exposed to such an ultra-maximum remain alive only for i-r| hours
(HiLBRiG, 1900). The more the maximum is exceeded the more rapidly does
death ensue. It is obvious that the absolute degree of temperature sufficient to
cause death will show wide variations, since it is closely related to the maximum
growth temperature. As examples of such specific differences it may be noted
that Vicia faba dies at a temperature of about 35° C., Secale at 44° (HiLBRiG.
1900), and other plants at about 50° C. (SACHS, 1864). That similar variations
occur in the case of organs of one and the same plant is shown by LEITGEB'S
observation (1886) that all the cells of the leaf of Galtonia, save the guard-cells
of the stomata, were killed in ten minutes by exposure to a temperature of 59°.
Further, there are many resting-stages of the plant, especially the spores of
Bacteria, which are insensible in a high degree to temperature, since many in the
resting-stage can stand the temperature of boiling water for long though not per-
manently. All parts of plants which can endure drought are uninjured by high
temperatures so long as they are dry. Seeds, spores, mosses, and lichens, can
often endure ioo°-i2O° in dry air. We are quite as ignorant of the causes of death
by heat as of the causes of thermotonus ; at all events we must not assume, as is
so of ten done, that it is due simply to coagulation of proteid ; indeed the fact that
death due to heat may occur at quite low temperatures militates against that view.

As in the case of supra-maxima so infra-minima of temperature may retard
growth, leading to a condition of 'cold rigor*. While some organisms are
killed rapidly by continued cold rigor, or in other words by freezing, others may
continue alive for months or even years in that condition. Death from cold takes
place in certain tropical plants (MoLiscn, 1897) at temperatures above o° C., in
other, cases far below o° C. In the case of turgid organs a formation of ice must
take place within them if the temperature be sufficiently low, and it has been
proved (MULLER-THURGAU, 1886) that many plants die the very moment the
formation of ice takes place. Potatoes, for instance, can remain alive at — 2°C.
if the formation of ice be prevented, while they die at — 1° C. if the ice be allowed

EXTERNAL CAUSES OF GROWTH AND FORMATION. I 301

to form. In such cases the formation of ice-crystals must be the cause of death.
This is all the more extraordinary seeing that other plants, such as our forest
trees and many herbaceous plants which grow during the^ winter (Stellaria
media, Senecio vulgaris), can endure alternate freezing and thawing many times
in succession. Frozen members are, however, at length killed by lowering the
temperature still further, and no turgescent cell can tolerate indefinite reduction
of temperature. Possibly death is due in many cases only to the withdrawal of
water in the formation of ice, a withdrawal which plants can tolerate only to acer-
tain degree. At least the fact is that certain anhydrous organs, such as seeds and
spores, are not killed by being subjected to the very lowest temperatures, e. g.
— 200° C. for five days (BROWN and ESCOMBE, 1895), or — 250° for a shorter period
(THISELTON-DYER, 1899). [MEZ (I9°5) nas shown that many plants do not
suffer injury by the withdrawal of water that takes place on the formation of
ice, and which always ceases at a temperature of — 6°C., but are killed by lower
temperatures. In this case death is due to actual cold, not to loss of water in
consequence of low temperature.] To
go further into the problems which this
subject suggests would carry us too far,
so that we may refer for further details
to the literature on the subject, and
especially to PFEFFER'S (Phys. II, §§ 65-
68) treatment of the question.

We have up to now learned that
temperature acts as a stimulus on the
thermotonic plant, either accelerating,
retarding, or altering the rate of growth,
and that these effects are most com-
monly exhibited during the period of
elongation. Notwithstanding differ-
ences in the rate of growth, the absolute
size and a similar shape may be
reached, if only the duration of
growth alters correspondingly. This
is a phenomenon of quite general oc-
currence ; plants growing at optimum
temperatures do not show differences in
appearance from those which have been
cultivated at supra- or infra-optima. Changes begin to appear, however, when
the limiting temperatures are approached ; near the minimum the length of the
growing region increases, and shortens near the maximum (Popovici, 1900) ;
further, the internodes become shorter if the culture be maintained for long at a
low temperature, lengthened duration of growth does not affect the neces-
sary elongation. Temperature thus induces also a formative result ; such
results have been recorded, but they apparently play on the whole only a
limited part. VOCHTING (1902) has recorded a far-reaching effect of altera-
tion of temperature in the case of the variety of potatoes known as 'Mar-
jolin '. At from 6° to 7° C. tubers arise from the main shoot (Fig. 87, /), but at
20° C. these are replaced by normal leafy shoots (Fig. 87, //). Qualitative
differences such as these, however, are only rarely induced by temperature.

When we turn to light we are dealing with a factor which obviously must
always have a very great influence on the formation of the plant (compare p. 251).
The effect of light on growth is fundamentally different from that of heat.
Many organisms can pass through their entire life-cycle in darkness ; in other
cases, at least some parts, e.g. roots, can do without light, but light is generally not
a direct condition of growth. Indirectly it is certainly essential to the existence

Fig. 87.— Tubers of ' Marjolin ' potatoes. /, after
cultivation for four to five weeks at a temperature of
6-7° C.; //, after cultivation for seventeen days at
25 C. The roots are not represented in the drawings.
Vt main shoot, K. tubers, /,, etiolated leafy shoots.
After VOCHTING {Dot. ztg., 1902, PI. 3).

302 METAMORPHOSIS

of plant life, and indeed of all organisms, since it provides the energy whereby
the green plant builds up organic material, on which in turn all organized life
on the earth depends. We are not, however, concerned here with this aspect
of the case. If we provide for the supply of a sufficient quantity of nutrient
materials, then the great majority of plants and plant parts can grow without
light. Light is not generally essential to that condition of capacity for growth
which we may term phototonus, in analogy with thermotonus ; in other words,
withdrawal of light does not, as a rule, induce a darkness-rigor in the plant.
In many cases which we may describe as darkness-rigor, we are dealing
in reality with phenomena which are only indirectly dependent on the absence
of light (JosT, 1895). Only in a few cases can it be shown that light is an essen-
tial formal condition of growth. Thus it has been established that many seeds
germinate badly or not at all if they be kept constantly in the dark ; Viscum
album (WIESNER, 1894), Veronica peregrina (HEINRICHER, 1899), and Nicotiana
(RACIBORSKI, 1900) may be cited as conspicuous examples of this. This
phenomenon is especially common in the spores of ferns and mosses (BORODIN,
1868 ; LEITGEB, 1876). That we have not in this instance to deal with an
assimilatory action of light and the renewal of the necessary constructive
material, but with a specific stimulatory action, is shown by the fact that if
tobacco seeds soaked in water be exposed to light for an hour they can germinate
in darkness, further, that germination of the spores above mentioned may take
place in air from which carbon-dioxide has been excluded, and, finally, that
light may be replaced by other stimuli, such as a temperature of 32° C. in the
case of fern-spores, and a sugar solution in the case of moss-spores (HEALD,
1898 ; GOEBEL, 1896).

In such cases as these, which must be considered as exceptions to the rule,
we may speak of a minimum intensity of light essential to development. There
is a definite maximum, however, affecting the generality of plants, which, when
exceeded, first retards growth, and, finally, causes death. The position of the
maximum is again specifically very varied. It lies very low in shade-loving
plants such as we find abundantly in woods, or, more especially, in the sea.
Such plants are killed by direct sunlight. The same is the case with many
Bacteria, which are killed by a brief exposure to direct sunlight, or even diffuse
lig;ht. The light maximum is much lower still for many subterranean organs than
it is for shade-loving plants. Thus it is well known that the tubers of the potato
bud out readily in the dark, while such development is retarded in diffuse
daylight. Darkening has a similarly favourable effect on many, but not all
roots, as we shall see later on. Plants which naturally grow in sunny regions
are those best adapted to the highest light intensity, and the maximum in their
case is reached only when the sunlight is concentrated by means of a lens. The
different organs of the plant are not all equally sensitive ; thus the chloroplasts
are much more easily injured than the rest of the protoplasm.

During the course of development of individual organs the position of
the light maximum varies often very considerably. It is only while in the young
state that the shoots of the potato are strongly retarded in their growth by light.
We meet with very remarkable relationships in the Cactaceae, whose shoots in the
long run lose their power of growth as a result of illumination. When put in the
dark growth recommences and continues for a time after re-illumination. The
growing point may be stimulated to further growth if placed in the dark. Many
aquatics behave in a similar manner, e. g. Elodea, Ceratophyllum, Myriophyl-
lum, for MCBius (1895) found that the full-grown internodes of such plants
started growing once more when placed in darkness. On the other hand, light
exercises a retarding influence on the embryonic growth at the growing point
in the Cactaceae and on the growth in length of internodes in the aquatics
mentioned.

EXTERNAL CAUSES OF GROWTH AND FORMATION. I 303

Just as extreme variations occur in the position of the maximum and
minimum of illumination, both in the case of individual plants and individual
organs, so also it is impossible to make any general statement as to the position
of the optimum.

In nature, all the organs of a plant which are sensitive to light are subject to
frequent variations of light intensity, not only owing to the periodic alternations
of day and night, but to other causes as well, and it may be experimentally
shown that a variation of light intensity often, but not always, influences the
rate of growth not only in organs which require a certain amount of light for
their development, but also in those which can grow in complete darkness. This
fact becomes very obvious if we compare the rate of growth in darkness with
that taking place under feeble illumination. We may take a few examples
first of all from organs capable of growth in continuous darkness. STAME-
ROFF (1897) determined the increase in growth by means of a micrometer in the
following structures, which, under constant temperature, were exposed to the
light of an electric lamp and darkened every 10-15 minutes alternately.

Time of

exposure. Dark. Light. Dark. Light. Dark. Light. Dark. Light. Dark.

Mucor vegetative cells 10' 7 7 7 7 7 77 7 7

,, conidiophores 15' 10 9 9.5 8-75 9-25 8-5 9-25 8-25 —

Marchantia, roothairs 10' 6 4-5 6-25 4-5 6-25 4-5 6-25 4

Robmia, pollen-tubes 15' — 66 6 6 6-5 666

The growth of pollen-tubes and of the ordinary hyphae of Mucor was by ex-
periments of this kind shown to be unaffected by light, while the conidiophores
of Mucor and the rhizoids of Marchantia often exhibited, as a result of illumina-
tion, a very considerable diminution in growth. Similar retardations due to light
have been shown to occur in stems, leaves and roots of the higher plants by
SACHS (1872), PRANTL (1873), STREHL (1874), and KNY (1902). KNY compared
roots which had been exposed to lights of varying intensity with those which
had been kept in the dark continuously for the same time, and found that after
the lapse of several days the illuminated roots were markedly shorter than
those which had been in darkness. Most of the authors mentioned above have
compared — under constant temperature conditions — the increase in growth
taking place during the night with that occurring by day. The influence of
brief alternations of darkness and light in Phanerogams has been studied by
REINKE (1876), who found that the hypocotyl of Helianthus exhibited in every
quarter of an hour (in p) the following increments : —

Darkness. Light. Darkness. Light. Darkness. Light.

125 60 120 54 116 71

This diminution of growth in consequence of light, as already noted, may
amount in individual cases to a complete cessation of growth, and a sufficiently
great intensity of light may in the long run produce this result in every plant.
Generally speaking, however, ordinary daylight, if it be acting continuously,
produces merely a retardation and not an arrest of growth. Observations
made on plants in the arctic regions, as well as the experiments which BONNIER
(1895) has carried out in our own latitudes with artificial light, prove this con-
clusively.

It does not necessarily follow that the final shape and size of the plant
should be influenced by the accelerating and retarding stimuli which we have
hitherto dealt with ; but as a matter of fact they are often actually so affected,
and thus we may speak of a formative influence of light. At the same time we have
to discriminate between the intensity, the direction, and the quality of the light
which falls on the plant [especially the distribution of light on its upper surface].

304

METAMORPHOSIS

The formative influence of continuous darkness has been longest known
and is the most striking of the formative effects. Apart altogether from the
alteration in colour which frequently takes place, plants which are grown in
darkness exhibit special peculiarities in shape which are summed up under the
term ' etiolation '.

The pure etiolating effect of darkness is seen naturally only when light is

i n

Fig. 88. Dahlia variabilis. /, grown in light ; //, grown in darkness. From a photograph. Equally reduced.

withdrawn, other factors remaining constant. One of the indirect results of
placing a green plant in darkness is to cause a cessation of carbon-dioxide assimi-
lation and a constant stoppage of nutritive supply. In studying the various
phenomena of etiolation, therefore, we must arrange that no such absence of
nutrients takes place, and hence we employ plants for dark cultures which are
abundantly provided with reserves (e. g. seeds, tubers, trees). If now we com-
pare an etiolated shoot of Dahlia variabilis (Fig. 88, //) with one grown in light

EXTERNAL CAUSES OF GROWTH AND FORMATION. I

3<>5

(Fig. 88, /), we find that the internodes and petioles of the former have become
greatly elongated, while the leaf -blades remain small and undeveloped. Micro-
scopic investigation shows that the leaves remain in their embryonic condition,
the tissues exhibiting little differentiation. Further, the last phase of growth in
the stem is incomplete, for the mechanical elements are wanting, and hence
the etiolated shoot is quite soft ; apart from this the individual cells are very
much more elongated than in the normal shoot and their number is much
greater. The majority of Dicotyledons with long internodes behave in the
same way, but even in the case of the so-called ' rosette plants ', such as Semper-
vivum (WIESNER, 1891 ; BRENNER, 1900), etiolation results in the diminution
in size of the leaves, the elongation of the condensed internodes, and in an
opening out of the leaf rosette (Fig. 89, ///). These phenomena are not univer-
sal, however ; organs which grow normally in the dark, naturally react dif-
ferently from those which grow in light. Thus it would not be possible, for
example, to cause an extension in subterranean bulbs with condensed internodes.
Bulbous plants, e. g. bulbous species of Oxalis, exhibit totally different etiola-
tion phenomena. In their case elongation of the stem does not take place, but
the petioles of the leaves, on the
other hand, elongate very consider-
ably, while the leaf-blade remains
small (Josx, 1895). The petioles of
Oxalis deppei, for example, growing
in the dark, but not full grown, were
fifty-eight to seventy-eight cm.
long, while the controls which were
standing in a room in moderate sun-
light, had petioles from eighteen to
twenty-three cm. long.

Many Monocotyledons, whose
stems are outstripped by their
leaves in rapidity of growth, behave
in the same way as do species of
Oxalis. Such plants, both in light
and in darkness, form shoots of
about the same length, but in the
dark the leaves, owing to a con-
tinued capacity for growth in their basal meristems, exhibit a marked increase
in length in the dark, but are generally smaller than when grown in light.

It has been customary to distinguish these two types of etiolation as the
monocotyledonous and dicotyledonous respectively. In both of these groups
of plants, however, there are numerous exceptional cases, in which etiolation
does not take place or where the plants in question behave in the etiolated
condition quite differently from their allies. Among plants which do not
elongate their axis in the dark are certain climbers such as Humitlus and Dios-
corea, but their behaviour becomes intelligible when it is remembered that
climbing plants form, to start with, very long internodes in light, with leaves
which for a long time remain small in size. Further, plants are known in which
the leaf-blades are not actually smaller in the dark than in the light, such as
Beta, Taraxacum and Tragopogon. As already noted no elongation of the
shoot takes place in bulbous species of Oxalis, while among Monocotyledons
Tradescantia behaves quite like a Dicotyledon ; the leaves remain small and
the internodes become elongated. Again among the grasses the Paniceae —
e. g. Zea mais — have much elongated hypocotyls, and the leaves of hyacinths
are smaller and shorter in the dark than in the light. Finally in the Cac-
taceae shoots grown in the dark remain shorter, often considerably so, than
when grown in light (VOCHTING, 1894 ; GOEBEL, 1895).

Fig. 89. Sempervivum assimile. /, grown under nor-
mal conditions; //, grown in a moist atmosphere; ///, grown
in darkness. After BRENNER (1900).

306 METAMORPHOSIS

Etiolation is not limited to Monocotyledons and Dicotyledons ; it has been
observed also in Gymnosperms, ferns, mosses, Algae, and Fungi. As to some
of these cases we shall have something to say. later on — at present a few exam-
ples only, taken from the Fungi, need be referred to (compare PFEFFER, Phys. II,
102). The influence of darkness is very marked on certain species of Coprinus,
where a vigorous elongation of the stipe and a diminution of the pileus occurs.
In extreme cases, in some species, the pileus is entirely suppressed (e. g. C. ster-
corarius), a result which can no longer be considered as a case of etiolation.
An excessive elongation of the sporangiophore has been observed in many
Mucorineae (Pilobolus), and the stalk of the perithecium of Sphaeria velata has
been known in darkness to elongate to five times its usual length.

The instances of etiolated Fungi last cited are of especial interest when we
attempt to answer the question as to the causes of etiolation. In these cases
the accessory action of light (in carbon assimilation) is quite excluded from con-
sideration. We can certainly prove that darkening the higher green plant does
not induce etiolation by stopping carbon-dioxide assimilation. If we cultivate
an autotrophic plant in light, but in an atmosphere without carbon-dioxide, we
exclude all carbon-dioxide assimilation, yet no etiolation is to be observed.

The varied behaviour of the different organs of plants, as well as that of
different species, shows also that in etiolation we have to deal with a stimulatory
action of light, which under different conditions leads to the most diverse
results. In the first place we have an alteration in the normal correlation of
growth between organs ; the great development of the internodes hinders, in
Dicotyledons, the usual surface-growth of the leaf-blade. Similarly we can
induce the formation in darkness, in Phaseolus and Mimosa, of leaves of about
normal size ( JOST, 1895) if, by removing all lateral buds before their elongation,
the main shoot is saved from competition with them. PALLADIN (1890) has
obtained the same result by retarding growth in the internodes by appropriate
means. It is not, however, known in detail how illumination or darkening
operates on the growth of cells, and the numerous experiments which have
been made lay stress for the most part on one possible factor only, such as turgor,
elasticity of the cell-wall, &c., thus implying that the problem is simpler than it
really is. It must be remembered amongst other things that etiolation is not
a simple consequent of light activity, but is rather a complex result of several
secondary influences, especially the retardation of transpiration (compare p. 319).

We know much more about the biological significance of etiolation than
we do about its causes (GODLEWSKI, 1889; DARWIN, 1896). Looking upon
superelongation of certain organs as the most usual characteristic of etiolation
we may regard it as an adaptation on the part of the plant to escape from dark-
ness. From this point of view it is immaterial whether the internodes or the
petioles elongate ; the chief point is that the special organs which require illu-
mination should be lifted out of the darkened area. The leaves need not enlarge
if they are not to exercise their functions. Further investigation is required
to show how far the superelongation of the fruit-bearing parts of Fungi or
mosses is of service. In the case of experiments carried out in the dark etiolation
is certainly of no service ; in their natural habitat, however, originally under-
ground shoots or creeping parts, which are covered with earth or leaves, again
reach daylight in virtue of the effect of etiolation coupled with geotropism
(Lecture XXXIV). Owing to the combined action of etiolation and helio-
tropism (Lecture XXXVI), the plant is able to put itself in a position to reach
the most suitable light intensity. Etiolation is not induced by absolute dark-
ness only ; diminution of light has a similar effect, and generally speaking each
varying intensity of light to which a plant is subjected, impresses itself on its
structure. As light increases the leaf grows in size up to a certain maximum,
while higher illumination causes a diminution in size. The stem behaves con-

EXTERNAL CAUSES OF GROWTH AND FORMATION. I 307

versely, and BERTHOLD (1882) has demonstrated superelongation in Algae
placed in somewhat too bright light. Diminution, however, may clearly be
seen to follow further increase in light intensity, finally resulting in a cessa-
tion of growth. It is not difficult to prove an increase in leaf -surf ace as a con-
comitant of an increase in illumination if we compare etiolated with normal
leaves. STAHL (1883) has, however, calculated that in moderate light in shady
situations leaves become larger than in direct sunlight, and has shown that
beech-leaves, exposed to sunlight are only half, and leaves of the elder only
a quarter, the size of those grown in the shade. Thickness of the leaf stands
in close relation to area ; the thickness increases with the reduction of surface
and vice versa. It is known also that the anatomical structure of the leaf is
greatly affected by light. Elongated palisade tissue is
the characteristic feature of leaves exposed to sunlight,
spongy mesophyll of leaves which are shaded. While
many plants demonstrate themselves to be light or
shade plants by their leaf anatomy there are other im-
portant adaptations which are worthy of note (Fig. 90).

Experimentally it may be shown that each bud can
be made to unfold, and in doing so, if the light be suf-
ficient, it becomes a normal shoot, if insufficient an
etiolated one. In nature weakly illuminated buds do
not produce etiolated shoots, they simply do not develop
at all. Evolution of the bud takes place only if the
light be sufficiently intense, and the degree of intensity
varies greatly for different plants. WIESNER (1893-1900,
summary, 1902) has provided us with accurate measure-
ments on the subject which lead us to numerous impor-
tant results. A few of these only can be quoted on the
present occasion. [Compare WIESNER, 1904 and 1905 ;
CIESLAR, 1904 ; HESSELMAN, 1904.] WIESNER used the
BuNSEN-RoscoE method for measuring light intensity,
a method well adapted for estimating the highly re-
frangible rays only, i. e. those which are of most impor-
tance to the plant (p. 311). WIESNER has determined the
degree of light intensity under which a number of plants
will live in different surroundings. First he determined,
by the BUNSEN-ROSCOE method, the ' absolute photic ration ', and then
deduced therefrom the ' relative photic ration ' falling on the plant. If the
plant is able to live, on the other hand, in full sunlight, and also in a light
intensity reduced to one-tenth of the maximum, WIESNER says the relative
photic ration lies between one and one- tenth.

WIESNER gives the following data for Vienna :—

Fig. 90. Transverse sections
through the leaves of the cop-
per beech. /, illuminated ; //,
shaded. After NoRDHAUSEN
(I903, PI. 4).

Buxus sempervirens
Beech (enclosed)

„ (open ground)
Quercus pedunculate*
Betula verrucosa
Larix decidua

Relative photic
ration.

to ita
to -h
to ^
to &
to i
to

Minimum of absolute photic ration
calculated by the BUNSEN-ROSCOE
method.

•012

.015

.021

.050

.144

For one and the same species the higher the latitude or the altitude the
more the light that is required. Its absolute and relative photic ration in-
creases with the diminution of temperature. Thus the minimum of the relative
photic ration in the case of Acer platanoides alters from one-fifty-fifth near

X 2

308 METAMORPHOSIS

Vienna to one twenty-eighth at Drontheim, and to one-fifth at Tromso ; the
minimum of the absolute photic ration in Betula nana is 0-338 at Christiania,
0-386 at Tromso, and 0750 at Spitzbergen.

WIESNER draws the following conclusion from his observations : — ' Just as
the plant requires a certain amount of heat for the proper performance of its func-
tions, so also it needs a certain definite amount of light.' It has not, however,
been proved that the plant requires a certain amount of heat, we only know that
it needs a certain degree of temperature. Unless this be provided it is not only
impossible for it to thrive but all growth is impossible. Branches of the above-
mentioned plants can also grow without any light, and so light cannot be con-
sidered as an absolutely necessary condition of life in the same way as tempe-
rature (p. 301).

The formative influences of light hitherto discussed refer to disturbances in
the normal correlations of organs, and express themselves in increase on one side
and decrease on the other ; when total exclusion of light is effected and the plant be-
comes etiolated these phenomena are most clearly marked. When, however, we
speak of an etiolated plant yet another modification of the normal plant occurs to
us, viz. alteration in colour. Etiolated plants have white stems and yellow leaves,
since chlorophyll is not usually formed in darkness. We may with good reason,
however, distinguish between this colour change and real etiolation, by which we
understand only 'excessive elongation' and 'reduction,' since etiolation may
take place in cases where chlorophyll is present. Later on we shall become ac-
quainted with factors, other than darkness, which induce excessive elongation
without injuring the chlorophyll. Moreover, there are quite a number of plants
in which the formation of chlorophyll is independent of light and in which all
the same a marked etiolation takes place in darkness (ScniMPER, 1885). This
appears to be the case generally with Algae and mosses, while Pteridophyta
behave variously. The Equisetaceae, like Phanerogams, form no chlorophyll
in the dark, whilst the Filicinae do. The Gymnosperms are especially interest-
ing ; while the adult plant can form no chlorophyll in the dark, the seedlings of
Coniferae and of Ephedra have that power (SACHS, 1862 and 1864; BURGERSTEIN,
1900). In cases where the formation of a yellow-colouring matter only takes
place in chloroplasts in the dark, it is often quite sufficient to expose leaves which
are not too old for a short time to sunlight in order to obtain the green coloration.
If we again place the plant in darkness, after the light has acted for a time on
the etiolated leaf, but before the light has produced any visible effect, the
greening takes place in the dark as an after-effect. A very low light intensity
is sufficient for the purpose, greater intensities indeed tend to destroy the
chlorophyll.

All vegetable colouring matters do not behave like chlorophyll. SACHS
(1863) showed that many plants formed normally coloured flowers in darkness
(e. g. tulip, crocus, cucumber, &c.) ; in other cases, however, these floral colours
are formed only in light (ASKENASY, 1876). Similarly the formation of the red-
colouring matter, often found dissolved in cell-sap, depends entirely on the
presence of light (OVERTON). In all these examples we are dealing with effects
of light which may influence very greatly the general appearance of the plant,
but which are perhaps quite simple phenomena, to be explained by a sufficiently
thorough knowledge of the chemical composition of the colouring matter in
question. We have still, however, other formative influences of light to con-
sider which present more difficult problems for solution.

There are many plants of varied relationship which exhibit a different
form when young from that exhibited by them when more fully developed.
In many of these the juvenile form is produced by being exposed to light of less
intensity than in older stages. If the intensity of light remains low, the
later form is suppressed ; if the intensity of light diminishes after the adult

EXTERNAL CAUSES OF GROWTH AND FORMATION. I 309

form has appeared, the plant reverts to the juvenile condition once more. The
following may be cited as examples of this phenomenon. The alga Batracho-
spermum has a juvenile form named Chantransia (GoEBEL, 1889) ; protonemata
of mosses (KLEBS, 1893), and the elongated and circular leaves of Campanula
rotundifolia are examples of the same phenomenon (GoEBEL, 1896, Fig. 91).

Fig. QI. Campanula roiundtfolia. When feebly illuminated the flower-buds k are arrested ; a lateral shoot
A develops circular leaves. From GOEBEL'S Organographie.

Certain Cactaceae (Opuntia, Phyllocactus) may be added to this list, since their
shoots flatten only in light and revert to the original radial stem-like symmetry
in the dark (GOEBEL, 1895 ; VOCHTING, 1894).

Since the several organs of plants often bear the most varied relations to

310 METAMORPHOSIS

light intensity, the origin or development of an organ may be stopped by
insufficient illumination, or may be guided in other directions, so as to present
in the long run a totally different appearance. The Fungi form good examples of
this. The formation of sporangia does not take place in the dark in Pilobolus
microsporus, while the sterile sporangiophore grows far beyond the normal
length, and an analogous phenomenon inCoprinuswas referred to above (GRANTZ,
1898). Let us glance next at the behaviour of the roots in the higher plants.
In many cases these develop only in darkness, and hence aerial roots may appear
in etiolated plants, roots which are entirely wanting in plants when exposed
to light, even though they be cultivated in a damp atmosphere. On the other
hand, light stimulates the formation of buds and thus (WIESNER, 1895, 685) buds
are produced on the upper side of the downwardly-bent branches of the willow,
which are mostly illuminated from above, and on the underside of the upright
branches of the poplar, which are more illuminated from below. Very remark-
able effects of light have been observed in the case of subterranean shoots. The
offsets of Circaea form scale-leaves in the dark but foliage-leaves in the light
(GoEBEL, 1880) ; the stola of Adoxa may be made to grow greatly in length if
exposed to light, while darkening them at once induces a stoppage of longitudinal
growth and a formation of tubers (SxAHL, 1884). Further, the development of
potato tubers is associated with darkness, and it is possible by appropriate
means to induce the formation of tubers even on the top of the leafy shoot
(V6CHTING, 1887). Perhaps the most interesting case is that described by
BERTHOLD (1882) ; he was able by weak illumination to induce the formation
of roothairs from the apex of the shoots of many Algae (Callithamnion, Bryopsis).
In Bryopsis the pinnae also were transformed into roots in the dark (NoLL,
1888 and 1900 ; WINKLER, 1900 a).

We shall have another opportunity of studying the effect of the intensity
of light on flower formation, at present we have still to glance at the influence of
the direction of light on the structure of organs. Unilaterally incident light,
i. e. illumination of different parts of the plant with light of unequal intensity,
often produces formative results well worthy of notice. Very frequently in
plants which have developed polarity, light-direction determines which end
is to be base and which apex, which root and which shoot. In Equisetum,
according to STAHL (1885), the first plane of division in the germinating spore
is formed at right angles to the path of the incident ray, thus separating a shaded
root-cell from the illuminated prothallus-cell. WINKLER (1900 b) has observed
a similar phenomenon to occur in the egg -cells of Cystoseira barbata. He was
also able to determine the time required to develop this result ; thus unilateral
illumination for three hours had no effect, but four hours' exposure was sufficient
to produce a result after the plant had been many hours in darkness. Germination
and a differentiation of root and shoot also occurred, though delayed, without
any such unilateral illumination. Many lower plants behave in the same way,
while in others, and especially in the higher plants, polarity is undoubtedly
independent of external conditions.

The symmetry of the plant body is also dependent on the direction of light,
being radial when the light falls equally on all sides and dorsiventral when the
light is unilateral. Thus in Antithamnion cruciatum the branches are approxi-
mately decussate in diffuse light, but if illuminated on one side only they all
arrange themselves in one plane perpendicular to the path of the incident ray.
Other structures are always dorsiventral and the light determines only which
side shall be the upper, which the lower. Examples of this are seen in the
shoots of Lepismium radicans and Hedera helix, where roots are formed on the
shaded side only ; the rhizome of Caulerpa forms roots on the shaded and shoots
on the illuminated side, in fern prothalli roothairs and sexual organs are formed
on the shaded side, and in the thallus of Marchantia rhizoids are formed under-
neath and assimilatory parenchyma above. The direction of the light determines

EXTERNAL CAUSES OF GROWTH AND FORMATION. I 311

dorsiventrality in higher plants also, e. g. in Thuja (FRANK, 1873), and Begonia
{ROSENVINGE, 1889). In most cases it is possible by altering the direction of
the light-incidence to produce corresponding changes in the dorsiventrality;
thus fern prothalli may be made to form sexual organs on the upper side by
illuminating the lower, but the dorsiventrality in Marchantia is so fixed by
heredity that new growths are no longer affected by external factors, but follow
in their development the plan of structure already laid down by the fully-
developed regions.

We must content ourselves with these examples and in conclusion inquire
as to the significance of the quality of light, the colour or wave-lengths of the
rays. We arrive at this most important fact that in growth and formative
processes the more refrangible rays are those which are operative. In many
cases it has been shown that the less refrangible rays which do most of the work
in carbon-dioxide assimilation act morphogenetically like darkness. We see,
therefore, how little the cessation of carbon-dioxide assimilation has to do with
etiolation, since green plants become etiolated in red light in spite of assimilation
taking place at the same time. The formation of chlorophyll goes on equally
well in light of all wave-lengths (REINKE, 1893). [Other pigments behave
differently ; in Oscillaria, according toGAiDUKOw's (1903) experiments — which
wait confirmation however — a so-called complimentary adaptation of colour
takes place, that is to say, the cells become red in blue light and blue in red light.]

For long it was believed that ultra-violet light had quite a special effect
on the external formation of the plant. SACHS'S (1887) statements on the point
have not, however, been confirmed (compare p. 364). The influence of Rontgen
and of other recently-discovered rays, has been studied, but without leading
to results of any physiological value. [Compare KORNICKE, 1905.]

Bibliography to Lecture XXIV.

ASKENASY. 1876. Bot. Ztg. 34, i.

BERTHOLD. 1882. Jahrb. f. wiss. Bot. 13, 569.

BONNIER. 1895. Revue gen. de bot. 7, 241.

BORODIN. 1868. Bull. Acad. Petersbourg, 13, 432.

BREFELD. 1877. Bot. Untersuchungen iib. Schimmelpilze, 3, 93.

BRENNER. 1900. Flora, 87, 23.

BROWN and ESCOMBE. 1895. Proc. Roy. Soc. 62, 160.

BURGERSTEIN. 1900. Ber. d. bot. Gesell. 18, 168.

[CIESLAR. 1904. Centrbl. f. d. Ges. Forstwesen. i.]

DARWIN, F. 1896. Journal Roy. Hort. Soc. 19 (Bot. Ztg. 1896).

FRANK. 1873. Jahrb. f. wiss. Bot. 9, 147.

[GAIDUKOW. 1903. Scripta Horti Petropol. 22, 7.]

GODLEWSKI. 1889. Biolog. Centrbl. 9, 481.

GOEBEL. 1880. Bot. Ztg. 38, 794.

GOEBEL. 1889. Flora, 72, i.

GOEBEL. 1895. Ibid. 80, 96.

GOEBEL. 1896. Ibid. 82, i.

GRANTZ. 1898. Einfl. d. Lichtes auf die Entw. einiger Pilze. Diss. Leipzig.

HEALD. 1898. Bot. Gaz. 26, 25.

HEINRICHER. 1899. Ber. d. bot. Gesell. 17, 308.

[HESSELMAN. 1904. Zur Kenntniss des Pflanzenlebens schwedischer Laubwiesen.

Jena.]

HILBRIG. 1900. Einfl. supramax. Temp, auf d. Wachstum. Diss. Leipzig.
JOST. 1895. Jahrb. f. wiss. Bot. 27, 403.
KLEBS. 1893. Biol. Centrbl. 13, 641.
KNY. 1902. Jahrb. f. wiss. Bot. 38, 421.

KOPPEN. 1870. Warme u. Pflanzenwachstum. Diss. Moskow.
[KORNICKE. 1905. Ber. d. bot. Gesell. 23, 324 and 404.]
LEITGEB. 1 876. Sitzungsber. Wiener Akad. 74.
LEITGEB. 1886. Mitt. a. d. bot. Instit. Graz, i, 123.
[MEZ. 1905. Flora, 94, 89.]
MOBIUS 1895. Biol. Centrbl. 15, i.

312 METAMORPHOSIS

MOLISCH. 1897. Unters. iib. das Erfrieren d. Pflanzen. Jena.

MULLER-THURGAU. 1886. Landw. Jahrb. 15, 453.

NOLL. 1888. Arb. bot. Inst. Wiirzburg, 3, 466.

NOLL. 1900. Ber. d. bot. Gesell. 18, 444.

NORDHAUSEN. 1 903. Ber. d. bot. Gesell. 21, 30.

OVERTON. 1899. Jahrb. f. wiss. Bot. 33, 171.

PALLADIN. 1890. Ber. d. bot. Gesell. 8, 364.

POPOVICI. 1900. Bot. Centrbl. 81, 33.

PRANTL. 1873. Arb. bot. Inst. Wiirzburg, i, 371.

RACIBORSKI. 1900. Bull. Inst. de Buitenzorg, No. 6.

REINKE. 1876. Bot. Ztg. 34, 143.

REINKE. 1893. Sitzungsber. Berliner Akad. 527.

ROSENVINGE. 1889. Revue gen. de bot. i, 153.

SACHS. 1862 and 1864. Flora, 45, 186 ; 47, 505.

SACHS. 1863. Bot. Ztg., Beilage.

SACHS. 1864. Flora, 47, 8.

SACHS. 1872. Arb. bot. Instit. Wurzburg, i, 99.

SACHS. 1887. Ibid. 3, 371.

SCHIMPER. 1885. Jahrb. f. wiss. Bot. 16, i.

STAHL. 1883. Jen. Zeitschr. f. Naturwiss. 16.

STAHL. 1884. Ber. d. bot. Gesell. 2, 389.

STAHL. 1885. Ibid. 3, 334.

STAMEROFF. 1897. Flora, 83, 135.

STREHL. 1874. Langenwachstum d. Wurzel u. des hypocotylen Gliedes (Diss.

Leipzig).

THISELTON-DYER. 1899. Proc. Roy. Soc. 65, 362.
VOCHTING. 1878. Organbildung im Pflanzenreich, I. Bonn.
VCcHTiNG. 1887. Bibliotheca botanica, Heft 4.
VOCHTING. 1894. Jahrb. f. wiss. Bot. 26, 438.
VOCHTING. 1002. Bot. Ztg. 60, 87.

WIESNER. 1873. Sitzungsber. Wien. Akad. Math.-nat. Kl. 67, 1,9.
WIESNER. 1891. Ber. d. bot. Gesell. 9, 46.

WIESNER. 1894. Sitzungsber. Wien. Akad., Math.-nat. Kl. 103, 401.
WIESNER. 1893, J89S, 1900. Photometr. Untersuch. auf pflanzenphys. Gebiete.

Sitzungsber. Wien Akad. Math.-nat. Kl. 102 (1893), IO4 (I895), 109 (1900).
WIESNER. 1902. Biologic d. Pflanzen. Vienna.
[WIESNER. 1904. Sitzungsber. Wiener Akad. 113, T. 469.]
[WIESNER. 1905. Ibid. 114, T. 77.]
WINKLER. 1900, a. Jahrb. f. wiss. Bot. 35, 449.
WINKLER. IQCO, b. Ber. d. bot. Gesell. 18, 297.

LECTURE XXV
EXTERNAL CAUSES OF GROWTH AND FORMATION. II

IN addition to heat and light, gravity must be considered as a factor fre-
quently influencing the growth and shape of plants. To begin with it may be well
to illustrate by means of a few examples that the weight of the entire plant or of
its parts is not only not injurious, but is even helpful to vitality. Great weight,
absolute or relative, is naturally, for purely mechanical reasons, a disadvantage
in the distribution of seeds, and hence we find developed in many cases floats,
wings, or other adaptations of the most varied character. In aquatic plants
also we find the vegetative organs provided with aids to flotation, for the water
has to support the weight of the organs which terrestrial plants themselves
support. Land plants possess, indeed, special mechanical tissues which are
absent from aquatics. Twining and climbing plants exhibit special adaptations
(Lectures XXXV and XXXVIII), e. g. holdfasts of various kinds which render
the development of a special strengthening skeleton more or less unnecessary.

All these adaptations are special peculiarities of the plant as such, which,
are in no respect due to the direct influence of gravity on the individual.

EXTERNAL CAUSES OF GROWTH AND FORMATION. II 313

More important for us are the accessible effects of gravity as shown by ex-
periment. Further, in these effects also it is often that of the weight of an
entire organ that is concerned, a tension or pressure, and in this case gravity
is of course replaceable by other forces. On the other hand, gravity exerts
a specific influence which, even when the weight of the entire organ is eliminated
by being supported by a prop or by immersion in water, makes itself felt both
in cells and cell parts. We will begin our studies with a consideration of such
specific effects of gravity ; the effects of pressure and tension we may postpone
till later.

The specific effect of gravity exhibits itself especially in the direction which
plant organs assume in space, and to which they return if they be displaced.
These movements are produced by changes in shape, which we will consider in
another connexion (Lectures XXXIV and XXXV). It may be remarked, how-
ever, that both in these movements and also in the effects on growth and for-
mation to be described here, gravity may be replaced by centrifugal force,
from which we may conclude that it is the * mass -acceleration ' which
directly or indirectly affects the plant. The fact that gravity may be replaced
by centrifugal force suggests the question as to the effect of the intensity of
the acceleration. ELFVING'S (1880) and SCHWARZ'S (1880) experiments have
shown that growth is not affected by increasing or reducing the acceleration, but
disturbances in the arrangement of the cell-contents are induced by great centri-
fugal force (MOTTIER, 1899), and disturbances in growth are also to be expected.

In nature the question of the intensity of mass-acceleration is of little conse-
quence, because the differences in the value of gravity on the earth's surface are
too small to be worth notice. The direction in which gravity acts on the plant
is of far more importance. Very frequently the symmetry of the plant part
depends on how it lies with regard to gravity, being radial when its long axis is
parallel with the direction of gravity, and dorsiventral when it is not so. Dorsi-
ventrality exhibits itself in the distribution and formation of the lateral branches ;
on the dorsiventral stem the branches and leaves are usually produced either
on the upper side only, or if they can develop all round, those on the under
side are distinguished by size from those on the upper (anisophylly). Roots
also appear to be developed only on the underside of dorsiventral organs.
Dorsiventrality may be recognized either at the growing point by the position
of the primordia of organs, or later on in the course of development of such
organs. The latter phenomenon is very common and is especially well seen in
the cuttings of the willow for example (VocHTiNG, 1878). If such a cutting
be suspended in its normal position in a moist chamber it forms at its apex
radially arranged lateral shoots, and similarly disposed roots at the base,
but the median region develops no lateral branches. If the cutting be placed
horizontally branching takes place at apex and base as before, but a number
of lateral shoots also appears on the upper surface from the apex backwards, and,
further, numerous roots arise from the under surface ; in other words the branch
has become dorsiventral. Finally, if the branch be suspended upside down, its
apex downwards and base uppermost, the largest shoots and roots appear as
in the first experiment, at the apex and base respectively, but both types of
organ develop progressively towards the opposite pole as in the normal
orientation. In this case a very obvious effect of gravity is disclosed, but it has
no power to alter the pre-existing polarity. This is very well shown in the case
of certain cultivated arboreal forms, e.g. the so-called 'weeping trees'. The
pendent branches of these plants, in spite of their inverted position, continued to
form lateral branches at their apices (VOCHTING, 1878).

As yet we know of no case where gravity has induced polarity in the growing
point or in the ovum, as, for example, light is capable of doing in Equisetum ;
the possibility of such an effect, however, cannot be denied. The examples

3H METAMORPHOSIS

quoted might be largely added to, still, on the whole, the morphogenetic influence
of gravity, much over-rated in HOFMEISTER'S time, is very limited. Gravity
has nothing like the effect on plant shape that light has.

Growth in length, on the contrary, is in general markedly influenced by
gravity. Thus it has been clearly established that Chara and Phycomyces
(ELFVING, 1880 ; RICHTER, 1894) grow more slowly when inverted than when
in the normal position, and other plants behave in a similar manner. [HERING
(1904) has shown that this retardation of growth in consequence of inverted
orientation is a phenomenon of widespread occurrence.] When shoots and
roots are placed at an angle to the direction of gravity their upper and under
sides grow at different rates, with the result that these members bend in a way
which we shall have to study later (Lecture XXXIV).

Growth in thickness of trees is also not uniformly affected by gravity.
In all sloping branches the upper side grows in thickness at a different rate
from the under. In Coniferae (and in Aesculus also) the under side grows more
in thickness than the upper side, while the converse takes place, at least at first,
in Dicotyledons (WiESNER, 1895-6). Extensive investigations undertaken by
HARTIG (1901) on the Coniferae show that the increased growth of the under side
may occur in the main stem also when it is placed horizontally even when ap-
propriately supported so that the action of mere weight on the under side is
neutralized ; the effect can be attributed under these conditions only to the direct
influence of gravity. Further, the under side exhibits not merely an increase
in secondary thickening, but also a special anatomical structure in the wood
formed ('redwood '), attributed also to the direct action of gravity by HARTIG.

HARTIG further attempted to show that the increased growth and formation
of ' redwood ' arose from pressure longitudinally exerted on the cambium,
which must arise in general in horizontally-placed branches owing to the influ-
ence of mere weight. Similarly, feebler increase in thickness and characteristic
anatomical structure of the wood on the upper side may be accounted for by
the tension in the longitudinal direction to which the cambium is exposed in
that region. HARTIG has given us no direct proof of this conjecture, but no one
can deny that this explanation is an extremely probable one, for it has often
been observed that tension and pressure have a distinct influence on growth.

Pressure exerted on growing cells must retard their growth and may finally
completely stop it. Cells so prevented from growing exert on their part a pres-
sure on the surrounding tissues, often leading to quite obvious mechanical
results. As PFEFFER (1893) has shown, this outward pressure is brought about
in this way — the cell-wall becomes relaxed through surface-growth, and the
whole osmotic pressure is thus directed against the external opposing layer. In
individual cases an increase in the osmotic pressure may also be observed under
such conditions. Frequently the plant is able in this way to overcome ex-
ternal resistance, as, for example, in the splitting of rocks by roots.

Tension of necessity acts on cells in the reverse manner to pressure. An
increase in growth in the direction of the tension is to be expected, and that
such takes place is easily proved if a stem be stretched by a weight. The pull
has, however, at the commencement quite a different effect ; it acts as a stimu-
lus and induces a retardation in growth, followed later on by the acceleration
mentioned (HEGLER, 1893).

The new cell-walls formed in cell division show many relations to tensions
and pressures exerted on them ; the planes of division appear parallel to the line
of action of the pressure and transversely to that of the tension, provided there
be no other circumstances to prevent that result (KNY, 1901). A further stimu-
lative effect of tension is seen in the mode of formation of the internal tissues.
In ripening fruits under natural conditions, and in experiments on other organs,
the amount and also the thickness of the mechanical elements increases in pro-

EXTERNAL CAUSES OF GROWTH AND FORMATION. II 315

portion to increase in weight. [KELLER (1904) has recently shown that in the
peduncles of ripening fruits there is an increase in the tendency to rupture, but
he (as well as WIEDERSHEIM and BALL) has shown that this is not due to in-
creased weight.] Hence we obtain morphogenetic responses as the result of
the action of mechanical factors, responses which are frequently observed in other
cases. Thus, by bending of the main root the development of lateral roots
from the concave side may be prevented, they then arise only on the convex or
stretched side (Fig. 92, NOLL, 1900). Very often we may observe results of
pressure manifesting themselves with different intensity in adjacent parts
of the same organ. Such ' contact sensitivity ' is illustrated by roothairs,
which exhibit a diminution of growth when in contact with the soil particles,
accommodating themselves in the most complete way to the inequalities of
surface. Contact responses of a special kind are exhibited by the tendrils of
species of Ampelopsis which form holdfasts by pressing their apices in close
contact with some solid body (LENGER-
KEN, 1885). Lastly Mucor stolonifer pro-
duces stola whose apices, when they come
in contact with the substratum, attach
themselves to it by means of rhizoids,
the plant thereafter proceeding to form
sporangiophores (WORTMANN, 1881).

The examples cited must suffice to
illustrate the influence of pressure and
tension on growth, and we must now
turn to the consideration of certain
other external influences on growth
which act sometimes chemically, some-
times mechanically. Since all growth
is dependent on the supply of nutri-
tive substances, it follows that the
essential nutrients we have already re-
ferred to may be considered as con-
ditions of growth, and each of these
must be present in a certain minimum
quantity before growth takes place.
An excess of the other nutrients does not
•compensate the plant for the deficiency
in one. Under such starvation con-
ditions growth results for the most part
in a diminution in the size of the whole
plant. HEINRICHER (1896) has described plants of Sinapis nigra, which when
sown closely on bad soil reached a height of only 18 mm., but which formed
both flower and fruit. Similar dwarf plants were obtained by LUPKE (1888) in
cultures without potassium ; a similar case is shown in Fig. 93. The reduced size,
which many plants exhibit in the absence of nutritive materials, may be con-
sidered as a phenomenon of adaptation, since the organism is able in this way to
complete its life-cycle, while, by employing the nutrients available for the forma-
tion of organs of normal size, only a single leaf perhaps could be formed, and
the development would then come to an end. The dwarfing of all the organs,
i. e. harmonic dwarfing, as we may quite correctly term it, is, however, not the
only response given by the plant to absence of nutrients ; in some cases inhar-
monic growth also results. We find, for example, that when nitrogen is absent
roots, roothairs, and internodes undergo excessive elongation (NOLL, 1901 ;
BENECKE, 1903). We may term this ' etiolation ', and biologically it is obviously
•closely related to etiolation due to absence of light. As the individual nutrients

Fig. 92. Young lupin with a curved principal root.
The lateral roots are developed exclusively on the
convex sides. After NOLL. From the Bonn Text-
book.

METAMORPHOSIS

increase in amount we at length reach certain quantities of each which produce
an optimum effect, and when these amounts are exceeded injurious results
follow, due either to osmotic or to chemical influences, and resulting in death
if a certain maximum be exceeded.

Amongst the substances which are essential to most plants oxygen is of
special importance. It is not a ' food-stuff ' in the ordinary sense of the term,
since it is associated, not with constructive, but with destructive metabolism,
e. g. with respiration. Growth is affected in a most marked manner by the
degree of concentration of oxygen present. If care be taken that the air-pres-
sure as a whole remains unaltered, decrease in oxygen pressure induces an
acceleration of growth, so that the normal amount of oxygen in the air may be
considered as supra-optimal so far as growth is concerned. In many cases,
however, by increasing the partial pressure of oxygen
an increase in the rate of growth may be observed, so
that there would appear to be two optima as regards
concentration of oxygen. Possibly this may be an in-
direct result of experimental conditions. In all cases,
however, we are able to increase or decrease the
amount of oxygen present in the air to such a degree
as to retard growth and finally bring about death
(maximum and minimum oxygen pressures). After
what we have already learned as to the oxygen -
requirements of different plants, the specific differences
in minima and maxima of that gas are easily under-
stood. A concentration which is subminimal to an
ordinary aerophilous plant may be supramaximal to
aerophobous ones such as anaerobes [compare PERODKO
(1904)]. As transitional forms between these two ex-
tremes, the sulphur Bacteria are of special interest,
because they have a very low oxygen-optimum, although
oxygen is absolutely essential to them. While typical
anaerobes require as a condition of growth a very low
partial pressure of oxygen, genuine aerobes as a rule do
not exhibit growth when the conditions are such as
to induce mtra-molecular respiration (WiELER, 1883
and 1901), or at most they show only a very insignifi-
cant increase in length (NABOKICH, 1901-2).

As already noted, many of the substances essential
to plant life act injuriously when certain definite con-
centrations are reached, and if the injury be referable
to the chemical action of the substance we may term
it a ' poison '. Many metabolic products found in plants

are in this sense poisonous both to the plants which produce them and to
others as well. [NIKITINSKY (1904) has also discovered metabolic sub-
stances in Mould-fungi which accelerate growth in these plants.] Plants are
in general capable of resisting the action of their own metabolic products, but
only if they occur in limited amount. Retardation of development results
in the long run from the presence in excess of alcohol or acids produced
during fermentation, and so also in the higher plants the products appear-
ing in them may act injuriously if they be not used up, e.g. carbon-dioxide,
or be not made insoluble and hence innocuous, as, e.g., when oxalic acid
unites with calcium. There are also substances, however, which never occur
in plants and with which they do not come into contact in nature, which are
virulent poisons, retarding growth even in very dilute solutions. It is unneces-
sary to enumerate these poisons here ; we need only note that many substances

F»g' 93- Seedling rose
grown in the Strassburg Bo-
tanic Garden, which after
forming a few leaves pro-
ceeded to form flowers in the
first year of growth (nat. size).

EXTERNAL CAUSES OF GROWTH AND FORMATION. II 317

are equally poisonous to plants and animals, while others affect even closely-
related organisms in entirely different ways. This is explained in part by the
fact that protoplasm is not identical in all organisms, but more especially that
the relative rate of entry of the poison into the protoplasm is most varied. Thus
PULST (1902) has shown that sulphate of copper, which is generally a virulent
poison, like most of the salts of the heavy metals, has no effect on Penicillium,
only because it is not absorbed by this fungus. It is quite inexplicable, how-
ever, why sugar and peptone, which are valuable nutrients to the majority
of plants and not in themselves in any sense injurious, should be extremely
poisonous to nitro-Bacteria (p. 229).

It is of special interest to recall the fact already mentioned that many
poisons have not only no injurious effect in dilute solution, but are actually of
service to the organism by stimulating its respiratory and metabolic activity.

We may also recognize that many substances act as chemical * stimuli' which
are not poisonous ; even nutrients may act as such, though that is not their
chief function ; at all events this aspect of their activity is not to be associated
with their nutritive value. Examples of these phenomena are seen in cases
where growth is initiated by the presence of substances whose nature is in some
cases known, in some cases unknown, and more especially in the germination
of spores and pollen-grains. For example, the pollen-grains of certain species
of Mussaenda germinate in distilled water, according to BURCK (1900), but only
when a small portion of the stigma is added to the water. Apparently the
stigma contains levulose, for of all the materials experimented with, and especi-
ally sugars, this was the only one which acted in this way even when merely very
minute traces were present. It is not easy to understand why dextrose should
not act in the same way if it be merely the addition of some material needed
for growth that was wanted. If, however, levulose be considered merely as
a stimulant to growth, then the extreme specialization becomes intelligible.
Further, closely- allied forms show great differences in this respect ; the pollen of
Pavetta javanica germinates only in an extract of the stigmas of that plant or of
Pavetta fulgens, but not in that of other species! Finally, it may be noted that
according to DE BARY (1884) spores of Completoria9Protomyces, andSynckytrium
germinate as a rule only on their host-plants, and that Orobanche and Lathraea
develop their haustoria only in the neighbourhood of the roots of their hosts.
It cannot be doubted that in these cases also some growth stimulant of a definite
chemical nature is given off from the host-plant, but such substances have not
as yet been isolated.

That chemical stimuli are also able to act in a formative manner need not be
further exemplified, since we have already seen (p. 249) how Basidiobolus behaves
under such conditions. Later on (galls, p. 320) we shall have an opportunity
of studying some morphogenetic results of chemical stimuli.

We may conclude this account of the effect of materials on growth by
considering water, which in addition to its chemical effect has also undoubtedly
an important physical influence on imbibition and turgescence, and hence on the
elasticity and pressure phenomena of the plant. When water is withdrawn,
as a rule all vital activities, including growth, come to an end, but certain plants
retain capacity for life even in the desiccated condition. Many mosses, lichens,
and even species of Selaginella are able to endure air-drying without suffering
permanent injury, and to start growing again as soon as water is once more
supplied. The majority of plants, however, die when once dried in the vegetative
state. The capacity for withstanding desiccation is very general in the resting
stages of plants, e. g. ' spores ' and seeds ; and in many of the lower plants the
actual formation of such bodies is dependent on the withdrawal of water. These
resting stages can frequently withstand much higher degrees of desiccation than
can be obtained by simple air-drying ; many seeds, for example, can withstand

3i8 METAMORPHOSIS

drying induced by a temperature of 100° to 110° C. without being killed, while
many mosses are killed by drying in a desiccator. As in general, so also in
plants which live under special conditions of life, special peculiarities are de-
veloped, whilst plants whose seeds under normal conditions are never desic-
cated, frequently do not possess the power of withstanding drought.

Long before air-dryness is reached turgidity is destroyed, making itself
evident by the wilting of the plant members. The capacity to endure wilting is
also very varied among plants. Certain succulents can endure a loss of as much
as 90 per cent, of the water they contain without suffering permanent ill effects,
other plants again can only stand the loss of half the water they normally hold.
When turgor is destroyed, growth over all comes to an end. The loss of water
may be due either to transpiration under conditions when the supply of water
is insufficient, or to osmotic action due to the presence of salt solutions which
themselves produce no chemical effect. The results are not identical in the two-
cases, and this is quite intelligible for the reason that when water is withdrawn
osmotically, as has been already pointed out, by entry of the salt or by renewal
of the osmotically active material, a reaction follows such as is not possible in
a wilted plant. Further, it is difficult for a wilted plant to retain definitely
a reduced quantity of water ; it will either re-absorb water and recover or give
off more and die. Yet there are many plants which may be preserved in the plas-
molysed condition for a long time without death taking place. Algae, for
example, may be so treated and preserved alive for many weeks. No growth,
however, is observable in them, though new cell-walls may be formed ; every
plasmolysed cell in the long run dies. Each alteration in concentration or of the
osmotic pressure of the external medium induces injury, and even the periodic
changes which take place, for example, in the water in the estuary of a river, de-
pendent on ebb and flow of tides, are such as few Algae can tolerate (OLTMANNS,
1891).

It will be seen from these remarks that the rate of growth and the final
size attained by every plant depend on the amount of water it contains, and this
in turn depends on the relation subsisting between absorption from the soil and
transpiration into the air. In addition to many other factors the amount of
moisture in the air, as well as the quantity of water and salts in the soil, plays an
important part. The minimum, optimum, and maximum for different plants,
and even for individual organs of a single plant, vary greatly. The researches
of TUCKER and SEELHORST (1898) on the influence of water on the relationship
between the roots and sub-aerial organs of the oat are of special interest. A
limited amount of water in the soil excites active growth in the root ; it cannot,
however, in spite of its great extent of absorbent surface, supply the aerial parts
with sufficient water, and hence these remain small, the relation between the root
and the whole plant being I : 7-4 while it reaches 1 : 16-6 when there is plenty of
water in the soil. In this latter case the root remains small, its optimum being
already exceeded. Thus between the root and the shoot there are appropriate
correlations.

We may now turn to the effect of moist and dry air as factors in deter-
mining form in the plant. It is impossible for us to enter into detail on this
question — a statement of the general results must suffice, because the material
for discussion is so abundant. It has been shown that retardation and accelera-
tion of transpiration very often form a self -regulating mechanism ; the plant
in dry air develops adaptations for reducing transpiration, in moist air for
accelerating it. The variability of the plant, and especially of the higher plant,
has been shown to be far greater in this respect than any one would have
imagined twenty years ago. These adaptations are seen not only in external
form, but also in anatomical structure. Plants grown in damp atmospheres
have longer internodes and petioles, and larger but thinner laminae. The
weakly transpiring leaves of Tropaeolum are, according to KOHL (1886), five times

EXTERNAL CAUSES OF GROWTH AND FORMATION. II 319

as large as those of plants grown in dry air and soil. Thus the outlines of organs
become less irregular in damp atmospheres (KoHL, 1886), that is to say the projec-
tions from the leaf become less marked, the ribs on the stem tend to disappear,
and the development of hairs becomes reduced. The anatomical differences
are even more remarkable. Increased transpiration tends to thickening of the
cuticle and induces the development of collenchyma and sclerenchyma, the
vessels are larger and more numerous, and abundant palisade parenchyma is
formed in the leaf. Critical researches are still required, however, to deter-
mine for us how far the observed results are to be put down simply to the dif-
ference in the amount of water present in the plant, and how far to the differ-
ences in the rate of transpiration ; if the latter factor be the important one it
will be necessary in the next place to find out whether the giving off of water
as such acts as a stimulus, or whether the supply of nutrients which stand in
close relation to transpiration is of greater significance.

When plants which normally live in dry regions are cultivated in a moist
chamber very remarkable results are obtained. LOTHELIER (1893) found that
the formation of thorns was inhibited in a very damp atmosphere ; for example,
Berberis developed leaves in place of thorns and Ulex developed ordinary leafy
branches. GOEBEL (1898), who repeated these experiments, was not, however,
able to confirm these results entirely ; he was able to observe retardation, but not
complete inhibition, of thorn formation. Still more remarkable results were
obtained by BRENNER (1900) from succulents. Fig. 89 (p. 305) shows the habit of
Sempervivum assimile, at I under normal conditions, and at // after lengthened
culture in damp air. As transpiration becomes weak the elongation of the
internodes destroys the radical rosette but later on a new rosette is formed.
The leaves grow more vigorously on their upper sides and hence bend towards
the ground in arches ; at the same time they become markedly thinner. This
diminution in thickness, the resolution of the rosette (which has been observed
in other plants under the same conditions by WIESNER (1891)), and finally certain
anatomical alterations, e. g. the bulging out of the epidermal cells, were cor-
rectly considered by BRENNER as adaptations to aid transpiration.

Many of the alterations in form and anatomical structure observed to take
place in plants grown in damp air remind us of those which occur in plants kept
in darkness. Sempervivum assimile, for example, loses its rosette form in dark-
ness and its leaves are markedly smaller. Increase in dampness of the atmo-
sphere is, however, very often associated in nature with diminution in light, and,
conversely, strong insolation is often mostly associated with more active transpi-
ration. In nature it is very rarely that we get a single factor determining the
shape of the plant. If we find, for example, that subterranean branches are
different anatomically and morphologically (CoSTANTiN, 1883 and 1886) from
aerial branches, we are led to believe the true reason lies not only in differences
in illumination, but also in the amount of water present in the medium, and
possibly even in the fact that such branches are in intimate contact with soil
particles. Similarly, the characteristic structure of water-plants is concomitant
not only with retardation of transpiration, but also with the changed relationship
to light, oxygen, carbon-dioxide, &c. According toMAcCALLUM(i902), the active
cause of the aquatic form of Proserpinaca palustris is due entirely to retardation
of transpiration ; if transpiration be compensated by the osmotic absorption of
a concentrated salt solution, typical aerial leaves arise. We await confirmation
of these results. [The studies of BURNS (1904) show that this phenomenon is
not so simple as it seems.] Similarly, several factors contribute to the forma-
tion of the alpine (BONNIER, 1895) and the halophytic (ScniMPER, 1891 ; STAHL,
1894) types of plant life. It is impossible for us to enter into further detail
here, so we will content ourselves with noting that the structure of the plant is
not predetermined once and for all, but that it is capable of being modified by
external conditions.

320 METAMORPHOSIS

We have yet to glance at the effect of other organisms on plant shape. The
plant can no more escape from the influence of such associations than it can
from heat or gravity, since, wherever organisms occur, there we find a competi-
tion for space, for light, and for food. The result of this contest is that many
of the organisms are killed off because the survivors have prevented them from
obtaining the necessary supplies of food material or light. The action of the
organism in these cases is only to induce chemical or physical alterations in the
surroundings, and these, in their turn, bring about such effects as we have already
made ourselves acquainted with. On the other hand, if an organism injures the
plant, if it be partly eaten by an animal, for instance, changes occur in the plant
body not merely in consequence of the injury as such, but as the result of the re-
action of the plant to it. The direct effect of injury needs no elucidation — the in-
direct effects will be treated of in the next lecture. The relations existing between
the plant and animal worlds are well known, such as those associated with the
transference of pollen by insects and the phenomena seen in myrmecophilous
plants. There can be no doubt that plants are adapted to the visits of insects,
and have become altered in shape accordingly ; but this is a phylogenetic phe-
nomenon on which as yet no experimental research has been carried out, so
that we may omit any consideration of it.

In addition to the physico-chemical influences of the organism on the
environment above referred to and the related effects of the presence of other
organisms, there remains for consideration the direct influence of one organism
on another, as when two organisms live together symbiotically or antibiotically.
Here also the alterations in structure and appearance, often very remarkable,
may be referred ultimately to chemical and mechanical factors, but it is im-
possible to treat of these problems comprehensively, because we do not know
how these factors act individually, and also because in nature they emanate
from the organisms themselves. We may, however, quote a few examples of
the morphogenetic effects which arise from symbiosis or parasitism.

Variations from the normal plant shape are induced by the action of certain
parasites, and known as galls. It is impossible to cite more than a fraction of the
extensive literature on galls; the following references must therefore suffice : —
HOFMEISTER, 1868 ; GoEBEL, 1898 ; ECKSTEIN, 1891; KusTER, 1903. Fungi are
the most prominent causes of galls in plants, and in addition Bacteria (p. 238),
Myxomycetes, and Algae ; parasites of higher rank are usually not counted as
gall-producers, although they also may induce Variations from the normal plant
shape '. Among animals we take account first of all of gall- wasps and gnats, but
there are other insects, as well as worms, which also produce galls. Let us con-
sider first of all fungus-galls. The effect of the fungus on its substratum may
be to destroy the immediate region affected or, in the long run, the whole plant.
That this is due to the activity of a poison given off by the fungus is obvious, and
there are cases known where the poison has been isolated, as, for example, oxalic
acid (DE BARY, 1884 ; REINHARDT, 1892). Such a radical procedure on the part
of the fungus is, however, not to the purpose, since rapid growth of the fungus
exposes it to injury by destroying its host-plant and finally itself. The behaviour
of other Fungi, however, is much more to the point since they do not injure
their host-plants at all or may even stimulate them to more vigorous growth.
Many Uredineae, and also Erysiphe guttata, stimulate the formation of chloro-
phyll in the host ; the Synchytrideae cause the epidermal and neighbouring
cells to grow vigorously in size, while others cause swelling of entire internodes,
leaves, fruits, &c. Where such hypertrophies are induced we speak of genuine
gall-formation. Anatomical investigation shows a great enlargement of the
parenchyma cells in these hypertrophies and often also an increase in number
of these, and the development in all of them of abundant protoplasm and starch.
These alterations are all to the benefit of the fungus as supplying it with extra

EXTERNAL CAUSES OF GROWTH AND FORMATION. II 321

nutritive material. This active development of parenchyma is accompanied
by a reduction in the amount of sclerenchyma and collenchyma. It may be
assumed that these galls are induced by some substances excreted by the fungus
which operate in such a way as to stimulate growth in the host-plant just as
many poisons do.

The alterations in form effected by other Fungi are even more peculiar than
these comparatively simple hypertrophies. Thus the whole appearance of cer-
tain species of Euphorbia is altered byUromyces pisi, andMelampsorella cerastii
produces the well-known ' witch-brooms ' on the silver fir, altering dorsiventral
into radial shoots and perennial into annual leaves. Both of these forms of
Uredineae ripen their aecidiospores on the metamorphosed plant, and these
germinate on another host, producing in it no modification worth speaking of.
Whether that be due to an alteration in the fungus or to peculiarities in the
second host cannot at present be determined. Further examples of such modi-
fications of form are given by Peronospora violacea, which alters the stamens
of Knautia arvensis into petals forming a ' double flower ', and by Ustilago anthe-
rarum, which stimulates growth in the otherwise dwarf stamens of the female
flower of Lychnis vespertina, and so apparently brings about the formation of a
hermaphrodite flower ; the anthers are, however, filled with the
reproductive organs of the fungus only and produce no pollen.

The greatest modifications produced by Fungi are the
new formations, e. g. the spherical outgrowths of the alpine
rose which remind one of Cynics-galls, but are really due to
the attacks of Exobasidium vaccinii, and especially the witch-
brooms induced by Taphrina laurenciana, that is to say, the
adventitious shoots with abnormal leaves which arise from the
foliage leaves oiPteris quadriaurita. [A very thorough study
of the anatomy of fungus-galls has been carried out by

GUTTENBERG (1905).]

The galls induced by insects are far more varied and, in
extreme cases, more complicated than fungus-galls. We may
note first of all cases where the organs of the plant attacked are
altered into others, and where the organs develop normally but
in unusual situations. Lima juncorum causes the altera-
tion of foliage-leaves in species of Juncus into scale-leaves, and
Chermes and Lonchaea lasiophthalma induce similar abnor-
malities in the spruce and in Cynodon dactylon (Fig. 94) re-
spectively.

The transformation of floral-leaves into foliage-leaves
was induced by PEYRITSCH (1882) by inoculating species of Arabis with
aphides, and the same investigator was able to induce the formation of
vegetative-shoots in the flowers of Cruciferae and Valerianaceae by inoculation
with Phytoptus.

Other types of gall arise by local hypertrophy in otherwise unaltered organs.
An example of this is seen in the bladder-galls of Viburnum lantana (Fig. 95),
where we meet with an enlargement only of the cells already formed ; usually,
however, active cell division takes place along with excessive superficial growth
or growth in thickness. Local surface-growth takes place on leaves in the common
bladder-galls ; local growth in thickness is illustrated by the very interesting
Cym'/>s-galls, which deserve more detailed treatment. These are interesting in
the first place because they exhibit specific characters both as to form and size,
which become more prominent the larger they grow, and because they exhibit
peculiarities of internal structure which are unmistakably of the greatest impor-
tance, not to the plant, but to the insect causing the gall. We will, therefore,
study a characteristic example of a Cynips-ga.ll both from the point of view of

Fig. 94. Lonchaea-
gall on Cynodon dac-
tylon. About half
natural size.

JOST

322 METAMORPHOSIS

structure and of development, and will select for the purpose the leaf-galls so
well known as occurring on the leaves of the oak, a detailed acquaintance with
which we owe to the splendid researches of BEIJERINCK (1882).

The galls of Dryophanta folii are green spheres changing to red, from
one to three cm. in diameter, attached to the ribs on the under side of the

F'£- 95- Part of a transverse section through the bladder-gall of Viburnum lantana \ a part of the unaltered
leaf is seen on the left-hand side. After KusTER (Pathol. Ptlanzenanat. Jena, 1903).

oak leaf. Beneath the green non-stomatiferous epidermis we find certain
spherical cells, also green. These merge inwardly into a mesophyll with un-
usually large intercellular spaces, characterized by possessing abundant tannin.
In a cavity in the interior, situated centrally, we find in autumn the insect
surrounded by a shell of thick-walled parenchyma. It then gnaws for itself
a canal out to the exterior, and breaks through in November at the onset of colder
weather. All the insects hatched are female, and lay eggs without fertilization,
a not infrequent phenomenon in the Insecta. The insect selects a resting bud
at the base of the older branches to lay its eggs in ; it bores through the bud-
scales (Fig. 96, 7) and lays its egg exactly on the apex of the growing point,
fastening it there with a drop of slime. From the position where the egg is
laid one may conclude that something more than a leaf-gall will arise. The
egg in springtime becomes enclosed by the growing apex, and comes
to lie in the middle of a mass of merismatic tissue of considerable size which
has arisen from the growing point and its adjacent outgrowths (///). The gall
then arises from the bud, and is in its full-grown state about 2 mm. thick
and 4-5 mm. long. Its general appearance is represented at Fig. 96, IV. We

EXTERNAL CAUSES OF GROWTH AND FORMATION. II

323

A'a'

notice an apical elongation — the gall proper — and basally a number of unaltered
bud-scales. A longitudinal section through a young stage is shown at Fig 96, V.
In the centre lies a space containing the larva ; round it there is a layer of cells,
characterized by their large nuclei and abundant proteid 'and fatty contents.
This we may term the nutritive layer, for on it the larva feeds. The whole
of the space between the nutritive layer and the papillate epidermis is com-
posed of greatly thickened amyliferous cells which remain in the ripe gall.
Finally vascular bundles branch abundantly through the cortex of the gall from
below upwards.

At the beginning of June the insects escape ; this time some are male,
some are female. The latter are like the leaf-wasps but are much smaller.
These insects were known by the name of Spathegaster taschenbergi before
their relation to Dryophanta /o/n was known,
and hence even yet the galls are spoken of
as ' Taschenberg galls '. The female ' spa-
thegasters' after fertilization betake them-
selves to the under side of immature leaves,
bore deeply into one of the larger veins with
their ovipositors and lay eggs in the canal so
formed. A gall then arises from the phloem
of the neighbouring vascular bundle which
soon bursts the cortex (Fig. 97, /), and in a
central space the young larva develops after
being released from the egg-shell (//). The
gall developing thus endogenously, like a
root, grows until it becomes an externally
visible sphere, fastened only by a short pe-
duncle to the inner edge of the leaf- vein. It
shows a differentiation of tissue (///) into
three concentric layers. The innermost
forms a nutritive layer for the larva, then
follows a sclerotic layer, and externally a
thick cortical region plentifully supplied with
vascular bundles. At this stage, however,
the gall is by no means fully formed. Later
on the sclerotic layer and the cortex increase
markedly, and solitary thin-walled cells, as
well as the thinnest-walled sclerotic cells,
grow greatly and become at the same time
filled with reserves. BEIJERINCK terms this
the secondary nutritive layer, since it, like
the primary layers, serves for the support of
the larva ; the largest cells are always to be found just where the larva feeds.

Obviously the larva excretes something which acts as a stimulus to cell
growth. This stimulus may be either chemical or mechanical, and the question
comes to be what is the stimulus which induces the gall-formation as a whole.
First of all it is certain that the wound caused by the puncture made by the
insect need not be taken into account. Further, the movements of the larvae,
looked at as mechanical stimuli, cannot be made answerable for the formation
of the gall, since gall-formation commences while the larva is still completely
enclosed in its egg-shell. Some definite substance must diffuse out from the
larva which stimulates the cells to hypertrophy. In certain cases, e.g. in Ne-
matus-gafts on willows, according to BEIJERINCK (1888) the female during the
process of laying the egg excretes a substance which induces gall-formation,
so that small galls may develop without the development of a larva, but

Y 2

Fig. g6. Galls of Spathegaster taschenbergi.

same after enclosure of tfie egg; gW;
investing tissue; LK larva; AY, cavity,

iv,

starch layer; ep, epidermis.

324

ME TAMORPHOSIS

W. MAGNUS (1903) was unable to confirm this. In the majority of cases the for*
mation of the gall is entirely dependent on the development of the larva. It is
quite unknown, however, how the differentiation of the tissues of the gall takes
place, whether, for example, the formation of the nutritive layer depends on
substances other than those which form the sclerotic layer.

In general, complicated galls, like those we have been considering, arise
from embryonic tissues ; these are at least more readily transformed than full-
grown ones. We must leave undetermined the question whether, as BEIJERINCK
holds, the gall actually arises from the phloem or whether the cambium takes
part in the process. It is important to note that the gall-formation arises
often from quite uninjured cells, so that the stimulating substance is obviously
diffusible. In favour of this view is the fact that galls are in general concentric
to the larva and that the action of the stimulus appears to cease at a certain
distance from the centre. HOFMEISTER (1868) also held that the cause of the
gall was a fluid excreted by the insect.

The gall-insect thus provides definite chemical substances which act as
stimuli, and the plant responds by forming a gall . These chemical stimuli produce
different galls in different plants ; thus the gall produced by Cecidomyia
artemisiae on Artemisia campestris differs from that on A. scoparia. From this

Fig. 97. Development of the gall of Dryophanta folii Transverse section through the midrib of the oak.

t egg cavity ; Ik, larval chamber ; ng^ nutrient layer ; ss, sclerotic layer.

it follows that not only the insect but the plant also plays an important role
in gall-formation. It is all the more remarkable that the plant itself makes
no use of the galls.

On the other hand the use of the gall -structure to the animal is obvious
The differentiation of tissues seen in Dryophanta-galls occur also in others;
tissues, especially nutritive ones, serving mechanically or chemically as a
protection to the insect, are common. Frequently cell-forms appear which
are entirely wanting not only in the normal plant but in any of its near
relatives. We also find other adaptations whose functions in relation to the
parasite are even more obvious, such, for example, as the formation of lids to the
galls of Cecidoses eremita (KERNER, 1891, II, 526, Fig. 5). The inhabitants of the
galls make use of their hosts to the fullest extent without giving any equivalent
in return. The plant, too, makes no attempt to get rid of its parasite, it renders
up willingly all the material wanted by it, it even builds it a house, and, in short,
treats it as though it were one of its own organs. We may, therefore, conclude
that the plant cannot differentiate between the materials formed by the larvae
and by its own members, and that, in the normal course of development also,
the material influences of one part on another must be of great moment.

Relationships of another but equally interesting type are to be met
with, finally, in lichens, which we may cite as an example of symbiosis.
As has already been mentioned (p. 243), each of the organisms which com-
bine to form the association obtains some advantage from the union, but

EXTERNAL CAUSES OF GROWTH AND FORMATION. II 325

it is not always easy to prove this. We have already gained some acquain-
tance with the stimulating effects of genuine parasites, but possibly the algal
constituents of lichens do not always benefit in that way ; at all events, we
notice one injurious result, viz. that they do not produce fruit when so
united. As to the form of the federation we may distinguish important differ-
ences. In many lichens, e.g. Ephebe, the alga is the predominant partner
and the lichen has essentially the appearance of the alga without the fungus.
Cora, on the other hand, one of the Hymenolichenes, has exactly the appearance
of the constituent fungus, one of the Telephoreae. In the majority of cases
the symbiotic union results in an entirely new form, with all the characters of a
single independent organism. In these cases obviously each organism influences
the other, and sometimes the one, sometimes the other, partner is predominant.
According to HOLLER'S (1893) researches, this is the case with Dictyonema,
a lichen the fungal constituents of which is the same species of Telephoreae which
occur in Cora. If the alga be a member of the Chroococcaceae, the lichen Cora
is the result, but a Dictyonema if it be a Scytonema. The filaments of Scytonema,
however, are much more vigorous than the unicellular Chroococcaceae, and thus
produce more arbitrary growth forms. The alga and the fungus carry on a sort
of struggle to determine the ultimate form of the compound organism. If the
lichen be developed in the air the fungus is the dominant partner (true Dic-
tyonema form), but if it be transferred to a solid support, the alga gains the
upper hand and it alone determines what the form of the compound organism
shall be (Laudatea form), and the fungus becomes merely an accompaniment
(MOLLER, 1893). The fungus can also live apart from the alga as a member of
the Telephoreae but may, when the suitable alga is met with, assume in the
course of its development either the Cora, Dictyonema, or Laudatea form.

Bibliography to Lecture XXV.

[BALL. 1903. Jahrb. f. wiss. Bot. 39, 305.]

DE BARY. 1884. Morphologic u. Biologic d. Pilze. Leipzig.

BENECKE, W. 1903. Bot. Ztg. 61, 19.

BEIJERINCK. 1882. Beobachtungen lib. die ersten Entwicklungsstadien bei Cyni-

pidengallen. Amsterdam.
BEIJERINCK. 1888. Bot. Ztg. 46, i.
BONNIER. 1895. Annales Sc. nat. vn, 20, 217.
BRENNER. 1900. Flora, 87, 387.

BURCK. 1900. Kon. Akad. van Wetensch. Amsterdam. Proc.
[BURNS. 1904. Annals of Botany, 18, 579.]
COSTANTIN. 1883, 1886. Annales Sc. nat. vi, 16, 5 ; vn, i, 135.
ECKSTEIN. 1891. Pflanzengallen und Gallentiere. Leipzig.
ELFVING. 1880. Acta Soc. Fennicae, 12.
GOEBEL. 1898. Organographie, i. Jena.

[GuxTENBERG. 1905. Beitr. z. phys. Anatomic der Pilzgallen. Leipzig.]
HARTIG. 1901. Holzuntersuchungen. Altes u. Neues. Berlin.
HEGLER. 1893. Cohn's Beitr. z. Biologic, 6, 383.
HEINRICHER. 1896. Ber. naturw. Verein. Innsbruck, 22.
[HERING. 1904. Jahrb. f. wiss. Bot. 40, 499.]
HOFMEISTER. 1 868. Allgem. Morphologic d. Gewachse. Leipzig.
[KELLER. 1904. Einfl. von Belastung, etc. auf Ausbild. d. Gewebes in Frucht-

stielen. Diss. Kiel.]

KERNER. 1891. Pflanzenleben. Leipzig and Vienna.
KNY. 1901. Jahrb. f. wiss. Bot. 37, 55.
KOHL. 1886. Die Transpiration. Braunschweig.
KUSTER. 1903. Patholog. Pflanzenanatomie. Jena.
LENGERCKEN. 1885. Bot. Ztg. 43, 337.
LOTHELIER. 1893. Revue gen. de Bot. 5, 480.
LUPKE. 1888. Landwirtsch. Jahrb. 17, 912.
MACCALLUM. 1902. Bot. Gaz. 34, 193. Abstr. Bot. Ztg. 1903, p. 69.

326 METAMORPHOSIS

MAGNUS, W. 1903. Ber. d. bot. Gesell. 21, 129.

MOLLER. 1893. Flora, 77, 254.

MOTTIER. 1899. Annals of Botany, 13, 346.

NABOKICH. 1901. Ber. d. bot. Gesell. 19, 222.

NABOKICH. 1902. Beihefte Bot. Centrbl. 13, 272.

[NiKixiNSKY. 1904. Jahrb. f. wiss. Bot. 40, i.]

NOLL. 1900. Landw. Jahrbiicher, 29, 361.

NOLL. 1901. Sitzungsber. niederrh. Gesell.

OLTMANNS. 1891. Sitzungsber. Berliner Akad. d. Wiss.

[PERODKO. 1904 Jahrb. f. wiss. Bot. 41, i.]

PEYRITSCH. 1882. Jahrb. f. wiss. Bot. 13, i.

PFEFFER. 1893. Druck u. Arbeitsleistung. (Abh. Kgl. Gesell. Leipzig, 20, 233.)

PULST. 1902. Jahrb. f. wiss. Bot. 37, 205.

REINHARDT. 1892. Jahrb. f. wiss. Bot. 23, 479.

RICHTER. 1894. Flora, 78, 423.

SCHIMPER. 1891. Indo-malayische Strandflora. Jena.

SCHWARZ, F. 1 88 1. Unters. Bot. Inst. Tubingen, i, 53.

SCHWARZ, F. 1883. Ibid, i, 135.

STAHL. 1894. Bot. Ztg. 52, 117.

TUCKER and SEELHORST. 1898. Journal f. Landw. Reviewed in Biedermann's

Jahresber. 28, 269.

VOCHTING. 1878. Die Organbildung, i. Bonn.
[WiEDERSHEiM. ioo2. Jahrb. f. wiss. Bot. 38, 41.]
WIELER. 1883. Unters. aus d. bot. Inst. Tubingen, i, 189.
WIELER. 1901. Ber. d. bot. Gesell. 19, 366.
WIESNER. 1891. Ber. d. bot. Gesell. 9, 46.
WIESNER. 1895. Ibid. 13, 481.
WIESNER. 1896. Ibid. 14, 180.
WORTMANN. 1 88 1. Bot. Ztg. 39, 368.

LECTURE XXVI
CORRELATIONS

SINCE, as we pointed out at the end of the last lecture, an alga living sym-
biotically with a fungus can markedly influence the mode of growth of the other
member of the partnership, although a give-and-take of soluble materials is
almost the only relation that subsists between them, and, further, since an
insect, which presumably operates only by excreting some chemical substance,
can induce the formation of galls, and, though a foreign organism, bring about
far-reaching modifications in plant form, it follows that individual organs, bound
together by intercellular protoplasmic threads into one organic unity, must
also influence each other in a very marked manner. Such relationships of
plant-organs, which may be termed 'growth-correlations ' (GoEBEL, 1880), are
very generally met with and claim our attention here. They stand on the
border-line between internal and external growth-factors, for if we regard the
individual cell or growing point as relatively independent, influences exerted on
it by other cells or other growing points of the same plant may be regarded as
external influences, since all parts of the plant, save that immediately under
consideration, are in the position of an environment to it. If, however, we look
upon the entire plant as a single unit then the action of one part on another
must be considered as that of an internal factor.

The cells of Spirogyra, which are all alike and which all perform the same
functions, do not apparently influence each other's growth, and it would appear
that, so far as the welfare of the individual cell is concerned, it is immaterial
whether they be united to each other or not.

If, however, different cells or, speaking more generally, different organs of
a plant-body differ in structure and function, then they necessarily influence

CORRELATIONS 327

each other, inasmuch as the performance of a definite function in one organ is
essential to the carrying out of other functions in other organs, although these
organs may be quite capable in themselves of performing such functions. In
normal ontogeny each functional member assumes a definite shape, and we
might readily suppose that it could not assume any other form. As a matter
of fact, however, we should be more correct in asserting that every organ
arising at the growing point may develop in a variety of ways ; the fact that
it is forced to assume a definite direction of development depends only on its
relations to other parts. Did no such regulation of the development of
parts exist and if every cell or every mass of embryonic tissue gave rise to such
structure as it was inherently capable of producing, the resulting plant would
be no longer an organism but merely a chaotic mass of living substance.
' Harmonious development ' is possible only if correlations exist, and to gain
some acquaintance with examples of these correlations must be our first task,
thus giving us the opportunity of considering a whole series of phenomena
which we have hitherto only imperfectly studied. In only a few cases is it
possible to draw conclusions as to correlation from observation only. BERTHOLD
(1882) has found that the lateral branches of many Algae may give rise to new
outgrowths at their bases on the convex side turned away from the chief axis,
the effect of the chief shoot being to cause the normally radial branches to become
locally dorsi ventral. Similar observations have been made on twigs of Cu-
pressus ; the lateral branches bear more numerous and larger proliferations on
the side towards the apex, although their disposition should be basiscopic.
Occasional anomalies may also be observed. Thus DE VRIES (1891) records
the case of a flower-stalk of Pelargonium which formed a leaf -bud, and which,
instead of dying off after the bud had opened, remained in existence for several
years, and by vigorous secondary growth formed a woody cylinder like an
ordinary stem. As a general rule the flower-stalk forms no cambium nor has it
any capacity for forming merismatic tissue ; it remains bare and no foliage-
leaves arise on it ; in other words, a correlation exists between the development
of foliage-leaves and the power of secondary growth in the stem. Similar
examples worthy of notice often occur among malformations. Experimental
studies on this subject are, however, of more importance and more pertinent.

From among such experimental researches we may select those which aim
at observing the effect on the rest of a plant of the removal of one of its organs.
This experiment necessitates, in the first instance, the formation of a wound
which the plant endeavours at once to cicatrize [MASSART, 1898]. In the second
place, the plant exhibits a capacity for replacing the organ which it has lost. We
have here to deal with a reaction on the part of the plant to external mechanical
stimuli, such as we have described in the last lecture, but these processes
become intelligible here for the first time in view of the light shed on them by
a study of correlations.

First of all a few words may be said as to the healing of wounds, and we
shall take our examples from the higher plants exclusively. The healing takes
place very differently according to the age and specific character of the tissue
affected. The injured cells and also the other cells adjacent to them die off, but
the layer of cells next below — if they still contain protoplasm — begin to react
so as to develop tissue over the wound. Many of the parenchymatous cells,
without actually growing, divide parallel to the surface of the wound, and their
walls become cutinized and thus interpolate a corky layer between the dead
and the living tissue. This reaction teaches us that fully-grown cells may still
retain the power of division, but we must not regard the destruction of the
connexion with neighbouring cells as the only factor inducing this reaction, for
materials from the injured cells, or other changes taking place during the inflic-
tion of the injury, might act as a stimulus.

328 METAMORPHOSIS

In addition to the simple healing of a wound by means of cork we meet
in other cases with a more complex method, viz. the formation of callus. The
cells which remain uninjured near the edge of the wound begin to grow actively
and curve over it ; cell division begins and finally develops there a thin-walled
large-celled tissue of irregular form to which the name of callus is given. All
cells containing protoplasm and nuclei, even epidermal cells, seem capable of
forming callus, but, naturally, young cells, and especially cambium cells, are
more vigorous than full-grown cells. [This capacity for forming callus disap-
pears in some cases early, in other cases late ; very rapidly, for example, in
the cells of the root cortex (MASSART, 1898 ; SIMON, 1904).] Small wounds,
such as pricks in leaves, may be completely occluded by callus alone, but in
larger wounds a development of the cortical cells takes place as well, accom-
panied by suberization of the outermost callus cells. In this way a replacement
of the epidermal cells is attained, always the first object aimed at. This is,
however, by no means the only purpose of the formation of callus. Without

attempting to offer a complete exposition
of the subject it will be sufficient if we
cite only a few examples here. Let us
glance first at callus formation on cuttings.
As is well known, gardeners propagate
many plants by very simple means, viz.
by cutting off twigs, placing them in wet
sand, and keeping them warm. If the
plant has the power of forming new roots
under these conditions then propagation
by cuttings is possible, otherwise not.

Fig. 08. Callus formation. / longitudinal sec- •»-/ /. •? •, e . , ...

tion through the base of a three days' old cutting of Bef Ore TOOtS are f Ormed f rOHl the Cutting

^S^^^^T^^^& there is formed at its lower cut end,
base of a one-year-oid cutting; m, medulla ; x^ imbedded m the sand, a callus, formed
£Th1oyenfe from the cambium, phloem, paren-

After STOLL (1874, Bot. ztg. 32, PI. 12). chyma, and pith, as shown in Fig.

98, /. It will be seen from the figure that

the cambiogenic callus cells overlap each other and thus the callus tissue comes
to form a hemispherical cushion on the cut surface of the slip (Fig. 98, //). Simi-
larly other callus masses develop on the cut surface, and by their fusion form
in the long run a single hemispherical mass of tissue. When the wound is
quite covered, corky layers appear in the callus which enclose the lower ends of
the vascular bundles, and separate them from the new tissue. On the outer
surface of the callus also a corky layer has already appeared. Later on, at
a certain distance from this layer, a cambium develops which unites with that
of the cutting and proceeds to form secondary wood inwardly and secondary
bast outwardly. In a one-year-old cutting, therefore, we find the secondary
tissue arranged at the base from within outwards as in the stem and in direct
continuity with it. We shall direct attention later on to the formation of
roots from the base of the cutting ; these are not shown in the figure.

The callus in this case is able to form secondary tissues, but in other
cases we find the callus capable of producing primary tissues. If, for
example, we cut off the extreme apex of the growing point of a dicotyle-
donous root, the callus which arises is speedily able to produce a new growing
point, which performs its functions quite normally and forms a new rootcap
[SiMON, 1904]. This property is, however, almost confined to the root ; it is
seldom possessed by the stem (PETERS, 1897).

As a rule, when an organ is removed its renewal does not take place from
the callus exactly at the same place, but in the neighbourhood ; this new organ
may be already present previous to the infliction of the wound as a more or less

CORRELATIONS 329

obvious rudiment, or it may be formed in the first instance from the callus.
Many transitions occur between these types which are difficult to separate in
practice. It is obvious, however, that the plant need not take the trouble to form
an absolutely new organ if in the vicinity there exist already the rudiment of one
identical with that it has lost. Thus, if the apex of a primary root be removed, the
nearest lateral root takes on the function of chief root, and in the case of the stem
there are plenty of buds which are only waiting their chance to take on the duty
of forming the main axis. Generally speaking the basal lateral buds of a shoot
are retarded in growth by the development of buds higher up ; yet frequently
in certain trees, such as the lime, the lateral buds high up inhibit the growth of
the terminal one, and hence arises the ' sympodial ' mode of growth of this tree.

When such ' reserve organs ' (GOEBEL, 1903) are absent, a fresh growth of
organs takes place from callus. This is of very common occurrence from the
stumps of felled trees, where a cambiogenic callus produces numerous buds.
Such a proliferation occurs in many plants on amputated portions of roots,
and segments of potato tubers are especially capable of forming such shoots.
Although in the majority of cases the origin of these buds is endogenous, as is
normally the case in the root, there are also cases known where the renovation
takes place from epidermal cells. The way in which plants of Begonia are propagated
from leaves is well known. If leaves which have had their principal veins cut
through be laid on wet sand a callus is formed at the end of each vein, in the
formation of which the epidermal cells take part. Growth and division of one of
the epidermal cells of the callus gives rise to a new shoot. It ought to be noted,
however, that at a certain distance away from the callus uninjured epidermal cells
may also give rise to new shoots, so that a normal epidermal cell can, without
doubt, retain the power of developing buds. We therefore see that callus forma-
tion is by no means an essential antecedent to the development of shoots.

Roots also may arise after injuries to roots, stems, or leaves, either
from rudiments of such already present in these situations or as entirely fresh
formations. On the other hand, the plant has, as a general rule, no power
of replacing leaves or parts of leaves. If the blade of a leaf be cut off, the petiole
usually dies and is thrown off, and the stem also, when its apex is amputated,
frequently dies off down to the node next below. HILDEBRAND (1898) has,
however; drawn attention to a case of genuine renewal of the leaf-apex, and
WINKLER (1902) and GOEBEL (1903) have recently investigated the matter more
closely. If a leaf -blade of a young plant of Cyclamen persicum be cut off — only
in a young plant is such an experiment successful — at a short distance from the
wound a development of tissue takes place on both sides of the petiole, and
this proceeds to form excrescences which, both in outward form and internal
structure, must be described as new leaf-blades. [GOEBEL does not agree with
WINKLER' s view that a genuine renovation of the leaf -blade occurs in this case;
he holds that all that takes place is a continued growth of the leaf base
previously correlatively inhibited. (Compare in this relation, as on all ques-
tions of regeneration, GOEBEL, 1905.)]

The instances we have quoted of the results of injury to the plant have
made us acquainted with a number of cases of correlation. The plant possesses the
power of replacing parts which have been lost, and thus cells, tissues, and higher
members "are formed anew, whose development by normal ontogenesis is impos-
sible, since their connexion with the rest of the plant is destroyed. Regenera-
tion appears not only in the amputated parts but also in the stocks whence the
cuttings have been taken. We cannot avoid the conclusion that the capacity for
manifesting vital activities of this sort exists in every cell containing protoplasm
and that it is in general kept in abeyance only by the influence of related parts.

We gain a closer insight into the factors which determine plant formation
when we examine more closely the situations where regeneration takes place. If

330 METAMORPHOSIS

the apex of a shoot be removed the bud nearest to the wound proceeds to develop ;
if the apex of the root be cut off the nearest lateral root takes its place. As we
have already seen, roots may appear on shoots and shoots on roots and even roots
and shoots on leaves, and the position of these is governed by definite laws. A
branch of willow bearing buds, if kept in a damp atmosphere, produces at its
upper end leaf-buds only, and roots only at the lower end, and the size of the
shoots increases the nearer they are to the upper end, and of the roots the nearer
they are to the base. As VOCHTING (1878) has shown, the distribution of organs,
as well as their relative size, depends on external factors, such as moisture,
gravity, and light, though not in the first instance conditioned by these factors.
On the other hand, the branch has an inherent polarity of its own, and this may
be influenced by external agents but cannot be completely overcome. In Mar-
chantia and allied Hepaticae, which exhibit remarkable powers of regeneration
( VOCHTING, 1885), renovation always takes place at the apex of the older part ; the
very smallest segments of the thallus show a differentiation into two poles, and we
can scarcely doubt that every cell possesses a distinct distal and proximal end.
Indeed, VOCHTING has also demonstrated the polarity of the individual cell in
the higher plant (p. 333). We must remember in this relation that in the lower
organisms the determination of what shall be base and what apex is frequently
settled by external factors — in Bryopsis, for example, by light — but we must
admit that the higher plants behave differently. In the phanerogamic embryo
and the gemmae of Marchantia the differentiation of base and apex is
already established, and depends entirely on internal factors. The phenomena
of regeneration teach us, however, that the differentiation into shoot and root
taking place after the first cell divisions in the embryo of Phanerogams does
not indicate a separation of the protoplasm into two parts, one with root and
the other with shoot characteristics, but that every individual cell must have per-
manently the capacity of forming both kinds of organ. Only the continuous
mutual influence of the parts can condition the formation of a shoot on the one
hand or a root on the other. It need scarcely be mentioned that the root
behaves in principle just like a shoot, producing shoots at its base and roots at
its apex ; further, that the polarity in regenerative processes exhibits itself,
very frequently at least, in the leaf, inasmuch as shoots arise on its morpho-
logical upper side and roots from its under side. Since this polarity is induced
only by correlation, we need not wonder that, in certain cases, it becomes obli-
terated where the growing point of the root throws off its cap and produces leaves,
in Neottia and Anthurium, for instance, apparently without any external inter-
ference, and in Ophioglossum, after separation of the ends of the roots. (For
literature see GOEBEL, 1898-1901, p. 435.)

The simplest cases of correlative influence are quantitative in their nature,
one organ determines to what extent another may develop (' compensation ' ;
GOEBEL, 1884, 1893-5). In the last-mentioned examples, however, we have to
deal with qualitative changes, and a few further examples of these may be given
here. We may deal first with GOEBEL' s researches on bud-scales (1880). By
removing the foliage-leaves at an appropriate time of the year it is possible to
inhibit the formation of bud-scales and to transform their rudiments into foliage-
leaves, and it is possible to bring about the change of subterranean buds into
leafy shoots, as in the potato and many rhizomes, by removal of the main leafy
shoot. We shall obtain a better conception of this phenomenon in the next lecture.

We may next select the spruce as an example of a far-reaching qualitative
correlation between the main shoot and lateral buds. The main axis is radial
and grows upright, the lateral shoots are dorsiventral and grow obliquely up-
wards. It we cut off the main shoot one (or often more) of the lateral shoots
highest up the stem grows round as nearly as possible at right angles, and becomes
radial in structure. It may be assumed that the dorsiventrality of many lateral

CORRELATIONS 331

branches and of laterally-placed flowers is also due to the correlative influences
of the main axis, and in this relation it should be remembered that normally
dorsiventral flowers, such as those of Orchidaceae and Scrophulariaceae, generally
become radial if they once become terminal (peloria).

We have now become acquainted with numerous correlations among pheno-
mena of regeneration appearing after the infliction of wounds, but these regenera-
tions are not the first or only consequences of injury. If a leaf-blade be cut off
serious disturbances must ensue in the petiole and the portion of the stem im-
mediately associated with it, which express themselves in reactions easily re-
cognized. Such reactions, consisting in the throwing off of the petiole in the case
of the leaf which has lost its lamina or the stem which has lost its leaves, have
already been referred to. If, however, all the leaves are not removed from the
stem there is no reason why the shoot should die off ; on the other hand, it no
longer develops conducting tissue to the removed leaves. There is also a relation
between the leaf and the leaf -base on the stem, which may be studied very readily
in the epicotyl of Phaseolus multiflorus. If we remove one of the two young
primary leaves at the first node of the epicotyl and at the same time carefully-
cut off the stem apex (whose further development would complicate the experi-
ment) we see a marked reduction in the diameter of the vascular elements on
one side of the epicotyl, from the base to the apex, while those on the side on
which the leaf still remains are developed normally and also show indications
of secondary growth (Josx, 1891).

The conclusion to be drawn from this experiment is undoubtedly that the
result is not due to the injury but to a suspension of function, and especially of
the growth in the leaf. This is evidenced by the fact that the same stem-structure
may be induced if the leaf be not cut off but only prevented from further growth
by enclosure in plaster of Paris. In other cases also the removal of an organ is
followed by purely mechanical growth retardation. Thus HERING (1896) has
shown that the cotyledon of Streptocarpus, which normally does not develop,
may be made to do so either by cutting off the one which does, or by enclosing
it in plaster of Paris ; WINKLER also showed that leaf-regeneration could be
induced in Cyclamen when he enclosed the leaves in plaster of Paris with-
out injuring them. Since in this case we are dealing with fully-developed
organs the plaster could not have acted as in Streptocarpus or Phaseolus by
retarding growth, but rather by inducing a cessation of the junction of the
organ. He obtained similar results by using instead shells of collodium and
shellac. We thus arrive at the conviction, which, however, still demands ex-
perimental confirmation in many points, that regenerations may be induced
not only by removal of an organ but also by rendering it inactive. [As to the
various factors concerned in regeneration compare KLEBS (1903), GOEBEL (1905),
and MACCALLUM (1905).] In the case of the plant it is difficult to distinguish
whether an organ is removed or merely has its functional activity retarded.

Organs which are not functional are usually abstricted, as we have already
seen in the case of petioles and stem-bases. Intact foliage-leaves in sensitive
plants, e.g. Mimosa, are rapidly thrown off if they be prevented from assimilating
by withdrawal of carbon-dioxide or by being kept in darkness ( VOCHTING, 1891 ;
JOST, 1895). Certainly all plants are not so sensitive ; evergreen leaves, for
instance, which live the whole winter through without manifesting any activity,
may be kept in the dark for months without dropping off.

We arrive at an exactly opposite result to that which we obtain with leaves
prevented from assimilating, if organs which have lost their original function
and are, in consequence, dying, take on new functions. Thus VOCHTING (1887) was
able by employing special artificial means to cause tubers of a potato which, owing
to germination, had dried up and were dying off to remain alive for a year more,
and he obtained even better results with Oxalis crassicaulis (1899), a plant

332 METAMORPHOSIS

which may be compared with the potato in all essential points of structure.
Normally the tubers germinating underground form several leafy shoots which
root at the base and become independent after the exhaustion of the parent
tuber. If the tubers be planted in springtime with only their lower ends in the
soil, the tubers themselves form roots while the leafy shoots arising at their upper
ends form none. The tuber thus is interpolated between the root and the shoot,
and it has therefore to perform not only the function of a storehouse of reserves
but also of a conductor of materials absorbed from the soil. It remains alive in
this condition the whole summer, grows considerably in thickness, and develops
cellular elements which are foreign to its nature but which are characteristic of
the normal stem, viz. large vessels, sclerenchyma, and wood parenchyma. The
need for elements to carry out the functions of conduction, support, and storage
acts here, as in cases of regeneration, as a stimulus which induces the satisfying
of this demand.

We owe to VOCHTING (1899) also a large number of experiments in which
tuberous plants were prevented from forming storage tissue. It is interesting
to note that these plants then proceed to deposit their reserves in other organs
and that these organs assume in consequence an entirely different structure and
function. Under normal conditions all phenomena of this kind are not to be
traced to the action of correlation. Two examples may be cited. If offsets of
Oxalis crassicaulis laden with reserves be cut off and kept in a moist chamber

they form normal tubers at their apices, but if all
the growing points be removed a tuber is formed
all the same either by cellular increase at one or
two internodes or by the swelling of the scale-
leaves. In this case a completely-developed
organ has taken on a new function and shape,
but such cases are rare. (Compare WINKLER,
1902, Ber. d. bot. Gesell. 20, 500, where in the
case described the factors in the process are
SurFfkgce9<Jf behoof™? °5o. ^Aftef unknown). Both structures are possibly with-
VOCHTING, 1892. out significance in this plant because it neither

possesses tubers nor can it form them.

Boussingaultia baselloides, which develops normal subterranean stem tubers,
is even more plastic. Such tubers may be induced to develop on every foliage-
bud if the stem be treated as a cutting, and the axillary-bud be kept in the dark.
If a cutting be treated so that its base, free of buds, is placed in the earth a large
tuber arises at the basal end of the axis from the callus, which lives throughout
the year without being able to produce a growing point. We need not enter into a
discussion here as to the varied histological alterations which take place in the
stem ; we need only note that roots swell into tubers since the plant has no
main axis in which to deposit its reserves. This takes place when leaves are
used as cuttings. All plants are probably not so variable, still VOCHTING' s
researches have made us acquainted with numerous interesting facts, into
which, however, we cannot enter further.

We have as yet only dealt with correlations which we may study by the
simple experimental method of comparison of the results of removing or in-
hibiting the functional activity of organs. We may now consider a third
method of demonstrating correlations, long known practically, and especially
illustrated by VOCHTING' s researches, viz. transplantation or artificial budding.
VOCHTING (1892) has made a thorough study of the simpler forms of trans-
plantation. He cut out a cube of beet and then replaced it in the original
wound. A rapid healing of the wound took place by appropriate fusions, the
cells which were not injured by the cut beginning to swell out, bud (Fig. 99),
and grow together where they touched each other (Fig. 100, ///). New

CORRELATIONS

333

vascular bundles originated, connecting the portion inserted and the main root,
and after a short time it was possible to determine only at certain places (gg in
Fig. 100, 7) where the transplantation had occurred and to observe there a
demarcation between the graft and the stock. Similarly we may cut out a
cortical block from a branch and replace it, allowing healing to occur. The
amalgamation takes place if a sufficient number of protoplasmic cells capable
of growing be present ; if these be absent, as in old wood, amalgamation is
impossible ; if these cells are present only in one region, e.g. in cambium, a local
amalgamation alone is possible.

If now the part cut out be inserted into the wound with different orientation,
if, for example, it be turned inside out or upside down, amalgamation still takes
place; but after a longer or shorter time a pathological condition ensues, for there
develops a swelling which in extreme cases may bring about the death of the plant.
The factor concerned in the tumour is polarity, which is functional in every cell and
not only longitudinally but also radially. [MiEHE (1905) has demonstrated this
polarity in individual cells of Cladophora in a very beautiful manner.] If we com-
pare a plug of beet inserted in the orthodox manner (Fig.ioo, /) with one inserted

Fig. 100. Mode of growth of transplanted portion of
beet. /, tangential section through the upper end of a
portion of tissue inserted in the normal orientation. The
limits of the tissue are indicated by the dotted lines gg:
The longitudinal lines indicate vessels. //, a similar section of
an inverted graft. The vascular bundles are developing almost
entirely in the long axis ; 1 and II about natural size. ///,
individual cells from the area of fusion ; /, intercellular spaces
and thickened cell-walls. X about 350. After VOCHTING,
1892.

Fig. 101. Two vessels
from Cydonia japonica.
One is united to the other
by an intermediate curved
vessel. x 120. After
VOCHTING, 1892.

upside down (Fig. 100, 77) we see that in the latter case the continuity of the
vascular system is destroyed, the polarity of the vessels is altered in so far that
their basal ends are unable to unite with the distal ends of those already present
in the stock. The new formations attempt to bend round and lay themselves side-
ways against older vessels so that they orientate themselves in the same direc-
tion. Fig. 101 shows more exactly the mode of union of the vessels, the polarity
of which is indicated by arrows. By no means all cells are able to effect an
accurate junction ; the disturbance caused by the severance is permanent,
and hence arises an active growth leading to a hypertrophy, suggestive of the
effect produced by the attack of a parasite.

Similar results are produced when we plant a piece of tissue cut out of a
certain region in another situation. It is possible to transplant in the same
way a portion of tissue on another individual of the same species or on another
species. Only experiments which aim at transplanting a portion of tissue
bearing one or more growing points are of especial interest. Such transplanta-
tions are frequently carried out in gardening operations, as when budding or
grafting of small twigs is resorted to. The part transplanted is spoken of as
the graft or ' scion ', and that to which the graft is attached as the 'stock '. In

334 METAMORPHOSIS

grafting the scion has the form of a disk of cortex subtending a bud, carefully
separated from the wood, and which is squeezed into the stock, close against
the cambium, after the lifting up of two flaps of cortex. The cambium of the
scion unites with the cambium of the stock and thereafter the bud develops.
Of the numerous methods of budding we need select only one, viz. budding by
clef ting. The end of the stock is split longitudinally and the wedge-shaped base
of the scion is sunk in the cleft. In this, as in the previous case, a good junction
must be obtained if a fusion is to result.

We may, as we have already seen, transplant the scion on to another species,
still it is impossible to graft any plant on another, for a certain degree of relation-
ship between scion and stock is requisite, although the capacity for amalgamation
runs by no means parallel with systematic relationship. Apples and pears,
for example, graft badly on each other, although they belong to the same
genus, while pears graft very readily on the quince, which is placed in another
genus. Similarly potatoes graft more readily on Datura and Physalis than on
many species of Solatium. We are able apparently to graft easily all other
species of Cactaceae on Peireskia aculeata, while other species of Peireskia
graft badly on this species. We must take cognisance of all these facts, al-
though we may not be able to explain them. The point of interest for us, how-
ever, in these cases of transplantation lies in the numerous correlations which
we may eludicate by their means. The interrelations between two species
thus fused together must operate in the same way as between two parts of one
plant since the union is a complete one, for STRASBURGER (1901) was able to
prove a fusion of the protoplasm of the stock and scion.

The interactions which take place between scion and stock may be shown
in the first place to be purely quantitative. There are plants which develop
better as scions on other species than on the same species, e.g. Physalis
on Solanum, Arabis albida on Brassica oleracea, Solanum dulcamara on
Lycopersicum. On the other hand, the development of the scion may be
retarded by the stock, and since retardation of vegetative growth is usually
accompanied by increase in flower-formation, grafting is for this reason fre-
quently resorted to in fruit-tree culture. Pears, for instance, which it is
desired to cultivate as dwarfs, are grafted on the quince, and apples on Malus
paradisiaca with the same object. Changes in mode of growth are often
accompanied by a change in the duration of life, thus dwarf apple-trees,
grafted on Malus paradisiaca grow only for fifteen to twenty-five years, while an
ordinary apple-tree may attain an age of about 200 years. Pistacia vera, grown
from a seedling, will live for 150 years, but if grafted on Pistacia terebinthust
for 200 years, if grafted on Pistacia lentiscus, for forty years only ; in the one
case, therefore, the duration of its life is increased, in the other reduced. Annual
plants cannot, as a rule, be made perennial by grafting, but LINDEMUTH (1901)
has found that the annual Modiola caroliniana lived for 3 J years when Abutilon
thompsoni was grafted on it, and similar results in other cases are extremely
probable.

If we turn now to the special qualitative effect we must note first of all
that as a rule (compare Lect. XXIX, p. 381) this effect does not go so far as to
cause a modification of the specific characters of the two united plants, and
on this knowledge rests the employment of the process of grafting in horticultural
practice. There are other qualitative changes, however, which make their ap-
pearance, e.g. fruit-trees maybe made, by grafting, to produce an increased yield
of flowers and fruit. To VOCHTING (1892) we owe a very interesting experiment
on the qualitative effect of grafting. He showed that large-leaved shoots arise
from the buds which spring from the base of the inflorescence of the beet in the
second year if they be grafted on a one-year-old stock, inflorescences if on a two-
year-old stock. These buds if left in their natural positions would, of course,
have disappeared in autumn, but when transplanted they are stimulated to

CORRELATIONS

335

further growth, and the way in which they grow depends on the stock. The
external form is thus correlated with the life duration, which in one case lasts
one year (i.e. the inflorescence), in the other two years.

Of equal interest is an experiment of LINDEMUTH (1901) on potatoes. The
potato may be grafted successfully on Datura, for certain buds of the stock
develop directly into horizontally placed aerial stola. In the course of the
formation of these stola the disposition of the plant makes itself felt in the
production of reserve stores which do not arise on the Datura stock. The stock,
however, encourages an active vegetative growth, doubtless owing to its great
capacity for forming roots, and the stola become not tubers but leafy shoots.
If, however, the scion be grafted on such a stock of Capsicum annuum,
which induces only feeble growth, the buds in question become tubers without
any formation of stola.

We have spent too long over specific examples of correlations where
attention has been paid more to the larger members (roots and shoots) and less
to the tissues and cells. These latter also present us with abundant illustrations
of correlation, although they have often not been experimentally established.
To take one case only, we may recall the
apposition of pits in cell-walls as an ex-
ample of perfect correlation.

We have now in conclusion to
summarize our conception of the opera-
tion of correlative influences ? In cer-
tain simple cases we may explain them
by a reference to nutritive factors. If
out of many buds only some develop we
must not assume that the non-develop-
ment of the others is directly due to a de-
ficiency in nutriment, since there is as
much nutrient there as is necessary to
permit all the buds to begin to grow. But
this would be extremely disadvantageous
for the plant. Obviously there is here an
adaptation of such a kind that no growth
begins when nutrition is feeble, and
the amount of nutritive material must
act as a stimulant in some way not known.

II

III

Fig. 102. Diagram of a plant of Phaseolus, on
which only one primary leaf is allowed to develop.
/-///; transverse sections at successive elevations;
g, vascular bundles.

In complicated cases of corre-
lation, such as the connexion bet ween leaf and leaf -spur mentioned above (p. 330),
we cannot avoid assuming the existence of complex stimuli, although we are
unable to determine their nature. We must look more closely into this case and
investigate how the stimulus may be transferred from the leaf to the next
lower internode. It is easily shown, in the first place, that the development
of the vascular bundle is quite independent of assimilation and the nutritive
processes connected with it, since the experiment may be carried out equally
well in light and darkness. We might further believe that the functional
activity of the vascular bundle on the side of the leaf — the passage of water
through the vessels and of plastic organic materials through the phloem —
provides the stimulus from which follows the secondary thickening (DE
VRIES, 1891). Such ' functional stimuli ' doubtless play a certain part in
ontogenesis, but in the cases already cited they are not decisive factors at all
events. We may, as shown in Fig. 102, split the epicotyl of Phaseolus longi-
tudinally and then cut through transversely the part of the vascular system
supplying the leaf left attached ; the result is that the leaf- trace bundles passing
backwards from the leaf develop quite as well as in the previous experiment
although the nutritive stream from the cotyledons no longer flows in them nor

336

METAMORPHOSIS

in their neighbourhood (Josr, 1893). We are here dealing with a quite special
kind of effect — a stimulus action — which comes from the growing organ and
propagates itself backwards. [Exceptions to this view are taken by MONTE-
MARTINI (1904).] We are unable to say, however, whether any definite chemical
substance exudes from the leaf which acts as a stimulus. The similarity
between the influence of the growing leaf and that due to the gall insect is worthy
of note, and this likeness would tend to suggest that we are dealing in this case

also with a chemical stimulus. Doubt-
less stimuli such as we meet with in
Phaseolus maybe frequently met with,,
and especially in the relationships of
secondary growth of the main axis to
the development of lateral organs,
relationships which are easily under-
stood from a biological point of view,
though from a causal aspect they are
not intelligible.

An attempt has also been made
to explain the phenomena of regenera-
tion causally. But a causal explana-
tion is naturally not provided by
putting forward the appropriateness
of the regeneration or by describing it
as a ' tendency to completion of the
body '. On the other hand, we more
nearly arrive at a causal relationship
after determining (compare p. 331)
that the inhibition of the function of
an organ may lead to regeneration as
B K;>'VJ readily as its complete removal. There

are two recent researches, one by
GOEBEL (1903), the other by KLEBS
(1903), which may be noted here as
throwing light on the subject. We
will select only one important item
from KLEBS' s paper. This author
starts from the position — which must
be accepted by all investigators — that
every organ laid down in the plant
must develop if all the conditions of
its growth are fulfilled. He recognizes
as one essential condition of the growth
of the root a certain amount of water
in the plant. Since a branch of a willow
cut off from the main stem is able by
developing roots to grow into an
independent plant, that, according to
KLEBS, can be accounted for only by
an extra supply of water to the root primordia. Again, roots may be induced
to shoot out at any place from willow twigs while they are still united with
the plant if we permit the entry of sufficient water to their primordia by
removal of the cork. Thus KLEBS was successful in getting roots to develop
(Fig. 103) at the apex of twigs of willow, although up till then an upsetting of
internal polarity had not been possible.

It is possible that the correlative inhibition of many buds by the chief

Fig. 103. Two cuttings of Salix fentandra^ the
upper ends freed from cork, the lower ends immersed in
water, cultivated in a very moist atmosphere at a tem-
perature of about 25°. A, in the normal position, with
callus above and roots below ; B, with roots developed
from the (inverted) apical region.

CORRELATIONS

337

ones is due to the fact that too much water is withdrawn from them by the chief
buds (compare WIESNER, 1889) ; but it is more probable that we have to con-
sider in this relation not merely water but to look generally for the factors in
such correlations in their relation to plastic materials. The case becomes much
more difficult when we come to deal with fundamental qualitative changes ; as
to the factors concerned in such changes we shall have something to say in the
next lecture.

From GOEBEL' s researches (1903) we may conclude that he was unable to
refer the determination of the position of innovations to a polarity as yet by no
means understood. According to GOEBEL, the direction of the circulation of plasta
is of much greater consequence. Roots and stems show innovations at their
apices and at other places towards which the stream of plasta generally trends ;
the leaf, however, shows regenerations at the base also, because the stream of
assimilation products is directed backwards. With the object of supporting
his view GOEBEL carried out a number of interesting experiments, but these
were by no means exhaustive ; much has yet to be done on the subject.
Still, we are acquainted with many facts which are not in accord with GOEBEL' s
conception. Basal innovations, for example, appear on the leaf and flower axis
of Achimenes if these be treated as cuttings, and yet the course of migration of
plasta in these organs is quite different (HANSEN, 1881).

Although we may be inclined, taking it altogether, to consider correlations
as due to plastic influences of some sort yet we must not say that mechanical
influences are entirely excluded ; at least in the case of organs which touch
each other such effects are quite possible. A distinction must be made between
a purely mechanical pressure and a stimulus effect such as we referred to in a
previous lecture on pressure, contact, &c. The present opportunity may be
taken of saying a few words on mutual mechanical influences of organs.

Very frequently we meet with pressure at the growing point in the course
of normal development, when the young members exhibit more vigorous growth
than the bud-scales, or, to speak more generally, than the external foliar organs
which enclose these young members. Many special bud arrangements are due
to internal factors exclusively, others are due to space relationships. The
crumpled petals of the poppy, for example, broaden out when the calyx is
removed, but remain crumpled if again enclosed in a shell of plaster of Paris or
other artificial calyx (ARNOLDI, 1900). Such unimportant and for the most
part transitory effects are limited to the mutual pressure of organs at the growing
point ; only rarely are they seen in full-grown organs, as in the inwardly directed
pressure of outer leaves of Agave on inner ones. As HOFMEISTER has shown
(1868, p. 638) there is no case known where the formation of an organ is essen-
tially conditioned by mechanical influences. What is true of formation is also
true of position. A mechanical theory of leaf -arrangement has been established
by SCHWENDENER (1878), and elaborated still farther by his pupils ; he believed
that mechanical relationships, especially the distribution of pressure at the
growing point, determine the point of origin of new organs, and further that
mutual pressure of organs may cause alteration in the primary arrangement.
Alterations in position, such as those SCHWENDENER seeks to explain, are,
however, nowhere observed, and they would also be absolutely unintelligible
(SCHWENDENER, 1883, &c. ; SCHUMANN, 1899 ; Josx, 1899, &c.).

Evidence in support of the assertion that the primordia of organs are
conditioned by pressure is as yet entirely wanting. In individual cases, e.g.
the formation of organs in axillary buds, the mechanical theory, although not
proved, appears to us to be simple and clear. Since a pressure may frequently
be produced on opposite sides of the growing point of the axillary bud by the
subtending leaf and the axis, and if new leaves arise at the point of least pressure,
they must appear laterally ; but the relationships of any growing point bearing

JOST Z

338 METAMORPHOSIS

the leaf-primordia are not so obvious as this, and they are certainly not
1 mechanical '. As to the distribution of pressure on the cone above the youngest
leaf -rudiments we know nothing. Recently LEISERING (1902), a pupil of
SCHWENDENER, has directly proved that in these cases there is no external
pressure exerted on the growing point. When LEISERING has recourse to
internal tensions induced by the base of the projecting leaf, the theory is placed
in a position where science surrenders the field to imagination. Any detailed
criticism of the mechanical theory need not be entered into here. (Compare
SCHWENDENER, 1883, &c. ; VOCHTING, 1894 and 1902 ; WINKLER, 1901 and
1903.) We need only add that in our opinion pressure has practically no
significance as regards the arrangement of organs. [Compare FALKENBERG (1901,
41), and BERTHOLD (1904, 133).]

In those cases also, such as axillary buds, where there is nothing against
the acceptance of the existence of pressure on the growing point, it is still an
arbitrary proceeding to regard this pressure as the only adequate factor and to
ignore all the other relations of the organs, which must exist on the analogy of
the numerous examples of correlation known. In addition it may be mentioned
that in not a few cases the same arrangement of lateral organs may be observed
when these are placed at a distance for those next older, where contact is impos-
sible (compare Fig. 65, p. 274), and where LEISERING' s internal tensions cannot
exist. If, however, in such well-established cases contact plays no part it is
not unnatural to suspect that it plays no part in others also.

Further, the arrangement of organs is not always determined by adjacent
organs ; the arrangement of lateral roots is known to be determined by internal
anatomical structure, and it may be imagined that at the growing point of the
stem, and in flower-buds especially, similar conditions occur. In Campanula
media the orientation of the carpels follows that of the calyx, and, according to
the number of the whorls which are interpolated between the calyx and the
carpels, the latter alternate with, or are opposite to, the stamens (EiCHLER,
Blutendiagramme, I, 295. How the carpels come to know the arrangement of
the sepals is the puzzle.

It may be doubted whether the mechanical theory is in the first place
purely mechanical, and in LEISERING' s most recent publication the possibility of
a stimulus effect of the supposed pressure is somewhat more clearly suggested.
That such activities may exist no one will deny, but proof of their existence at
the growing point is not as yet forthcoming. Although we may be unwilling
to accept the mechanical theory of leaf arrangement we must at the same time
admit that we have nothing better to put in its place, nor do we gain much
by referring the arrangement of organs to correlations of growth in general terms.
At all events it may be noted that there are certain regular arrangements of
organs in the plant which must also be referred to correlation, such as the dis-
tribution of the xylem groups in the transverse section of the root and of scleren-
chyma and assimilatory tissue in the cortex of the stem. No structures are
fixed hereditarily once and for all, they are in the highest degree variable.
Doubtless every cell in the periphery of the central cylinder of the root is
capable of forming a vessel, just as every cell below the growing point is capable of
forming a leaf ; nevertheless, only certain cells at regular distances from each
other become vessels and certain regions on the growing point become leaves.

The relations of the parts to each other and to the whole determine the
path of development of each individual plant organ, it is not predetermined
— that is the sum and substance of our interpretation of correlations.

It was remarked at the beginning of the present lecture that the phenomena
of correlation could be regarded as internal or as external factors in plant
formation. In the case of a complicated plant internal influences can scarcely act

CORRELATIONS 339

directly on the growing parts, at all events on the growing point of the shoot ;
light, heat, chemical and physical influences of the material environment never
affect the growing point itself. At the growing point or in its immediate neigh-
bourhood it is decided what the organ shall develop into, and although external
factors take part in this determination they can only do so indirectly through
neighbouring older parts, that is, correlations always play a part.

Whether, however, the external factors operate directly on the part in pro-
cess of formation or through the agency of older parts, they are always of the
nature of releasing energies, and not the actual causes of the formation itself ;
that must lie in the plant itself, or more exactly in the protoplasm. Hence it
is that plants have a cell structure and develop growing points, leaves, buds,
roots, &c. ; protoplasm is also responsible for the differences in the organs men-
tioned in different species. Although we may hesitate whether to consider the
correlations as due to internal formative factors or not, we know sufficiently
accurately that all organic formation in which protoplasm is concerned is condi-
tioned by internal factors. In this domain lie the genuine problems of develop-
mental physiology, from any insight into which we are as yet entirely excluded.
It is as well to note that although we may establish the existence of one of the
external factors in development we no more thereby attain an insight into the
subject than we do into the structure of a complicated steam-engine by know-
ing who opened the valve. Since we learn nothing as to the mode of working
of the machine by knowledge of this kind, it cannot be correct to designate the
modest beginnings of developmental physiology as ' developmental mechanics '.

A short time since G. KLEBS (1903) classified the factors in plant form in
three categories ; he distinguished external and internal conditions of the re-
sulting form and separated from the latter ' specific structure ', what we have
designated as 'those in which protoplasm is concerned'. Specific structure is
to a certain extent constant for each organism and on that depends everything
else. The external world never acts on the specific structure directly but
always on the internal conditions, on the quantity and quality of the substances
present in the cell, and on the physical characters of the protoplasm, vacuole,
cell- wall, &c. ; and these internal conditions in turn affect specific structure.
The internal conditions are to a certain degree open to investigation but the
results of their actions are open only to a limited degree.

Although KLEBS' s conclusions are, in principle, convincing, we still en-
counter difficulties in determining in special cases what depends on internal
conditions and what on specific structure, and we have in the lectures which
immediately follow, which were written before KLEBS' s paper was published,
distinguished only between internal and external factors ; in separating these
we make use of a sufficiently sharp but certainly superficial criterion. The
external factors are such as may be referred to the action of gravity, light, &c.,
while the internal factors are those whose direct dependence on the external
world cannot be demonstrated.

Bibliography to Lecture XXVI.

ARNOLDI. 1900. Flora, 87, 440.

BERTHOLD. 1882. Jahrb. 1 wiss. Bot. 13, 569.

{BERTHOLD. 1904. Z. Phys. d. pflanzl. Organisation. Leipzig.]

[FALKENBERG. 1901. Die Rhodomelaceen d. Golfs v. Neapel. Berlin.]

GOEBEL. 1880. Bot. Ztg. 38.

GOEBEL. 1884. Die gegenseit. Bez. d. Pflanzenorgane. Berlin.

GOEBEL. 1893-95. Flora, 77, 38; 81, 195.

GOEBEL. 1898-1901. Organographie. Jena.

GOEBEL. 1903. Biol. Centrbl. 22, 385.

[GOEBEL. 1905. Flora, 95, 384.]

Z 2

34°

METAMORPHOSIS

HANSEN. 1881. Abhdl. d. Senkenbergschen Gesell. 12.

HERING. 1896. Jahrb. f. wiss. Botanik, 29, 132.

HILDEBRAND. 1898. Die Gattung Cyclamen. Jena.

HOFMEISTER. 1 868. Allg. Morphologic. Leipzig.

JOST. 1891. Bot. Ztg. 49, 485.

JOST. 1893. Ibid- 5T» 89-

JOST. 1895. Jahrb. f. wiss. Botanik, 27, 403.

JOST. 1899-1902. Bot. Ztg. 57, 193 ; 60, 21 ; 60, II, 225.

KLEBS. 1903. Willkiirliche Entwicklungsanderungen bei Pflanzen. Jena.

LEISERING. 1902, a. Flora, 90, 378.

LEISERING. 1902, b. Jahrb. f. wiss. Bot. 37, 421.

LEISERING. 1902,0. Ber. d. bot. Gesell. 20, 613.

LINDEMUTH. 1901. Ber. d. bot. Gesell. 19, 515.

[MASSART. 1898. Memoires Acad. Bruxelles, 57.]

[MAcCALLUM. 1905. Bot. Gaz. 40, 97 and 241.]

[MiEHE. 1905. Ber. d. bot. Gesell. 23, 257.]

[MONTEMARTINI. 1904. Atti 1st. bot. Pavia, N.S. 10.]

PETERS. 1897. B. z. K. d. Wundheilung bei Helianthus. Diss. Gottingen.

SCHUMANN. 1899. Morphologische Studien. p. 238. Leipzig.

SCHWENDENER. 1878. Mechan. Theorie d. Blattstellungen.

SCHWENDENER. 1883, &c. Sitzungsberichte d. Berliner Akad. 1883, 1895, 1899,

1900, 1901, and Ber. d. bot. Gesell. 20, 249.
[SiMON. 1904. Jahrb. f. wiss. Bot. 40, 103.]
STRASBURGER. 1901. Jahrb. f. wiss. Bot. 36, 493.
VOCHTING. 1878. Organbildung. Bonn.

Jahrb. f. wiss. Bot. 16, 367.

Bibl. bot. Heft 4.

Bot. Ztg. 49, 113.

Ueb. Transplanation am Pflanzenkorper. Tubingen.

Jahrb. f. wiss. Bot. 26, 438.

Ibid. 34, i.

Ibid. 38, 83.

Jahrb. f. wiss. Bot. 22, 35.
Bot. Ztg. 47, i.

Jahrb. f. wiss. Bot. 36, i.

Ber. d. bot. Gesell. 20, 8 1.

Jahrb. f. wiss. Bot. 38, 501.

VOCHTING.
VOCHTING.
VOCHTING.
VOCHTING.
VOCHTING.
VOCHTING.
VOCHTING.
DE VRIES.
WIESNER.

WlNKLER.
WlNKLER.
WlNKLER.

1885.
1887.
1891.
1892.

1894.
1899.
I9O2.
1891.
1889.
I9OI.
I9O2.
1903.

LECTURE XXVII

PERIODICITY IN DEVELOPMENT. I

THE developmental processes in organisms are not always carried out with
the same degree of activity nor yet at an equal rate. If a plant consists of a
single spherical cell this cell can increase only to a certain specific size ; uniform
and continuous surface growth with retention of the spherical form is quite im-
possible. In the simplest organisms growth after a certain point is followed by
cell division, resulting in the formation of two organisms, each of which proceeds
to develop in the same way as did the parent. Growth and division follow each
other with regular periodicity. Even plants, which as a rule show no cell
division (e. g. the Siphonaceae), do not grow uniformly in one direction, but from
time to time form lateral branches. The more complicated the organism is the
more pronounced do periodic variations in its developmental activity become,
due sometimes to more or less recognizable external factors, sometimes to purely
internal conditions. One of the most remarkable of these periodicities is the
death after a time in many plants of part of the organism, while usually a frag-
ment only remains alive and undergoes further development. Not less note-
worthy is the phenomenon of hibernation, where all development ceases often

PERIODICITY IN DEVELOPMENT. I 341

for weeks, months, or even longer periods, although the capacity for such develop-
ment is still retained. These three prominent conditions — rest, activity, death
— are in the highest degree characteristic of living things, and to the considera-
tion of these and other periodic phenomena we will devote the present lecture.

This is not the first occasion on which we have encountered such problems,
for we have seen elsewhere that under unfavourable external conditions, such as
too high or too low temperature, or a withdrawal of water, development comes to
a standstill, and finally death ensues. But plants have to endure a scarcity of
water sufficient to render development impossible at regular seasons in many
parts of the world, and even in our country some plants at least are periodically
affected in this way. The same is true of cold, to which our native vegetation is
subject every winter. Plants, we shall find, have adapted themselves in a variety
of ways, all having for their object the tiding over such unfavourable conditions
without suffering permanent injury. We have seen adaptations of this sort
already in the behaviour of lichens and mosses to drought ; such forms can
remain alive despite a withdrawal of water specifically different in each case but
which would at once cause the death of leaves or roots of plants higher than
them in the vegetable kingdom. The seeds and spores of these higher forms,
however, which, owing to internal causes, are detached from the plant, can
endure the withdrawal of water and dry up, remaining alive for a long time even
in this air-dry condition, or while containing a very limited amount of water.
It is obvious that we cannot describe a seed in the air-dry condition as dead,
since it may retain its capacity for germination for several years. The question
we have to answer is whether this state of ' rest ' is real and absolute or only
apparent. May the seed be compared to a clock wound up and only needing a
push of the pendulum to set it agoing, or is the quiescence in the dry seed not
really due to abolition of vital processes but only to a diminution of these to
such an extent that they cannot be recognized ? The first question that occurs
to us is what about respiration, a process which we have seen is essential to every
plant activity. Does it cease in dry seeds or only greatly diminish ? This prob-
lem has often been tackled, but, according to the latest critical researches,
carried out by KOLKWITZ (1901), has not as yet been solved. KOLKWITZ'S experi-
ments, which were carried out on barley, show most clearly what an important
bearing the amount of water present in the seed has on respiration, since i kg. of
barley kept at summer temperature gave off in twenty-four hours 3-59 mmg. of
carbon-dioxide, when 19-20 per cent, of water was present, 1-4 mmg. with 14-15
per cent., 0-35 mmg. with 10-12 per cent.

Since there is about 20 per cent, of water in freshly-gathered barley and
10-12 per cent, in air-dry seeds one must conclude that respiration decreases
very rapidly as dryness is gradually attained, and in air-dry seeds reaches a value
which is practically zero, for only about i per cent, of the dry weight of the seed
would be respired in 100 years (compare p. 192). It is true we can accelerate
the excretion of carbon-dioxide in dry seeds by raising the temperature. At 50° C.
KOLKWITZ obtained 15 mg. of carbon-dioxide from a kilo of barley containing
10-12 per cent, of water. Nevertheless, we can scarcely be wrong in concluding
from KOLKWITZ'S experiments that respiration is not essential to the continuance
of a vital capacity, since many seeds are not injured so far as their power of
germination is concerned by enduring a much more thorough desiccation, in
which cases respiration must be reduced to an amount which cannot be estimated,
and thus can have no physiological significance. It is possible to reduce the
amount of water in barley to 3, 2, or even i per cent., and SCHRODER (1886) has
shown that barley containing only 2 per cent, of water germinated quite well after
an interval of eleven or twelve weeks. No generalizations can be made from these
results, and it is probable that fresh experiments may acquaint us with seeds
whose power of germination ceases with the stoppage of respiration.

342 METAMORPHOSIS

Seeds of the grass type, which can withstand thorough drying, as a rule
retain their powers of germination only for a limited number of years, and such as
may do so for fifty or more years must be considered as exceptional. What the
loss of germinating power depends on in the long run is not known ; but when
we reflect that gradual alteration has been proved to take place in the proteid
reserves present in dry seeds tending to reduce their solubility, we may conclude
that specific protoplasmic bodies undergo as time goes on alterations calculated
to render them functionless. At all events it is quite out of the question to
suppose that death of the resting seed is brought about by using up of reserve
substances in respiration.

But we must not assume that chemical changes in the interior of the seed,
independent of respiration, alone are necessary to explain the ultimate death
of the dried seed; we must also bear in mind the fact that many seeds immediately
after reaching maturity are unable to germinate and only show power of develop-
ment after a certain period of hibernation. Thus, according to KIENITZ (1880),
the seeds of the ash, hornbeam, and of Pinus cembra begin to germinate the year
after they ripen, and it is known in the case of other plants that individual
differences occur among the seeds themselves (WINKLER, 1883) ; thus seeds of
Euphorbia cyparissias germinate in the course of four to seven years. It is now
definitely known that these variations depend on varying degrees of permea-
bility of the testa for water, but we know nothing further as to why seeds which
have imbibed water are prevented from germinating (WiESNER, 1902, p. 55) ; at
most we may draw analogous conclusions from the behaviour of resting vege-
tative buds, a subject of which we shall have to speak later on. Undoubtedly
internal factors play the chief part in determining the initiation and cessation
of the resting period in seeds, whilst the hibernation of lichens and mosses would
appear to depend entirely on external conditions.

If the seed be provided with the external and internal conditions necessary
for germination, the plant resulting from its development exhibits numerous
periodic phenomena, which are partly autonomous, partly dependent on its
relation to the environment, e. g. daily and yearly periodicities.

Thus we may observe a daily periodicity in longitudinal growth, which
is perfectly intelligible if we remember that plants are subjected to certain
external factors which actively affect growth and which themselves vary peri-
odically ; thus we have a daily periodicity in illumination, in heat, and amount
of atmospheric moisture. These factors act, however, so unequally and even
antagonistically that it is impossible to calculate their combined effect before-
hand. Even if we omit from consideration atmospheric moisture, an in-
crease in which as a general rule accelerates growth, and confine our attention
to light and heat, we still find that changes in these two factors may have the
most varied results ; the plants, in a word, may grow more rapidly either by
day or by night. On a midsummer day a high temperature, approaching the
maximum point, in conjunction with bright light retards growth, and increased
growth in the evening may be due not merely to darkness but also to cooling
down to somewhere near the optimum temperature. Conversely, in springtime
the great reduction of temperature at night limits growth so much that the
maximum growth occurs by day owing to the higher temperature, in spite of
the retarding effect of light on growth.

In the experimental treatment of this question, where two variables are
under investigation, we must naturally deal with them one at a time. Ex-
periments in this direction have been carried out by SACHS (1872) and GODLEWSKI
(1898, 1890), who studied the effect of variations in light intensity under constant
temperature and moisture conditions. SACHS found that the rate of growth of
the stem reached a maximum in early morning after sunrise, that it decreased
hourly towards evening, and increased once more as darkness came on, often

PERIODICITY IN DEVELOPMENT. I

343

before sunset ; the increase continued until sunrise when the maximum was again
reached. This growth curve is easily explained if we assume the increasingly
retarding effect of light in the course of the day, and the effect of the withdrawal
of light at night. This hypothesis has, however, no experimental foundation
and the results obtained by SACHS have no general application. GODLEWSKI,
experimenting with epicotyls of Phaseolus, obtained entirely different results ;
he found that growth was greater by day than by night and that the maximum
was reached between six and eight p.m., the minimum in early morning. He
further showed that the transition from darkness to light acts as a stimulus, in
consequence of which a sudden but temporary decrease in growth took place.
A distinction must at least be drawn between this influence of change in light and
the influence of constant illumination or darkness. Corresponding stimulatory
activities due to changes in temperature appear to be generally non-existent
(TRUE, 1895), but we shall see in Lecture XXXIX that they do occur in special
cases.

Just as we are unable at present to understand fully the alterations in
growth taking place under the influence of a simple light change, so we have even
greater difficulty in explaining the after-effect of daily periodicity which was
demonstrated by SACHS and BARANETZKY (1879) as taking place in darkness
under constant temperature. These investigators observed in certain cases that
variations in growth, exhibited during light variations, continued with the same
periodicity in darkness all day long, and there can be no doubt that a causal
connexion existed between them. PFEFFER (1881) has advanced the following
explanation of these after-effects. It is based on certain phenomena which we
shall have to study later (nyctitropism, Lect. XXXIX), and assumes that after
a single illumination not only does a simple retardation of growth take place
but that this retardation is necessarily followed after a time by an acceleration ;
darkness operates in the same way. If, therefore, darkening sets in at a time
when, owing to illumination, growth acceleration has already taken place, the
effect of the single stimulus is added to the after-effect, and if the total effect is
maintained all day long, the after-effects will become all the more established.
Evidence is not forthcoming, however, to show that a double alteration in rate
of growth takes place at every application of the stimulus, and, further, where it
has been observed the second alteration does not make itself apparent till about
twelve hours after. This must be the case, however, if, in nature, the new
stimulus is to be added to the after-effect of the old.

Under these conditions it is important to note that periodic movements,
especially those with daily rhythm, occur in plants which grow under quite
constant conditions, in which an after-effect of any kind is out of the question.
Thus BARANETZKY has observed a daily periodicity in beets grown in the dark
under a constant temperature, resulting from internal factors only, and therefore
only accidentally showing a twelve-hourly periodicity. GODLEWSKI also demon-
strated in the case of beans germinated in the dark a regular daily periodicity in
growth, but this does not always take place, and is entirely absent in the case of
certain kinds of seeds. We thus arrive at the conclusion that a daily periodicity in
longitudinal growth cannot be due to external factors and their after-effects only.

Among the phenomena of yearly periodicity the resting period, seen in trees
and shrubs, and briefly referred to above, is of special interest. Selecting ex-
amples from among our native plants, we readily notice that the quiescent period
occurs as a rule in the winter months, the active period in the summer, and one
would naturally attribute this to the direct influence of external conditions, and
more especially to the annual rise and fall of temperature. Closer examination
shows, however, that this view cannot be correct, or at least that the relation
between the plant and the environment is not so simple as it appears.

The winter buds of many trees exhibit the rudiments of an entire shoot,.

344 METAMORPHOSIS

which develops in the following year — all its leaves having been laid down in
the autumn. The development which occurs in spring resolves itself essentially
into a longitudinal growth, leaves and internodes acquiring their definite
length, and this elongation may take place in the case of the oak, beech, &c., in
the course of a few days (Josr, 1891 ; KUSTER, 1898). We may describe this
as a spasmodic evolution, and it is evident that a certain degree of temperature
is needed to give the impulse for the commencing of this development, but that
the cessation of this evolution depends not on external but on internal relations.
Although new buds obviously begin to form in the axils of the leaves, or termin-
ally, by May or June, the leaves of these buds unfold only in the following
year; it is impossible, however, for any one to believe that the external conditions
are unsuited for their immediate development. We may readily attribute there-
fore the non-unfolding of these rudiments to correlation of growth. If a shoot
be deprived of its leaves in early summer, in most cases a second evolution of
leaves and buds takes place (GoEBEL, 1880), and many trees, such as the horse-
chestnut, develop shoots a second time in autumn if they have lost their leaves
owing to unfavourable circumstances such as drought. Correlation between
the fully-developed leaves and the rudiments of next year's growth, prevents
an immediate evolution of the latter. In this case a mere retardation of the
functional activity and not complete removal of the leaves is all that is neces-
sary to induce the correlation to make itself apparent. Hence one sees that
chestnuts or maples grown in darkness (Josr, 1893) proceed to form from the
terminal bud, not merely a second, but even a third shoot, and we cannot
doubt that by appropriate means, e. g. by accumulation of a sufficient amount
of materials, an even greater number of shoots may be induced to form. A second
shoot, the so-called ' Lammas -shoot ', certainly appears regularly in many trees,
e. g. in young oaks, if the foliage leaves are fully active. In such a case, owing
to internal relations not fully known, the correlative inhibition is removed earlier
than usual. The characteristic feature, however, of this entire group of plants
is that they may be made to increase their production of leaves but never to
keep on doing so. Every shoot formation is followed by a period of rest, however
short, and the successive shoots are separated from each other by scale-leaves.
We have to deal with a peculiar and unmistakable periodicity in the plant which
manifests itself not only in the periodic succession of leaf-development but
also in quantitative variations in the leaves and internodes. (Qualitative
variations we shall refer to later.) Very frequently one sees that the internodes
in a yearly increment increase from below upwards and then decrease : —

Length of internodes in cm.
Aesculus 3.5 6-5 7.5 0-5

Acer pseudoplatanus 0-5 4-5 10-5 9-5 7-0 2-5 2-5 o-o
Fagus sylvatica 0-4 i-o 2-2 3-2 4-7 7-0 80 8-0 8-0 4-8 4-5

Similarly the size of the leaves varies (beech) or they may (Acer, Aesculus)
decrease regularly from base to apex (compare TAMMES, 1903).

Another type of shoot evolution is seen in those plants which go on de-
veloping leaf after leaf the whole summer through, and in which the cessation of
leaf-development is probably induced by external factors. Thus roses grown in
a greenhouse go on forming leaves until late in December. This type is natu-
rally of less interest ; it need only be noted that it is related by transitional
forms with the type previously described. In Forsythia, for example, there are
short shoots which have completed their leaf-formation in early summer, and also
long shoots which develop leaves right on into autumn and finally die off at the
apex without forming any terminal bud. Between these extremes there are
shoots which, after a short rest, shoot out anew, and others which after only
forming short internodes once more proceed to form long internodes. The deter-

PERIODICITY IN DEVELOPMENT. I 345

mining factors which lead to this behaviour in individual branches are unknown,
and the conclusion that it is due to a periodicity in internal causes can scarcely
be avoided.

There are many observations, however, which scarcely support the view that
internal factors play an essential part in this periodicity, they rather go to prove
that the periodicity in the plant still stands in a definite relation to the change
in the seasons of the year. If, as is clear from what has been said, this is often
incorrect as regards the commencement of the resting period, it is so for the
close of this period, for the opening of the buds takes place as a general rule in
spring, when the temperature is rising. Previous to the institution of scientific
experiment on the subject gardening experience had shown that the close of the
resting period, in these latitudes at all events, was dependent in many plants only
on such a rise in temperature, and that it is possible artificially to induce leaves
and flowers to appear in the middle of winter if what may be termed an ' artifi-
cial premature spring ' be provided. This precocious development, however, has
limits ; in the case of the elder it is possible to bring about an opening of the
principal buds and subsequently of the flowers by raising the temperature shortly
before the beginning of November; in the summer months preceding, these buds
cannot be made to develop although the organs concerned have been laid down
in the bud. On the other hand, the development of these buds (but without
flowers) may be induced, as already noted, in early summer, immediately after
the first shoot has formed, by removal of the foliage leaves. Between these two
dates there is a period of rest, i.e. from July to October, when raising the tempera-
ture has no effect. This forms the special period of rest which depends on
internal factors only but which may be easily lengthened by external factors,
but is with difficulty shortened. According to ASKENASY'S (1877) researches
a complete rest, where growth is at an absolute standstill, does not take place in
the buds. In the flower-buds of the cherry a continuous but feeble embryonic

frowth goes on from summer till the next spring. Syringa also behaves
oubtless in a similar way. We do not know why this embryonal growth at
definite times, viz. from July to October, is not followed by growth in length.
Certain observations which we have yet to speak of will provide us with
a starting-point for further research.

JOHANNSEN'S (1900) ingenious method of shortening the resting period by
etherization is especially worthy of notice. Plants which are near the beginning
or completion of their resting period may be made to send out shoots by exposure
for two periods of twenty-four hours each to ether vapour. It is possible that the
action of the ether is that of an anaesthetic, i. e. that certain functions which
tend to retard growth in the plant are inhibited by the ether. It is, however,
more likely that we have not to deal with a specific action of the ether itself but
that other poisons may produce a similar effect. What we have to think of is
the stimulatory action which we have seen poisons to possess when below that
degree of concentration which is fatal, an action which affects the incitation of
metabolic activity and increases respiration more especially. We have every
right to assume that active metabolic processes go on in the plant during the
resting period. A. FISCHER'S (1890) researches, based on the valuable observa-
tions of Russow (1882), have added much to our knowledge of these processes.
According to them the reserve substances in trees undergo very extensive
changes in the course of the winter. In autumn the parenchyma is filled with
starch ; in October this starch begins to undergo dissolution, fats and, in part,
also glucoses taking its place. In some trees this transformation takes place in
the rind only, the starch in the wood remaining unaltered, whilst in others (Tilia,
Betula, Pinus) all the starch undergoes transformation, and hence no starch can
be found in these trees from November to the end of February. Starch is re-
formed in March and shortly before the development of the young shoots it is

346 METAMORPHOSIS

present in quantities almost as great as in autumn ; this starch is then dis-
solved and employed in the construction of new shoots. The close of the
special resting period coincides in a remarkable manner with the date when the
amylaceous contents are at a minimum ; only when all the starch is dissolved
do we succeed in inducing the development of shoots, and apparently every
increase in temperature at this moment induces a progressive re-formation of
starch. Conversely, the dissolution of starch in autumn can be induced by
lowering the temperature. Of course these transformations of reserves must
not be looked on as the causes of periodic rest, they are to be referred to a
common cause, possibly to an alteration in the protoplasm itself, which governs
all metabolic processes.

Detailed research on our native trees would certainly conduce to the
extension of our knowledge of this interesting problem in many ways, since none
of the various cultivated varieties of the same species behave identically as
regards their resting periods QOHANNSEN) ; so much the greater then must be
the differences between different species of trees. It is also well known that
different organs of the same plant exhibit striking variations in their resting
periods. Secondary growth in thickness, which commences usually about the
same time as the unfolding of the buds, lasts considerably longer than leaf forma-
tion, at least in those trees whose buds open spasmodically, and xylem ceases
forming in summer earlier than phloem (STRASBURGER, 1891). The entire
process, however, always exhibits a definite periodicity such as that described,
viz. activity in summer and rest in autumn and winter, the close of the period of
secondary thickening certainly not being directly dependent on external condi-
tions. The roots of trees differ greatly in this respect from shoots. Owing to
the obvious difficulty of research the problems connected with root growth have
been as yet little elucidated, and investigators (RESA, 1877 ; WIELER, 1893 ;
BUSGEN, 1901 ; HAMMERLE, 1901) have not been successful in making a perfect
comparison of the two types of organ. This much is certain, however, that
in many roots growth begins in March and continues till November or Decem-
ber; in the middle of summer a marked decrease in growth may be frequently
observed, which never amounts, however, to a complete stoppage. No experi-
mental researches are as yet forthcoming, especially on the influence of external
conditions, such as heat and moisture, but such researches are absolutely essen-
tial before we can arrive at any decision on the periodicity of growth in the root.

Many perennials may be compared with trees as regards the mode of
development of the leafy shoot. A peculiarity, however, appears in our spring
flowers, where apparently the resting period is transferred to the dry season of
the year, the actual summer. The commencement of the new growing period
generally occurs in these plants in autumn, and shows itself first in the formation
of new roots. The buds also begin to develop in October and November, but do
not as yet come above ground. Further development is retarded during the
winter cold, and may be temporarily at a standstill. This hibernation is, however,
an induced one, and on the temperature rising all these plants are easily induced
to shoot in winter. In nature the formation of flowers and foliage takes place
in the early spring, according to the species, from February to May. Early in the
summer the leaves fall off so that in midsummer the plant is reduced to its sub-
terranean parts only. Colchicum holds a special position among spring flowers
because the flowers appear, as is well known, from August to October, at the same
time as the new roots, while the leaves appear for the first time in the following
spring. Precedence of foliage by flowers is a phenomenon of widespread occur-
rence among spring plants, the peculiarity in the case of Colchicum, however, lies
essentially in this, that external factors prevent a rapid sequence of the foliage
shoots. In arctic countries and on high mountains the simultaneous forma-
tion of leaves and flowers is, however, a normal occurrence (WiESNER, 1902).

PERIODICITY IN DEVELOPMENT. I 347

More exact investigation shows us that the differences between spring plants
and trees are by no means deep-seated. The summer rest of the former may
be readily compared with the absence of leaf-unfolding in summer in all spas-
modically-budding trees ; the foliage leaves of such plants die off much sooner
than in the case of trees, but it can scarcely be doubted that embryonic growth
continues during the whole summer in the subterranean organs. Nor is the early
commencement of vigorous growth in the buds in autumn so very extraordinary,
since, according to ASKENASY, the buds of trees do not cease developing during
the winter. The characteristic feature of spring plants lies only in this, that their
periods are somewhat moved forward and that their leaves are very short lived.
They also exhibit a resistance at certain times to artificial forcing ; thus tulips
and hyacinths cannot be made to flower before December. In all probability the
formation of shoots is more readily possible in such plants just before the beginning
of the resting period ; at least certain results obtained by SCHMID (1901) may
be interpreted in this sense. This author has also shown that the special resting
period in the case of the potato may be shortened by external causes.

The observations which have been quoted, as well as the fact that members
of one and the same plant are in a state of rest at different times, teach us that
under given external conditions only individual functions come to rest or
exhibit reduced activity, and that these resting periods cannot be directly due
to unfavourable external factors. This may be proved even more readily in the
case of many tropical plants which show periodic growth even though external
conditions be constant and favourable. We are indebted to SCHIMPER (1898) for
exhaustive studies on this subject. According to his researches a large number
of trees exist at Buitenzorg in Java which at longer or shorter intervals, once
to six times annually, throw off all their foliage and, often after only a few days,
produce fresh foliage once more. Other plants behave even more remarkably,
e.g. Amherstia nobilis, in which the individual branches act independently of each
other, so that we may find on a single tree at the same time branches with ter-
minal resting-buds and shoots in all stages of development. Trees transplanted
from a temperate to a tropical climate behave just like Amherstia, inasmuch as
they lose, not periodicity itself, but its relation to seasons. The appearance of
Magnolia yulan in December and January in the Hill station at Tjibodas near
Buitenzorg is described by SCHIMPER (1898, 266) in the following terms :
' Individual branches bare of leaves but with leaf and, here and there, flower
buds ; others with young leaves and full blown flowers ; others with adult
leathery leaves and withered flowers ; others with leaves with autumn colouring
readily deciduous.'

In other tropical climates, always warm and moist, such as the Brazilian
rain forest near Para, we find examples of periodicity, e.g. Hevea braziliensis.
According to the observations of HUBER (1898) the full-grown tree frequently
produces only one or often two shoots in the year after the leaves have
more or less completely fallen off. The young trees behave in an especially
interesting manner. One of these formed, during the rainy period of 1896-7,
five shoots, each of which was in full leaf in thirty days, and then remained
quiescent for about ten days. Each shoot had at first short internodes, then
longer, and then finally shorter internodes once more. It bore scale-leaves at
the base, then foliage-leaves, and terminated in a bud enclosed in scales destined
to be the next shoot. The five shoots opened out on Dec. 10, 1896, Jan. 20,
March 12, April 25, and June 6, 1897, respectively ; the succeeding shoots were not
observed exactly, but it was established that three had been formed during
the remainder of 1897 and three more in the first half of 1898. Since, however,
different examples are always in different phases of development, it cannot be
doubted that the periodicity is due entirely to internal causes. As the plant
gets older the number of shoots decreases, and, finally, as already noted, only one
appears each year. This rhythmic alternation of activity and quiescence which

348 METAMORPHOSIS

appears in tropical plants is an adaptation to special periods in the year, so that
the resting period is associated with the cold or dry season and the active period
with the warm or wet season. This periodicity, however, cannot be at once
adapted to other alternations of seasons. If plants from the southern hemi-
sphere be brought into such a climate as ours, they are able to bring their perio-
dicity into harmony with our climate only if they become thoroughly acclima-
tized. In the case of our indigenous flora periodicity may be altered only
gradually, since the plants maintain their accustomed rhythm with consider-
able tenacity. It has already been pointed out that we can induce the forma-
tion of shoots by certain stimuli, but we cannot on the other hand lengthen the
resting period indefinitely without injury to the plant. Manifestly, activity is
followed by a resting period in the plant ; conversely, activity follows of necessity
a period of rest, and the periodic alternation between them is maintained with
greater or less constancy. [KLEBS (1903) has shown that many plants which
have well marked resting periods under natural conditions may, by artificial
means, be made to grow continuously. The chief point to note in this relation
is that one prevents the occurrence of the arrests, which increase during the course
of the resting period, and that already the conditions for the development of
shoots are provided at the commencement of theresting period. Although we do not
in the least degree question the correctness of KLEBS' s very interesting obser-
vations, still we must take exception to the framing of any general conclusion
upon them. The conditions existing in trees especially, which KLEBS has
scarcely at all considered, compel us to hold the view that the resting period is
often not to be considered as an inevitable consequence of growth. If KLEBS' s
view has a general application we should be able to produce a continuous leaf
formation in trees which exhibit periodic vegetative activity.]

The examples we have hitherto given of daily and yearly periodicity illus-
trate in reality only quantitative changes in growth ; there are, however, also
qualitative differences, so that at different seasons different kinds of organs are
produced. The contrast between vegetative and reproductive organs is a case in
point, but even the vegetative organs themselves do not always appear in the
same forms. In the highest plants, for instance, we recognize a regular succes-
sion of scale-leaves, foliage-leaves, and bracts. These qualitative changes in the
productive activity of the plant stand in close relation to the quantitative types
treated of above, since in this case also a yearly periodicity is very well marked.
The factors which make plants produce leaves of dissimilar form are naturally
internal, inquiry into which is by no means easy. Certainly, the succession
of these organs is not unalterable, for we can influence the succession to a certain
extent. The insight which we gain thereby teaches us nothing more than
that correlations exist between the individual organs, disturbances in which
induce disturbances in the normal succession. A few examples will make this
clearer. The normal leafy shoot of a tree, after producing a larger or smaller
number of foliage-leaves, proceeds to form scale-leaves, forming a covering under
which the next year's shoot is constructed as a terminal bud. Again, similar
buds are formed in the axils of the leaves, which also commence with scale-leaves.
The scale-leaves have a quite different function from the foliage-leaves, and hence
we find them to possess a distinct form and structure. They do not possess large
surfaces exposed to light. Chlorophylliferous tissue, permeated with vascular
bundles, is wanting ; they are small, compact, and closely pressed together.
In their first beginnings, however, they differ in no respect, as GOEBEL (1880)
has shown, from foliage-leaves (Fig. 104), and they exhibit, as these do, a differen-
tiation into leaf -base (G) and blade (L). While in the case of the foliage-leaf it
is especially the blade which develops greatly, in the bud-scale it does not do so
as a rule, the leaf -base developing instead. If, some time in the spring, we
remove the foliage-leaves from a developing shoot, the leaf-organs, which would

PERIODICITY IN DEVELOPMENT. I

349

in the normal course of development have formed bud-scales, develop in to foliage-
leaves. The lowest of these are stimulated to become foliage-leaves although they
have already advanced to a greater or less extent towards the scale-leaf forma-
tion, and all intermediate conditions occur between foliage and scale-leaves.
Further up the shoot quite normal foliage -leaves are developed. As a result
of this experiment we may conclude that as a general rule the scale-leaves
develop under the influence of the foliage-leaves, that a certain number of foliage-
leaves have their activity diverted to the formation of scale-leaves. The same
is true of the reproductive-leaves. The simplest form occurs in certain ferns, e. g.
Blechnum and Struthiopteris. The reproductive-leaves are in this case foliage-
leaves which, owing to the formation of reproductive organs (sporangia), have
taken on another function and ap-
pearance. While in many ferns
these sporangia arise on ordinary
foliage-leaves, which retain their
assimilatory function,in those men-
tioned above a division of labour
takes place, some leaves devoting
themselves only to the production
of sporangia and having their as-
similatory parenchyma reduced.
GOEBEL has shown that if the
foliage-leaves of Struthiopteris be
cut off the reproductive-leaves
which appear later (sporophylla)
may change into ordinary leaves
bearing no sporangia.

The sporophylla of the ferns
are represented in the higher plant
by the stamens and carpels, which
are distinguished also by bearing
sporangia. Further, we can deter-
mine here also tracts as forming
transitions between foliage and the
essential floral organs. Numerous
observations and experiments
have been made which show that
reproductive-leaves may become
altered into foliage-leaves owing
to unknown causes, or, in indi-
vidual cases, owing to the influence
of insects, not as yet investigated
(PEYRITSCH, 1882), even although
it is quite obvious, both by their
position and structure, that they are intended for floral organs. We must there-
fore conclude that in the normal plant development there are internalbnt by no
means unalterable causes which induce a periodic alternation or 'metamor-
phosis ' of leaves.

These factors are, in detail, quite unknown to us. One hypothesis by
way of explanation has been advanced by SACHS (1880-1). According to this
author there are in the leaf, in addition to the products of assimilation pre-
viously spoken of, specific constructive materials which pass away from the
leaf in all directions and which collect in certain quantities where a definite
organ has to be developed. Thus the flower would be formed out of flower-
building material, roots out of root-building substance and so on.

Fig. 104. Acer platanoides. /, foliage leaf, reduced. //,
scale-leaf. ///, young scale-leaf (magn.). IV, young foliage-
leaf (magn. and schem.). G, leaf-base ; 6", stalk ; Z,, blade.
From GOEBEL, Organographie.

350 METAMORPHOSIS

This hypothesis very conveniently explains anomalies and regeneration
phenomena, and this has won it a certain amount of acceptance. Closer examina-
tion shows, however, that the difficulties are not thereby removed, but only
shifted elsewhere. If we ask what these specific constructive bodies are, why
they pass to certain definite places, and how their morphogenetic activity is con-
ditioned, we are compelled to say we have no knowledge ; hence it has been said
that SACHS'S hypothesis is merely a paraphrase of the facts, which moreover is not
correct. A criticism of this hypothesis is to be found in VOCHTING (1899),
PFEFFER (Phys. II, 234), and KLEBS (1903) [also GOEBEL, 1905]. According to
our view, the specific constructive substances for each organ cannot be trans-
ported from place to place, they consist of protoplasm,\vhich remains stationary.
We must assume, however, that the protoplasm of every young cell has the power
of constructing the most varied type of organ. That it forms one organ rather
than another is determined by a releasing force unknown to us and not by any
primary constructive material.

Side by side with the external 'metamorphosis' in the shoot is another
' metamorphosis ' in stems, branches and roots, whose seat is not the apical
meristem but the intercalary meristem known as cambium. As is well known,
this cambium produces different tissues inwardly and outwardly, which are
termed secondary wood and secondary bast. The construction of the wood
takes place in such a way that, in the course of the vegetative period, the
cambium does not always produce similar elements, those formed in the begin-
ning of spring are different from those formed in the height of summer, and
these we term spring wood and autumn wood respectively (for literature see
WIELER, 1891, 1892, 1897 ; Josx, 1891, 1893) Spring wood merges gradually
into autumn wood but the transition from autumn wood to spring wood is more
abrupt, and hence the various annual rings or single year's growths are often
clearly visible to the naked eye. The difference between spring and autumn
wood in the simplest cases, e.g. the Coniferae, lies merely in the decrease in the
diameter of the tracheides and the increase in the thickness of their walls.
This simple condition had long since suggested an explanation of annual rings.
Sometimes purely mechanical factors, such as the pressure of the cortex, some-
times nutritive influences were given as the causes. No one believes any longer
in any real influence of cortical pressure on annual ring formation, but it is un-
deniable that cells may develop very differently under different nutritive con-
ditions ; so long, however, as we have no experimental data available as to the
influence of nutritive substances on simple objects any explanation of secondary
increase in thickness must remain imperfect.

These explanations are, besides, quite inadequate when we think of the annual
rings of Dicotyledons, where, in addition to quantitative differences, qualitative
differences also occur between autumn and spring wood, as, for example, in the
more abundant or exclusive formation of large vessels in spring. It is naturally
impossible to give a definite answer as to the effect of nutrition in such specific
cases, and so no theory has been hazarded as to the special problem of annual
ring formation. We must simply accept the view that this periodicity in the
annual ring is due to internal factors, just as we have regarded the yearly
periodicity in longitudinal growth also as autonomous. But just as in opposition
to the rule there are conditions when a second shoot in the same year may be
observed, or induced to form experimentally, so it is, too, with secondary
growth ; a second annual ring may be seen in the horse-chestnut in autumn
when supplementary shoots appear. A certain relation, in fact, subsists
between the annual ring and the annual shoot. If the second shoot appears
before the autumn wood has been formed, one would not expect a second
annual ring (oak, Lammas-shoot) ; the relations between annual rings and
annual shoots do not, however, appear to be of the nature of correlations as
was previously believed (Josx, 1891), so that the formation of leaves may not

PERIODICITY IN DEVELOPMENT. I

be the direct consequence of spring wood formation, both phenomena may
depend on common factors, factors which operate in such a way as to induce
the formation, after a certain resting period, of a new shoot with foliage-leaves
and a new zone of wood with broad vessels. This concurrence is always
appropriate, since with every new leaf a rise of transpiration takes place, to
replace which increase in the water conduits is necessary. As to the annual
rings in tropical trees so little is as yet known that it is needless for us to consider
them (HoLTERMANN. 1902 ; [URSPRUNG, 1904]).

In addition to the daily and yearly periodicity in the plant a few words
may be said in conclusion as to periodicity in its entire life-cycle. There are
plants like Stellaria media, Senecio vulgaris, &c., which go through their entire
life-history, from the germination of the seed to the ripening of new seed, in
a few months, in which also each seed germinates at once, so that several genera-
tions may be formed in one calendar year without any rest and unrestricted by
the time of year. After a certain number of seeds have been formed the plant
dies off, but the seeds provide for the maintenance of the species. Similarly, but
more intimately associated with the seasons are numerous annuals, and with
these forms may be associated other monocarpic or single fruiting plants, in
which the formation of seed is preceded by a two to many year stage of purely
vegetable growth with or without interpolated resting periods. The proba-
bility is that in all these cases the formation of fruit is the cause of the death of
the vegetative organs, since life in the latter may be markedly prolonged by
preventing the setting of seed. To a certain extent in opposition to these types
there are plants, such as our native trees, which fruit frequently, and whose
continued existence is not determined by the formation of seed. In all perennial
types another periodicity, in addition to the annual periodicity, makes its ap-
pearance, which we will study only as it is represented by trees. A tree is at the
commencement a seedling with very limited power of growth, as is seen in many
annuals; it gradually gains in strength, however, and by growth in length and in
thickness, and by the development of the constituent elements of the xylem
it attains ever increasing size until a maximum is reached. Similarly there
arises also as a natural necessity a descending curve, which finally ends in death
after the tree has gone on for many years producing seeds for the maintenance
of the species. Long before the specimen in question as a whole succumbs,
individual parts of it die off. Thus the leaves die off after they have performed
their functions for one or more years, and although external factors co-operate
in the leaf-fall of deciduous trees, still leaf-/<z// is as much an organic process
as leaf- formation. As a rule, certain cells are produced at the base of the leaf-
stalk whose function it is to cut the leaf off. These cells form a separating layer,
the swelling up of a certain middle lamella of which brings about the separation
of the dying part from that which still remains alive. As in the case of leaves, so
also entire branches may be abstricted, or, without any such separation, may die
and gradually rot away where they were developed. All the older tissues of the
stem die in the long run ; the peripheral tissues become transformed into bark,
falling off or forming a protective sheath to the parts within ; centrally the wood
becomes transformed into duramen involving death of the elements. Only the
apical and intercalary merismatic regions, as also their youngest derivatives, re-
main alive in an old tree. We thus see that every cell which has lost its embryonic
characters dies after a longer or shorter period if it does not assume these
characters anew, for reasons of which we have spoken in Lect.XXVI. Whether,
however, a cell remains or becomes embryonic depends on its relations to the
whole plant and to its different parts, for the organism provides for the per-
sistence of some cells and the death of others. But this is not true of all organisms ;
where there is no differentiation into embryonic and somatic cells, as in unicellular
types, there are no cells to die off of their own accord, all remain alive so long as
they are not injured by accidents from without. We will refer again to this

352 METAMORPHOSIS

conclusion in the next lecture. We need only add at present that the capacity
for life in embryonic cells is retained only if they be active and have the power
of growing and dividing ; if this capacity be lost they are doomed sooner or
later. We have already shown that in seeds and similar structures the inactive
protoplasm for long (but not indefinitely) retains its power of development ;
active merismatic regions die off much more rapidly if they be prevented from
growing. Root apices, for example, confined within plaster of Paris died off in
about ten weeks according to PFEFFER'S experiments (1893, 356).

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BARANETZKY. 1879. Mem. de 1'acad. de St. Petersbourg, vn, 27.

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FISCHER, A. 1890. Jahrb. f. wiss. Bot. 22, 73.

GODLEWSKI. 1889-90. Anzeiger d. Akad. in Krakau.

GOEBEL. 1880. Bot. Ztg. 38, 753.

GOEBEL. 1887. Ber. d. bot. Gesell. 5, p. LXIX.

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WIELER. 1891. Allg. Forst- u. Jagdzeitung (March).
WIELER. 1892. Tharandter Forstl. Jahrb. 42, 72.
WIELER. 1893. Cohn's Beitr. z. Biologic, 6, i.
WIELER. 1897. Tharandter Forstl. Jahrb. 47, 172.
WIESNER. 1902. Biologic d. Pflanzen. 2nd ed. Vienna.
WINKLER, A. 1883. Ber. d. bot. Gesell. i, 452.

LECTURE XXVIII
PERIODICITY IN DEVELOPMENT. II

REFERENCE was made incidentally in the last lecture to the periodic alter-
nation of foliage and reproductive-leaves in ferns and flowering plants, and the
reason for the difference in form and anatomical structure of these two types
of leaf, we found, lay in the different functions fulfilled by them. The repro-
ductive-leaves have assigned to them the formation of sporangia, the specific

PERIODICITY IN DEVELOPMENT. II 353

organs of reproduction. What is the factor which all at once determines this
change in the organism, viz. the arrest of the formation of vegetative organs and
initiation of the development of reproductive-leaves ? This is the problem in
developmental physiology which forms the subject of consideration in the
present lecture. First of all, let us be sure that we understand what ' repro-
duction ' really means.

The current conception of reproduction has been derived from the con-
sideration of the phenomena as exemplified by the higher plants, and especially
by animals. In animals the individual is said to reproduce when it gives rise to
individuals like itself. The conception of the ' individual ' as understood by the
zoologist cannot be applied to the plant world, for in many growth-phenomena it
is extremely difficult to say whether we are dealing with continued growth of
the parent or with the formation of a new organism. No one would dream of
applying the term ' reproduction ' to the formation of a lateral branch on the
stem of a willow, for the young branch is exactly like the parent, and obviously
is merely adding to the extent of the original tree ; but suppose this very branch
were torn off by a gale of wind, and suppose it rooted itself and developed into
a sapling, we have to decide now whether this is a reproductive phenomenon or
not. The isolation of twigs, which may in this illustration be purely accidental,
occurs in many plants quite definitely and regularly. Thus in many rhizomes
the branches become isolated by decay of the older parts behind, and these
isolated branches give rise to new and independent plants. In such cases
what must one call this process ? At the beginning it is obviously merely
growth and branching, later on it becomes reproduction. Where must we
draw the line of demarcation ? The difficulties do not decrease when simple
forms such as Algae are considered. Spirogym, for instance, consists of cylin-
drical cells united into cell filaments. Each cell is completely independent ; it
grows and divides, and thus the filament increases in size. Under definite
external conditions, however (BENECKE, 1898), .the filaments break down into
their constituent cells, each of which has the power of developing into a new
filament. Now if we regard the cell- filament as a unity, reproduction consists
in the segmentation of the thread into individual cells, but if each cell be an
individual we have no reproductive process represented here at all.

With these examples before us we are justified in saying that often in
nature no hard and fast line can be drawn between vegetative growth and re-
production. It will serve our present purpose better, however, if we institute
an arbitrary limit and regard the process of reproduction as always involving
the formation on the part of the plant of special organs whose duty it is to give
origin to new individuals of the same species. Such organs must of necessity
be capable of separating from the parent plant ; they must contain a certain
amount of protoplasm capable of development and with possibilities of access
to reserves of food-material required to render further development possible.
This protoplasm may be in the form of a single cell or a cell-complex, whether
occurring in higher or in lower plants, and the cell-mass may take a form as
complex even as a growing point or bud. Let us commence with a simple
example.

Ulothrix zonata is an alga whose cylindrical cells are combined to form a
simple unbranched filament, but it differs from the majority of the forms of Spiro-
gyra to which we have previously referred in being composed of cells which are not
all alike ; one of them, the basal cell, exhibits the peculiarity of acting as a fixing
organ (Fig. 105, A, r). Reproduction in the alga takes place as follows. The
contents of a cell subdivides into two or more cells (B), each of which contains,
in addition to protoplasm, a chloroplast and a nucleus. Further, each is provided
at its colourless anterior end with four delicate protoplasmic filaments or cilia,
by whose vibrations the naked cell moves through the water (C) after its escape
from the mother-cell through a cleft in the membrane. It has been customary
OST A a

354

METAMORPHOSIS

to designate the reproductive organs of the lower plants as spores, so that we
may describe these motile spores as ' swarmspores '. Such a swarmspore after
a few hours' motility comes to rest, throws its cilia off, envelops itself in a cell-
wall, and grows into a new cell-filament. Another type of swarmspore also
occurs to which the name of gamete is given. These swarmspores are dis-
tinguished from those already described not only by the fact that they are
smaller and possess only two cilia (E) but also especially by their subsequent be-
haviour. After escaping from the mother-cell (D) they come together in pairs

and fuse with each other, forming the
so-called zygotes (F-H) . The zygote as-
sumes a thick investment (/), and, after
a long resting period, germinates, giving
rise to four ordinary swarmspores (K).
Let us now compare the reproduc-
tive phenomena as seen in Ulothrix with
those of the freshwater alga, Oedo-
gonium. Here also we meet with a
cellular filament fixed at one end to the
substratum, also exhibiting two types of
reproduction, i. e. by swarmspores and
by fusion of cells. The swarmspores in
this case originate, as a rule, one in each
cell,'so that the whole of the cell-contents
goes to form one swarmspore ; in this
case no increase in number is associated
with reproduction. It is of no conse-
quence for our present purpose that the
swarmspores of Oedogonium are struc-
turally different from those of Ulothrix,
all that we need lay stress on is the fact
that, as in Ulothrix, after a certain time
the movement in the swarmspore ceases
and a new cell-filament is formed. So far
as the other method of reproduction is
concerned considerable difference exists.
In Oedogonium the two cells which unite
to form the zygote are quite different
from each other, they originate in differ-
ent cells, and often in different filaments.
The contents of certain cells, distin-
guished by their greater size, and
known as oogonia, contract, and the
cell-wall develops a special opening
(Fig. 106). The cell-contents do not
escape, however, but fuse in situ with
another cell which enters into it through

this] opening. These other cells have the form and power of movement
of the ordinary swarmspores, but differ from them in their smaller size,
reduced amount of chlorophyll, and their mode of development (Fig. 106,
///). The large cell is known as the ' ovum ', the small cell as the ' sperma-
tozoid'. After these two cells have fused, the zygote, or, as it is here termed,
the 'oospore', becomes surrounded by a thick membrane, passes through
a hibernating period, and then germinates, giving rise to ordinary swarmspores.
The terminology we have used, viz., ovum and spermatozoid, indicates that we
have here to do with a sexual act comparable with that met with in the higher
animals. The ovum is the female, the spermatozoid the male element, and it

j

Fig. 10"?. Ulothrix sonata. A, young filament
with rhizo'idal cell, r ( x 300). B, portion of a filament
with escaping swarmspores, two in each cell. C,
a swarmspore. D, formation of gametes and emptying
of a portion of the filament. E, gametes. T7, Gt con-
jugation of gametes. H. zygote. /, zygote after the
end of the resting period. A~, zygote whose contents
have divided into swarmspores. B-K, X 482. After
DODEL-PORT, (from the Bonn Textbook).

PERIODICITY IN DEVELOPMENT. II

355

is only after their fusion that further development can take place ; at the same
time, however, there is one point of difference between the two cases, viz. fusion
in the plant is followed by a resting period, entirely absent from the animal.
It is impossible to doubt, however, that the reproductive cells in Oedogonium
have arisen from ordinary swarmspores, and that the plant itself has been derived
from forms characterized by the possession of motile gametes which fuse in pairs
as is still the case in Ulothrix. These cells were originally alike, and a differen-
tiation has gradually appeared in them, to which we apply the term ' sexual'.

It was only natural that the discovery of sexuality in the lower plants
(PRINGSHEIM, 1855), comparable to the only form of reproduction in the higher
animals, should have created an immense sensation, and that from that time
onward an attempt was always made to determine, in the first instance, in every
account of reproduction whether the process was sexual or asexual (vegetative),
for it was the custom to refer the extremely varied forms of organisms to one or
other of these two categories. This view was, however, incorrect for two reasons.
In the first place, it was shown that sexual reproduction had arisen at several
points in the plant world and that possibly
it had not the same physiological sig-
nificance in all groups. Under vegetative
reproduction, also, were included a variety
of processes which could not well be
homologized. It is perfectly obvious that
the ' vegetative ' swarmspores of Oedo-
gonium have far more points in common
with the ovum and spermatozoid than
with a slip of a willow, and yet it is still the
custom to treat of such swarmspores side
by side with ' cuttings ' in a chapter on
4 vegetative propagation '. Unfortunately,
it is not possible at present to replace this
old-fashioned classification with one based
on the knowledge now in our possession,
although attempts have already been made
in this direction. Thus HANSTEIN (1877)
distinguished between reproduction by
' embryos ' and by ' buds ', and MOBIUS
(1897) has attempted to carry out this
subdivision more fully. Without giving
the reasons for the view we hold, it appears to us that this classification does
not meet the case, so that we must continue to use the old nomenclature in spite
of its defects. We are entitled to look forward to rapid progress in this
subject as a result of the renewed interest taken lately in problems connected
with the physiology of reproduction (compare KLEBS, 1900 a), although at the
same time we must not expect that the infinite variety of reproductive pro-
cesses in nature may ever be brought under one uniform system.

Let us inquire now as to the factors which render reproduction in general
possible, and those which are essential to the appearance of the different types
of reproduction more especially in these Algae. For a long time it was imagined
that these factors were essentially internal, and that in the lower plants, more
especially Algae, just as in the higher plants, asexual and sexual reproduction
alternated regularly. It was assumed that reproduction was a necessary result
of internal development when the plant had reached a certain size or age. It
is owing to KLEBS' s (1896 onwards) researches that our insight into this question
is now deeper and more exact. The chief result of KLEBS'S investigations is
briefly that under appropriate external conditions growth and division only take

A a 2

/, Oedogonium boscii. Fertilized
The ovum is already surrounded

Fig. 106.
oogonium.

by membrane ; above, a spermatozoid. After
KLEBAHN (Jahrb. f. wiss. Bot. 24, PI. 31. x
175. II, Oedogonium boscii. Young, just ooened
oogonium, in and in front of the opening, mucilage.
After KLEBAHN (ibid.), x 400. ///, Oedo-
gonium 'landsboroughi. Spermatozoid forma-
tion. In the lowermost cell the spermatozoids
are forming by bipartition; in the cell above it
they are escaping by a fissure in the wall. After
HIRN (1900).

356 METAMORPHOSIS

place, but also that reproductive organs are formed only under definite con-
ditions, and that the nature of these organs is not determined by internal
causes alone. Internal factors, here as everywhere, play, doubtless, an important
part, inasmuch as they bring about the state of activity of the plant which
enables it to react to the outer world in definite ways ; the environmental factor
acts merely as a releasing stimulus.

Let us now examine more in detail the phenomena of reproduction as
seen in Ulothrix. This alga grows in flowing water, in good light and at low
temperatures. For these reasons it is not well adapted for laboratory research,
and hence KLEBS'S attempts to elucidate its reproductive physiology were not
altogether successful. He was unable to induce the alga to form swarmspores or
gametes at will, but he managed to determine that external factors did play a part
in the question. Under favourable natural conditions the alga produces no re-
productive organs, it simply grows vegetatively, and its filaments become 20 to
30 cm. in length. The principal inducement to the formation of zoospores
appears to be the cessation of the flow of water and a diminution of oxygen
in it. Such a condition of things takes place periodically in regions inhabited
by Ulothrix, and hence for the most part growth and swarmspore formation
go on at the same time. The alga may exist in this condition for years ;
gametes are formed only under special circumstances, apparently when the
water level falls and when the cells come directly in contact with air, or are
only occasionally sprinkled with water. The zygote can withstand drought,
and hence it is intelligible that they should be formed under dry conditions.

KLEBS (1896) obtained more satisfactory results with species of Oedo-
gonium, which are more easily cultivated than Ulothrix. The two species
investigated behaved differently, but one can only say that both demanded
a certain alteration in the surroundings before swarmspore formation could be
guaranteed. In -the case of Oedogonium capillare darkening had a quite marked
effect, which was increased by cultivation in a 4 to 10 per cent, solution of cane
sugar. After being cultivated in ordinary water, in which it assimilates
vigorously, it produced swarmspores on being transferred to a dilute nutrient
salt solution. In Oe. diplandrum, on the other hand, swarmspores were formed
when it was transferred from flowing to still water, from a lower to a higher
temperature, from an inorganic nutrient solution to pure water. This is all
the more remarkable seeing that both forms grow in similar localities. It is
very desirable that these two plants should be more closely studied under natural
conditions, so that we may get to know under what circumstances the forma-
tion of swarmspores takes place in nature ; at present we are acquainted with
the artificial conditions only. Sexual organs may be produced in both species
if the water be limited in amount, and if it contains little in the way of nutrient
salts, and if the plants be strongly illuminated. Oedogonium diplandrum re-
quires a much greater intensity of light than Oe. capillare.

Sexual reproduction is by no means essential to the persistence of Oedogonium,
for KLEBS has shown that Oe. diplandrum grows luxuriantly in many situations
without ever forming sexual organs, and that only the male form of Oe. capillare
occurs near Basel, thus entirely precluding the formation of zygotes. The
biological distinction between zygotes and swarmspores is perfectly obvious.
By means of the latter the plant is able to reach new locations and by the former
to tide over periods when external conditions are unfavourable to vegetative
growth. Put shortly, swarmspores subserve distribution, zygotes, continuity
of the species. Why a fusion of two cells is necessary for the formation of a
resting spore and why these two cells should exhibit sexual differences is not so
apparent. On reviewing the reproductive processes in Algae and Fungi we
find that although very often sexual fusion is followed by the formation
of resting spores, still there are cases of resting spores produced asexually
(e. g. Bacteria), and, on the other hand, products of sexual union which develop

PERIODICITY IN DEVELOPMENT. II 357

directly without any resting period (e. g. Fucus). If follows that sexual fusion
is not the cause of the need for a resting period ; indeed it has been observed
that the gametes of Ulothrix under certain circumstances (KLEBS, 1896, 321)
may develop into resting spores without any fusion, and the same phenomenon
has been for a long time known to occur in many species of Oedogonium (com-
pare HIRN, 1900, p. 39). If gametes are able to develop into resting spores with-
out any fusion, then conjugation must have some special significance of its
own, into which we shall go later on.

The examples quoted are sufficient to render clear the historical lines of
investigation on this subject. Fungi are, however, much better adapted for
studies of this kind than Algae, because we are more thoroughly acquainted
with their conditions of life, and because these conditions are for the most part
not so restricted as in the case of Algae. An example of this has been already
given in Basidiobolus ranarum, but without referring further to this case we
may rather turn to the general conclusions which KLEBS (1900 a) has arrived at
from his researches on Fungi. In that group, far better than in Algae, we
see that reproduction is not an essential result of vigorous growth, as one would
be inclined to assume from the regularity with which reproduction follows such
growth. As a matter of fact, growth may proceed to an unlimited extent with-
out reproduction, provided the necessary external conditions are fulfilled. In
addition to favourable general conditions the most important is continuously
good nutrition, e. g. in cultures, a frequent renewal of the nutrient solution before
its nutritive value has appreciably diminished or injurious metabolic products
have accumulated. If, from time to time, a fragment of the mycelium of
Saprolegnia mixta be taken from a good culture of the fungus and transferred
to a fresh nutritive solution, it may be kept growing vegetatively for two and
a half years without reproducing. * This phenomenon is illustrated far more
effectively by the case of yeast, which has, for hundreds of years, persisted in
the vegetative condition, and which can be induced to form spores only under
very special conditions.' Similar experiments have been carried out on a large
number of lower organisms, Myxomycetes, Bacteria, and Fungi, with the same
results ; it is more important to note that at least some higher types may be
similarly treated, e. g. Coprinus ephemerus, which may be cultivated for months
as a sterile mycelium without the formation of any pileus. Experiments cannot
be carried out so successfully on Algae because it is often the case that the most
insignificant alterations in the conditions induce spore formation ; it may be
noted, however, that Vaucheria geminata will remain sterile in flowing water for
as long a period as may be desired. The factor which most commonly induces
the appearance of reproductive organs is an alteration in nutrition, especially
a reduction in the absorption of food. In the case of certain Fungi which form
fruits only in the air, transpiration is also a determining factor ; a saturated
atmosphere interrupts the formation of spores, but a reduction in the amount
of moisture induces the formation of spores in abundance. Light also plays a
part in a few cases, as in certain species of Coprinus and in Pilobolus microsporus.
It is only when light is permitted to play on the terminations of the conidio-
phores of Pilobolus that conidia are formed. Illumination for a few minutes is
often sufficient ; if it be not illuminated the conidiophores grow on just like
ordinary vegetative cells, so long as nutritive materials can reach them.

The withdrawal of nourishment acts in the first instance as a stimulus to
the formation of reproductive organs, while, at the same time, it retards vegeta-
tive growth. A contrast would thus appear to exist between reproduction and
growth pure and simple, but it must not be assumed that reproduction begins
when nutritive conditions are unfavourable and growth only when they are
favourable. On the contrary, it appears that reproduction is all the more
vigorous the better the vegetative parts are nourished. It would thus appear
that growth is an essential precedent of reproduction. It is so, however, only in

358 METAMORPHOSIS

so far as growth is associated with vigorous assimilation, for it is obvious that
the construction of reproductive organs necessitates the previous accumulation
of a certain amount of nutritive materials, and these must be all the more
abundant the more complex the reproductive organs are. Thus simple repro-
ductive bodies may arise from other organs of propagation without growth,
provided these have a sufficient supply of constructive materials. The best
known example of this is seen in the swarmspores of Oedogonium which after
coming to rest may become swarmspores once more. KLEBS hasdrawn attention
to similar phenomena in many Fungi.

A more important difference between growth and reproduction lies in this,
that general vital conditions are not so restricted in relation to the former as the
latter. For example, growth may still go on at a very low or a very high
temperature, while formation of reproductive organs ceases. Oxygen, light,
concentration of the nutritive solution, the quality of the nutrient, all have
minima and maxima which are much closer together in the case of reproduc-
tion than in the case of growth. It must not be assumed, however, that the
optimum for the one process is also that for the other.

Observations similar to those which have been made on Algae have also
been carried out on Fungi, both as regards the occurrence of the reproductive
organs in general and of the various types of these in particular. Fungi far
excel Algae in the variety of their methods of spore formation. Interesting
as are the results achieved by KLEBS and his pupils on this subject we must not
attempt to describe them here, for their complex nature renders it impossible
to summarize them in a few sentences, and space will not permit of more.

If we turn now to the higher plants, at first sight we appear to meet with
an essentially different state of affairs. An oak, for example, has, like a higher
animal, only one type of reproduction, i. e. the formation of an embryo con-
tained in a seed. This embryo is the product of a sexual act, which takes place
when the plant has reached a certain age, and is seemingly induced by internal
causes. Closer study, however, reveals something entirely different. In order to
understand the phenomena we must study reproduction in the fern, for a
knowledge of the process in the latter is essential to a true conception of what
follows. The fern has the same complicated structure as the flowering plant,
consisting as it does of a root-system and a leafy shoot. On the under sides
of the leaves arise, in definite ways characteristic of the group, asexual spores
enclosed in special receptacles, the sporangia. The spores generally pass through
a resting period and germinate under favourable conditions. The plant which
arises from the spore resembles, however, a liverwort rather than a fern. It con-
sists of a cellular expansion a few millimetres in diameter, increasing by means
of an apical cell, and attached to the soil by rhizoids. This second generation
derived from the spore is termed the prothallus, and is incapable, as a rule, of
developing by simple growth into the first stage. When its reproductive period
comes on it develops sexual organs. Archegonia, corresponding to the oogonia
of Oedogonium, appear (Fig. 108), and each of these contains, as its essential
constituent, an ovum. The ovum is fertilized by a motile sperm, produced in
a special receptacle, an antheridium (Fig. 107), and from the product of the
fusion of these two cells there arises a new fern plant, which is at first dependent
on the prothallus, but later on, after the death of the prothallus, is capable of
independent existence. In the natural course of events the two generations
in the fern follow each other regularly, the asexual generation or sporophyte
being succeeded by the sexual gametophyte, and that in turn by the sporo-
phyte ; in other words, we have here a case of alternation of generations. Any
attempts to induce a spore to form a sporophyte or an oosperm to form a
gametophyte by external agencies have been as yet entirely unsuccessful. In
this respect the fern shows a marked difference from the Algae and Fungi.

This distinction cannot, however, be fundamental, for we know of several

PERIODICITY IN DEVELOPMENT. II

359

Fig. 107. Polypodium vulgare. A,
mature, ff, empty antheridium; £, cell
of the prothallus ; / and 2 parietal cells ;
3, cover -cells (x 240). C, a motile
sperm. Z>, the same fixed by iodine
solution ( x 540). From the Bonn Text-
book.

abnormal phenomena which occur in many ferns under conditions which have
not as yet been fully studied, viz. the phenomena termed apospory and apogamy
(for literature see GOEBEL, 1898-1901, p. 430). By apogamy is meant the
development of the sporophyte, or part of it, the sporangium, as a vegetative
outgrowth of the prothallus, without any intervention of the fertilized ovum.
Again when prothalli arise not from spores but
from sterile sporangial - cells, or from general
vegetative- cells of the sporophyte, we speak of
the phenomenon as apospory. Apogamy and
apospory show that the characters of the sporo-
phyte, as also of the gametophyte, lie latent in
every cell of the fern, and that the changes
leading to the formation of the other generation
do not occur first in the egg-cell or in the spore.
Apospory and apogamy lead us to hope that it
may yet be possible to discover the more im-
mediate conditions that determine the rhythm of
alternation so that we may then be able pos-
sibly to alter it at will.

Another variation from the regular alterna-
tion of generation occurs much more frequently
than apospory and apogamy. Many ferns have
in addition to spores and sexual cells other repro-
ductive organs which may be termed accessory.
These may occur on the sporophyte or on the
prothallus, but in neither case do they form stages in the alternation ; one
variety renews the sporophyte, the other the gametophyte (BowER, 1887).
The regularity in the alternation of generations may in this way be destroyed
and each generation may go on repeating itself directly an indefinite number
of times. Doubtless careful investigation may show that external factors
often determine whether the typical or the accessory reproductive organs
shall be produced in any special case.

If we now glance at the life-cycle of the other Pteridophyta and compare

them with that of the

ferns in the narrower sense

of the term, we find that

we are able to identify in

all of them the same two

generations, but not al-
ways with equal clearness.

The gametophyte in very .^£W^S£X_-^ Y^fflg^A JL*

many forms reaches only

a very limited size, and is

finally entirely enclosed

within the spore. The

wall of the spore opens at

one end, in the one case

to allow the sperms to

escape, in the other to

afford a means of entry

for the sperm to the archegonium and to the ovum. Concomitantly with

these reductions in size of the prothallus, further modifications occur in its

historical development. While both types of sexual organs are or may be borne

on the prothalli of ferns, in other types the prothalli are unisexual, and it depends

on the character of the spore and the sporangium whether the prothallus

Fig. 108. Polypodium vulgare. A, immature archegonium; K\
neck canal-cells ; A"", ventral canal-cells ; 0, ovum. B, mature and open
archegonium (x 240). From the Bonn Textbook.

360 METAMORPHOSIS

will afterwards produce male or female reproductive organs ; the sporophyte
produces megaspores in megasporangia and microspores in microsporangia,
the former female, the latter male.

This stage in the historical development of the Pteridophyta brings us
nearer to the condition found in the Phanerogams. As a result of HOFMEISTER' s
epoch-making investigations it is possible to compare the Gymnosperms in all
essential points with the Pteridophyta. We must, however, refer to the text-
books on botany or to the expositions of the subject given by GOEBEL in his
Organography of Plants for details of the comparison, since the plan of the
present course of lectures does not include a treatment of morphological relation-
ships such as these. We shall confine ourselves to a discussion of the Angio-
sperms, whose likeness to the Pteridophyta is not so apparent. Angiosperms
also develop sporangia on leaves which, like those of Pteridophyta, undergo
metamorphosis. These metamorphosed leaves and the sporangia which they

Eroduce are collectively spoken of as the flower. The metamorphosis of floral-
javes has been already referred to at p. 349, and it would appear that phylo-
genetically this metamorphosis has been intimately associated with the forma-
tion of sporangia, but whether the same is true ontogenetically in each
individual case is more than doubtful. Floral-leaves, at all events, have not
lost their power of becoming foliage-leaves, and after the application of appro-
priate stimuli flowers may be induced to take on a green colour.

This question, however, need not be considered here, for it is the sporangium
and not the leaf from which it arises that is more especially before us. We
may distinguish microsporangia (pollen-sacs) developed
on staminal leaves and containing microspores (pollen-
grains), and megasporangia (ovules) enclosed within
carpels, each containing, for the most part, one mega-
spore or embryo-sac. Very frequently we find micro-
and mega-sporangia united in the same flower, and their
mode of occurrence is determined exclusively by internal

Fig. 109. Tradescantta , , . .. J J

virgintca. Grain of poiien. factors, acting in such a way that the microsporangia are
«Bhf ve±,1"hceelLia'xaI^5 developed first, the megasporangia later.
From the Bonn Textbook. The case where the flowers are unisexual, and where

the sexes are distributed on different plants, requires

further investigation as to the factors which bring about this determination of
sex, but no definite conclusions on the matter have as yet been reached (SxRAS-

BURGER, 1900).

Let us now trace the further development of the spores. The microspore,
without any further growth, becomes transformed into a prothallus, dividing
into two cells of unequal size (Fig. 109). The small cell becomes the
antheridium, dividing later into two male cells, the large cell remains sterile and
has a special function to perform. This function becomes evident so soon as
the microspore reaches the stigma through the agency of wind currents or insects.
So soon as it has become securely fixed in that situation it proceeds to develop
a long hypha-like pollen-tube, which (Fig. no) forces its way deep into the
megasporangium, breaks through the wall of the megaspore, and sheds into
it the two sperms. The earlier changes which take place in the megaspore are
very remarkable. The megaspore remains imbedded in the tissue of the spo-
rangium, hence the necessity for the formation of a pollen-tube. Its nucleus
undergoes three successive divisions, so that finally eight nuclei are present in
the megaspore, and these are arranged in a definite manner (Fig. no). Three
of them are situated at the end of the megaspore where the pollen-tube enters,
forming there what is termed the egg-apparatus (ie, Fig. no; compare
Fig. in), each becoming surrounded by a layer of protoplasm obtained from the
mother-cell. The egg-apparatus thus consists of three naked cells. Similarly,
at the other end of the megaspore, three other cells (an, Fig. no) are formed, the

PERIODICITY IN DEVELOPMENT. II

antipodal cells, which, however, appear to have no special function to perform.
Finally, there remain over two other nuclei — the polar nuclei — which move
towards the centre of the megaspore, fusing sooner or later in that situation,
the product being known as the secondary nucleus of the embryo-sac. Which
of these nuclei or cells are to be considered as belonging to the prothallus it is
impossible to say ; this much, however, is certain, that the three cells of the
egg-apparatus, or, at all events, the one distinguishable by its size as the ovum,
represents the archegonium reduced to its simplest condition.

After entry one of the sperm nuclei fuses with the ovum, and the product
develops into the embryo. The two cells adjacent to the ovum disappear. The
other nucleus fuses with the embryo-sac-nucleus (see p. 370), and from the pro-

A

-f*

Fig. no. Pistil of Polygonum convolvulus, fu, the funicle
of the ovule, supporting the body of the ovule with its integuments,
ie and «, and containing the nucellus, ««. The latter encloses the
embryo-sac, £, in which is seen the egg-apparatus, «*, the embryo-
sac nucleus, ek, the antipodal cells, an. The ovule is enclosed
within the ovary,./z£>, which is continued into the style, g, and ends
in the stigma, n. On the stigma are seen pollen-grains, j>, one of
which is giving rise to a pollen-tube, PS. x 48. From the Bonn
Textbook.

Fig. in. Funkia ovata. Upper end
of the embryo-sac, showing the egg-
apparatus. A< before fertilization ; z?,
during fertilization. 0, ovum; j, syner-
gidae ; /, pollen-tube ; », nucellus. X 390.
From the Bonn Textbook.

duct of that fusion and from the protoplasm of the embryo-sac there arises
a cellular tissue — the endosperm — which becomes filled with reserve food
materials, and sooner or later is used up by the developing embryo. The whole
megasporangium now increases greatly in size and forms the seed, which when
mature enters on a period of rest.

Even in the very highest plants it is thus possible to demonstrate an
alternation of generations. The marked reduction in the gametophyte of the
fern is carried in this case so far that practically nothing is left of that stage
except the reproductive organs themselves, and the alternation becomes in
consequence completely hidden. It is obvious from a consideration of these
facts that there will be far less opportunity for variations in the regularity of
the succession of generations in the higher plants than in the ferns. Since the

362 METAMORPHOSIS

development of sexual organs appears to be a necessary consequent of the forma-
tion of spores physiological investigation must be limited to the interpretation
of the conditions of spore- and flower-production.

As in certain ferns, so also in many Angiosperms, we meet with accessory
organs of reproduction, at least in the sporophyte stage, and when we speak of
these as asexual or vegetative reproductive methods we do not mean the mega-
and micro-spores (although these also are asexual in character), but those special
accessory organs which invariably consist of a growing point or bud, giving
rise to leaves and roots after these or the organs bearing them have separated
from the parent. Great variations occur in the point of origin of these buds,
and in the way in which the necessary reserves are stored in leaf, stem, and
root ; these, however, we need not discuss here.

After this superficial sketch of the morphology of the reproductive organs
of the flowering plant we may pass to the problems requiring consideration from
a physiological standpoint, viz. what determines flower formation, what vege-
tative reproduction ; what relations exist between reproduction and vegetative
growth ; what is the special significance of reproduction, and more especially
of asexual and sexual reproduction.

We are not nearly so well acquainted with these subjects in connexion
with higher plants as in the case of the lower, and the experimental investigation
of the former is much more difficult. The first question we have to answer here
is whether or not unlimited growth is possible without reproduction. At the first
glance the answer would appear to be in the negative, but further consideration
shows that that answer is incorrect. A tree may be several hundred years old
and still go on growing ; in the long run it dies, but we cannot say that death
was due to internal factors and that perpetual vegetative growth was impos-
sible. Death may be due rather to external factors, e. g. the ever-increasing
difficulty of conducting water to the lofty growing points. KLEBS (1903)
attempts to account for the death of trees by assuming that certain decompo-
sition products migrate from the dead parts and gradually infect the living
organs. Our suggestion above might be made more general by saying
that when a certain size is exceeded nutritive disturbances (not merely inter-
ruptions in the water flow) generally ensue, which in the long run bring about
death. In many of the lower organisms also, e. g. in Hydrodictyon, con-
tinuous vegetative growth appears to be impossible. When the cells of this
alga come to exceed a certain size its central cells are badly supplied with food-
material and must in the long run die off unless the size of the body be reduced
as a result of reproduction. Internal factors are also known to be effective,
but they do not affect the parts of the tree which are of special interest to us.
The protoplasm of the growing points in an old tree has just as much power of
development as that of one a single year old. In proof of this view we point
to the numberless cultivated plants which are propagated by cuttings, and
which have been so propagated for hundreds of years. This is true, for example,
of the willow, poplar, sugar-cane, and many others ; at the same time, some of
these plants in their later years are often subject to diseases which have been
considered to be a consequence of degeneration due to perpetual vegetative
propagation. It has, indeed, been maintained that the life of cuttings is limited
by that of the mother-plant from which they are taken. MOBIUS (1897) has,
as it appears to us, demonstrated beyond criticism that this view is incorrect,
and we may refer directly to the evidence he has advanced. It will be as well,
however, if we cite one or two examples in illustration of the position we hold,
viz. that vegetative growth may be unlimited. At the commencement of the
present lecture we referred to the case of rhizomes which grew on from year to
year, in the main axis or in lateral shoots. Since they are always forming new
roots acropetally all difficulties in absorption of water are removed. The
older parts, however, die off, and hence it is not possible in nature to deter-

PERIODICITY IN DEVELOPMENT. II

363

mine whether such a plant has been in existence for years or for centuries.
Many of these rhizomic plants have lost their power of forming seed, and their
continued existence depends entirely on the growth of the rhizome. Acorus
calamus is a case in point. This plant has been known as a native of Europe
for at least 400 years, and there can be no doubt that during the whole of that
time it maintained itself in existence and increased by vegetative methods only.
[KLEBS has given some admirable examples (1903-4) of continued vegetative
growth experimentally induced. He was successful in causing Glechoma hede-
racea to remain growing in the form of creeping shoots for years.]

The examples quoted make it clear that the formation of flowers and
reproduction by means of sporangia is not generally necessary for the tribal
continuance of the plant, and that the plant, whether it be a flowering plant or a
Saprolegnia, can continue
in existence by vegeta-
tive means only. Inquiry
into the factors which
bring about flower forma-
tion becomes thus all the
more necessary.

When we examine
flowering plants under
natural conditions we find
that the flowers begin to
appear when the plant
is ' ripe for flowering '
just as sexual -cells ap-
pear in the animal when
it reaches a certain age.
But although flower for-
mation, generally speak-
ing, takes place at a cer-
tain age, which differs
with each species, still,
exceptions are known, as,
for example, the oak,
which normally is ' ripe '
in its sixtieth to eightieth
year, but which occasion-
ally produces flowers in
its first year and then
dies down (WiESNER,

1902, p. 75. Compare Fig. 93). VOCHTING (1893) has demonstrated in a very
striking manner that flower formation is influenced by external factors. The
plant he experimented upon was Mimulus tilingii, which left off forming
flowers in light of low intensity but still sufficiently strong to permit of vigorous
vegetative growth. The effect of the diminution in light makes itself felt even
when the inflorescence has been formed ; it arrests the further development of
the flower already laid down in the axils of the bracts (Fig. 112), and induces
a general evolution of resting axillary buds into vegetative shoots.

This illustration of the relationship between foliage and flower formation
recalls vividly that between growth and reproduction in Fungi. We have
established in relation to these forms that the limits of the general vital condi-
tions for reproduction are more restricted, and VOCHTING has shown in Mimulus
that the minimum of illumination for flower formation lies higher than for that
of vegetative organs. We are, however, as yet far from having reached a solu-

Fig. 112. Mimulus tilingii, after VOCHTING (1893, PI. n). /, TOJ
normal plant. //, Top of a plant cultivated in diminished light
development of inflorescence. The inflorescences (a, a) are stu
vegetative shoots have developed everywhere abundantly.

/, Top of a
" jht after
stunted ;

364 METAMORPHOSIS

tion of the problem, since the increase in the intensity of light obviously does
not play the same part in VOCHTING'S experiments as does the limiting of
the food supply in Saprolegnia. A certain light intensity is only one of the
conditions of flower development and not the chief releasing stimulus. Under
such a light intensity of course vegetative developments go on unchecked. There
are many observations and experiments which show that other external factors
may be of great importance in the determination of flower formation. Ivy
blooms only in a warm sunny aspect, and exhibits vegetative growth only in
woodland shade (WiESNER, 1902, p. 75). The quality of the light also has an
important effect on flower formation. According to SACHS (1887), ultra-violet
light must be especially considered in this relation (p. 311). In his experiments,
carried out under conditions when ultra-violet light was excluded, flower-buds
were formed, but they did not open ; apparently the intensity of the light was
too low. SACHS'S statements may be explained by VOCHTING'S experiments and
observations (1893) and by those of KLEBS (1900 b) and MONTEMARTINI (1903).
MOBIUS found (1897) certain Gramineae and species of Borago flowered better
on dry soil in places where nutrients were restricted in quantity than where there
were abundant supplies of water and nutritive salts. Perhaps with this is
related the fact that root-pruning of trees tends to induce an increase in flower
formation ; that ringing has also the same effect is probably due in the first
instance to the abundant supply of organic material to the axillary buds.
[KLEBS (1903, and especially 1904, 545) has established the external conditions
necessary for flower formation in a number of plants. No general laws on the
subject have as yet, however, been formulated ; on the contrary, every plant
seems to maintain relationships of its own with the environment.]

Although external factors are known to be of great importance in flower
formation, it is not to be expected that they play as vital a part there as in the
lower plants. Even though proof be advanced that flower formation stands
in definite relation to a definite external factor, that is not to say that that rela-
tion is so simple and so direct as in the case of a unicellular alga. If it be proved,
for instance, that a dry soil increases flowering, and that a wet one retards it, we
can only say that the character of the soil directly influences the roots and only
indirectly the aerial organs. The root may be looked upon as a part of the
environment of the shoot, just as every cell is a part of the environment of
another cell, but that does not affect the relations existing between different
parts of the higher plant, and calculated to affect the formation of flowers also.
In fact, the correlations between leaf and flower formation must always be kept
prominently in mind. All factors which tend to advance foliage development
are unfavourable to flower production and vice versa. In extreme cases flowers
are completely suppressed, and certain plants exhibit vegetative growth only ;
in certain aquatics GOEBEL (1893) showed that the luxuriant formation of
vegetative organs was the cause of the absence of flowers, and MOBius (1897,
p. 137) has brought forward evidence of a similar nature. We need not enter
more fully into these correlations since we cannot bring forward anything sub-
stantially new in their explanation. It is always important, however, to re-
member that in flower formation not merely external but also internal stimuli may
play an important part, and some of SACHS'S (1892) observations on this question
are very suggestive. In May SACHS made cuttings of Begonia in the usual way,
and found that the plants springing from such leaf -cuttings gave rise to flowers in
the beginning of November, preceded by a luxuriant formation of foliage-leaves.
If, however, the leaf-cuttings are taken from a flowering specimen in the end of
July, flowers appear on them in the end of September, but few leaves are
previously formed. Several other experiments of the same nature are recorded.
Thus GOEBEL (1901) found that leaves of Achimenes haageana in the flowering
condition, used as cuttings, proceeded to form flowering shoots at once, while

PERIODICITY IN DEVELOPMENT. II 365

cuttings from younger plants produced vegetative shoots. Such facts as these
SACHS attempted to explain by his hypothesis of specific constructive materials
(p. 349). He assumed that special flower-forming materials occurred in the
plant about to flower, and hence the rapid formation of flowers on cuttings
taken from it. Quite apart from the general criticisms we have brought
against SACHS'S hypothesis, there are by no means unimportant objections to
this special application of it. WINKLER (1903) found the shoots which arose
from the leaf cuttings of Torenia very liable to form flowers, and this tendency
was held in common by all leaves, and even by cotyledons. On the assumption
that the materials for the manufacture of flowers postulated by SACHS must
be especially abundant in the flowers themselves I prepared flower-cuttings of
many species of Achimenes ; though these produced buds in quantities none
of them formed flowers. The conditions necessary for the production of
flowers from leaf-cuttings must obviously be studied on broader lines ; still
these facts may be adduced as interesting evidence that internal factors play
a very important part in the formation of flowers.

In general, however, our information as to the factors which are essential to
the formation of flowers is very imperfect ; the facts which have been brought
forward prove, however, that the periodicity in blooming usually observed is not
fixed once and for all but may be experimentally influenced. On the other hand,
the alternation of generations in the Phanerogams is in so far fixed that the
formation of spores is followed necessarily by the formation of sexual organs.

Plants which, in addition to seeds, also form accessory organs of reproduc-
tion, such as tubers, bulbs, &c., require special mention, since it can hardly be
doubted that the conditions which are necessary for the formation of flowers
or seeds must differ from those which govern the formation of propagative
buds. On this point, however, little is known; so far only correlations
between them have been established. For example, it not infrequently
happens that flowers and propagative-buds are produced concurrently, but that
the setting of seed takes place only if the formation of propagative-buds is
prevented (compare LINDEMUTH, 1896). The cases where propagative-buds
arise in place of flowers are of especial interest, e.g. Poa bulbosa and many others,
for in these cases we do know that special but as yet unknown external factors
accelerate or retard the formation of such flowers or buds (HUNGER, 1887).
[Looking at the general results we have arrived at in this lecture we must admit
that the normal path of development, both in higher and in lower plants, is only a
special type of many developmental possibilities. That this normal sequence and
not some other is usually met with must be attributed to a normal sequence
in the external factors. It is to KLEBS especially that we owe this valuable
conception with regard to the factors governing developmental processes.]

A few words may be said in conclusion as to the biological significance of
reproduction. In the lower organisms this is especially obvious, for the impor-
tance to the organism of swarmspores and of conidia and resting spores capable
of distributing the plant in the air requires no explanation. The value of the
formation of fruit and seed in the higher plants is equally intelligible. It is
immaterial whether the organism forms these organs once and then dies, or
whether it continues forming them for several years, their production tends
not only to the persistence but also to the increase of the species. Every
organism does its best to distribute itself and to seek pastures new ; this
characteristic is so marked in living things that one gets the impression that
their entire existence is spent in producing their successors. We use the
term ' impression ' purposely, because attempts to answer such questions tend
to teleology ; they lead us at once to the ultimate problem of all, viz. what is
the final aim and object of the existence of the organism ? and to that question
no answer can be given.

Although we may understand in general the biological reasons for the

366 METAMORPHOSIS

occurrence of reproductive organs we must admit that we have as yet arrived
at no clear appreciation of the factors which determine the existence of several
types of these organs. There are many organisms which persist and multiply
exclusively by one mode of propagation. As above observed, MOBIUS recognizes
buds and germs, and distinguishes them by their mode of origin. Germs arise
by rejuvenescence, buds by growth and ordinary cell division. Rejuvenescence
is illustrated in the formation of the ovum, as also in the development of
swarmspores. It is not quite so obvious, however, why the origin of a new mem-
brane— a phenomenon which did not occur in Oedogonium — should be of so
much importance to the organism, since the old membrane was just as capable
of growth as the new one in this case. But in describing the phenomenon as
' rejuvenescence ' we are regarding not so much the cell-wall as the internal
rejuvenescence of the protoplasm. STRASBURGER (1880) has shown that the
protoplasm undergoes a reconstruction when swarmspores are formed in Oedo-
gonium ; the nucleus moves through the green protoplasm towards the outside,
and later on resumes its old position. BERTHOLD (1886, p. 289) has observed
similar rearrangements in other cases of cell-formation. It is not clear, how-
ever, how such processes bring about a rejuvenescence, and one idea underlies
all conceptions of this nature, viz. that protoplasm degenerates, gets worn out
by perpetual growth and cell division, and this conception is one which we have
striven to oppose. If protoplasm wears itself out, it has also no doubt the
power of regenerating itself.

There is one point which we have left undiscussed in our consideration of
these questions, viz. why must the formation of many resting spores in the Thal-
lophyta be preceded by a fusion of cells, a phenomenon which may in the ex-
treme case be regarded as a sexual act ? Why should the ovum in the Pterido-
phyta and the Phanerogams develop only after being fertilized by the spermato-
zoid ? We shall consider this question in the next lecture.

Bibliography to Lecture XXVIII.

BENECKE. 1898. Jahrb. f. wiss. Bot. 32, 453.

BERTHOLD. 1896. Protoplasmamechanik. Leipzig.

[BOWER. 1887. Trans. Linn. Soc.]

GOEBEL. 1893. Biolog. Schild. 2, 217. Marburg.

GOEBEL. 1898-1901. Organographie. Jena.

HANSTEIN. 1877. Botan. Abhandl. 3, part 3.

HIRN. 1900. Monographic d. Oedogoniaceen. Acta Soc. Sc. Fennicae, 27. Hel-

singfors.
HUNGER. 1887. Ueber einige vivipare Pflanzen u. d. Erscheinung d. Apogamie.

Diss., Rostock.

KLEBS. 1896. Ueber d. Fortpflanzungsphysiologie d. nied. Organismen. Jena.
KLEBS. 1898. Jahrb. f, wiss. Bot. 32, i.
KLEBS. 1899. Ibid. 33, 513.
KLEBS. 1900 a. Ibid. 35, 80.
KLEBS. 1900 b. Ber. d. bot. Gesell. 18, 201.
KLEBS. 1903. Willkiirliche Entwicklungsanderungen. Jena.
[KLEBS. 1904. Biol. Centrbl. 24, 257-614. Comp. also 1905, Jahrb. f. wiss. Bot.

42, 155-1

LINDEMUTH. 1896. Ber. d. bot. Gesell. 14, 244.

MOBIUS. 1897. Beitr. z. Lehre v. d. Fortpflanzung d. Gewachse. Jena.
MONTEMARTINI. 1903. Atti Istit. Bot. Pavia. N. S. 9, I.
PRINGSHEIM. 1855. Monatsber. d. Akad. Berlin.
SACHS. 1887. Arb. bot. Instit. Wiirzburg, 3, 372.
SACHS. 1892. Flora, 75, i.

STRASBURGER. 1880. Zellbildung u. Zellteilung. 3rd ed., p. 81.
STRASBURGER. 1900. Biolog. Centrbl. 20, 657.
VOCHTING. 1893. Jahrb. f. wiss. Bot. 25, 149.
WIESNER. 1903. Biologic d. Pflanzen. 2nd ed. Vienna.
WINKLER. 1903. Ber. d. bot. Gesell. 21, 96.

36?

LECTURE XXIX
FERTILIZATION, PARTHENOGENESIS, HYBRIDITY, HEREDITY

[In dealing with these phenomena we enter on a subject which, in spite of its great interest
to physiologists, has been investigated almost solely from the morphological point of view.
This renders its discussion very difficult as well as not infrequently onesided and subjective,
since the main problems still await decision by experiment. In the additional notes in the
present English edition only a selection of the recent literature can be dealt with.]

IN plants, as in animals, there is, as we have already seen, a characteristic
method of reproduction by the fusion of two previously separate cells, and in ex-
treme cases the fusing cells, like the organs which produce them, are so widely
different from each other that we may then speak of sexual differentiation and
of the ' fertilization ' of a female by a male cell, in harmony with the conditions
existing in the higher animals. In endeavouring to arrive at the meaning of
fertilization we may consider the process as it occurs in definite cases among
Archegoniatae and Angiospermae without inquiring whether fertilization has
the same value in all cases. Since, doubtless, the sexual process has arisen not
once only, but several times in the history of organisms, we must look on it as
possible that in different places it may have a different significance, e.g. in
Diatomaceae and Phanerogams (compare KLEBS, 1899).

In certain Algae the principal difference between egg-cell and sperm lies
in the much greater size of the former ; but both the gametes are normal cells,
each has protoplasm and a nucleus, and each may even possess chromatophores.
But in Phanerogams and ferns nearly all the protoplasm of the male cell dis-
appears and the cell consists finally of little more than a nucleus. The
protoplasm is never entirely absent, but, as far as we know, in the higher plants,
the chromatophores are always wanting. We may therefore assume that the
nucleus of the sperm is a most important organ, and is responsible for the final
results of fertilization.

During the process of fertilization the protoplasts of the two cells unite and
the two nuclei also undergo fusion, and thereafter the fertilized egg begins to
grow. Without fusion neither male nor female cell is capable of growth.
Thus the first result of fertilization is the removal of a developmental inhibition,
and we may conclude that the egg was in want of something which is supplied
by the sperm, and that its nucleus is destitute of certain substances which the male
nucleus possesses.

It is probable that further research may show a qualitative difference to
exist between the two nuclei, although at present such differences have not been
determined. On the contrary research has hitherto shown a similarity in the
materials composing ovum and sperm nuclei, and amongst these nuclein has
been regarded as most worthy of attention. Although ZACHARIAS (1901) in
general found no qualitative difference in the nuclein present he found a con-
siderable quantitative difference — the ovum-nucleus being much poorer in
nuclein than the sperm-nucleus. In certain cases, for instance Marchantia, this
difference becomes qualitative for no nuclein at all can be detected in the egg-
nucleus just previous to fertilization. On this has been founded the hypothesis
that the ripe ovum is incapable of development because it does not contain sufficient
nuclein9 a deficiency which is supplied on fertilization ; but that the sperm cannot
develop by itself because it is deficient in protoplasm.

Views entirely different from that just advanced are held, two of which may
be briefly referred to here ; one is based on the phenomena exhibited by the
nucleus of the cell preceding the formation of the sperm and ripening of the
ovum — phenomena resembling those seen in animals. It has often been re-
marked that the formation of male and female cells begins with a nuclear
division differing widely from the typical mode, and described as reduction-

368 METAMORPHOSIS

division. In the normal vegetative division the chromosomes, which occur in
a definite and, for each species, perfectly constant number, split longitudinally,
so that each daughter-cell contains the same number as the parent-cell. In the
division which precedes the formation of the sexual-cells a similar number of
chromosomes appears, but since they do not split longitudinally, both ovum and
sperm contain only half the usual number and only by fertilization is the full
complement again attained [compare Fig. 87 in the 7th ed. of the Bonn Text-
book]. A phenomenon similar to this reduction is also found in Phanerogams.
The nucleus in the embryo-sac and that in the pollen-grain show only half as
many chromosomes as do the vegetative cells. The reduced number is not
brought about, however, by suppression of the longitudinal division but by
the nuclein thread dividing into only half the regular number of chromosomes.
[Since these words were written a very comprehensive summary of the cyto-
logical literature has been published, showing that a reduction- division occurs
before the formation of sexual-cells both in plants and in animals (compare
KORNICKE, 1904 ; STRASBURGER, 1904 a, 1905, &c.]

Among zoologists the arrest of the developmental process has been
attributed to this reduction, and many botanists, e.g. JUEL (1900 b), hold the
same view as to the reduced number of chromosomes. Apart altogether from the
fact that the reduction in the number of chromosomes is brought about in quite
different ways there are other considerations which militate against this view.
In speaking of the genetic relationships between Pteridophyta and Phane-
rogams in the foregoing lecture we saw that the whole of one generation of the
fern — namely, the prothallial generation, on which the sexual organs are borne —
is reduced to a few cell divisions in the spores of the Phanerogams. In ferns
the prothallial generation possesses the half number of chromosomes and yet
it is capable of vigorous development. Chromosome reduction in the Phane-
rogams is manifestly an ancestral character handed down from earlier types —
perhaps fern-like in character — and has no direct relation to fertilization, as
appears to be the case in animals. Also in the case of apogamy in ferns, in
which the sporophyte arises on the prothallium without the intervention of an
ovum it seems that the increase in number of the chromosomes can come about
otherwise than by fertilization, for it can hardly be doubted that apogamous
shoots have the same number of chromosomes as the normal one. In apogamous
ferns, according to FARMER, MOORE, and DIGBY (Proc. Royal Soc., 1903, 71,
453), a fusion between the nuclei of two vegetative cells takes place, leading to
a doubling of the chromosomes, and the nucleus resulting forms the starting-point
for the apogamous growth. We ought, perhaps, to receive this statement with
caution, and may raise the question whether the authors determined that such
a fusion takes place only in apogamous prothalli, and also that such a fusion
actually does serve as the starting-point for the new growth. Can the passage
of a nucleus from the one cell to the other have been the result of the mode of
preparation as in MIEHE'S observations quoted on p. 170 ? [FARMER has as
yet given no further explanation, but DIGBY (1905) has published some
additional preliminary notes to which reference may be made. NEMEC (1904)
also appears to incline towards the interpretation we have given above of the
significance of the transference of the nucleus.]

For other consequences of the chromosome hypothesis we must refer to
P- 377 9 and turn now to the consideration of another theory of fertilization
which finds its chief advocate in BOVERI (1902). This hypothesis has grown out
of investigations carried out upon animals. The egg is supposed to be in-
capable of growth because it possesses no ' centrosome ', and the influence of
the sperm depends not on its bringing a nucleus but a centrosome to the ovum.
Hitherto we have not spoken of this organ of the cell, simply because it is entirely
wanting in the highest plants. The animal cell, on the other hand, very
generally contains small granules surrounded by protoplasm exhibiting radiating

PARTHENOGENESIS 369

striae. These granules are capable of division, and one is found temporarily
at each pole of the nuclear spindle. The idea is now very generally accepted that
these centrosomes play an important part in nuclear division, since they deter-
mine the arrangement of the nuclear spindle and fix the dynamic centres, at least
certain observations of BOVERI may be interpreted in this way. If then the egg
forms no centrosome at all, or no active centrosome, then we can easily understand
how its power of division is inhibited and how perhaps it cannot even grow. But
BOVERI (1902) has not taken into consideration the fact that his theory will not
hold in plants, where centrosomes are absent. If we follow STRASBURGER
(1900 a) in assuming that in the higher plants the centrosome may be represented
by a partly-defined portion of the protoplasm, the so-called kinoplasm, we may
extend BOVERI'S hypothesis to the plant world also.

One thing is certain, viz. that none of the theories mentioned above can
explain all the facts. All three agree in supposing that something is wanting
in the egg, and so far all these hypotheses are correct. The phenomena of partheno-
genesis however prove that this conception is not universally correct. True
parthenogenesis, that is to say, the development of an unfertilized egg into an
embryo, is not often met with in plants. Where it has been carefully investigated,
as in Antennaria alpina (JuEL, 1900 a), and in species of Alchemilla (MURBECK,
1901), it has evidently become the normal process for the propagation of the
species, and it is questionable whether the ovum in these plants is capable of being
fertilized at all. The question would well repay an investigation, for if normal
pollen does not occur in the species in question the experiment should be made
with that of another closely-allied species. The investigation should prove of
special interest, since JUEL has found that in Antennaria the egg contains the
full complement of chromosomes present in the somatic cells, and he holds, for
this reason, that fertilization is impossible. If KERNER'S belief that Antennaria
hansii, Kern., is a hybrid between A. alpina $ and A. dioica a* be well founded,
then JUEL'S position will be untenable (compare FOCKE, 1881, 194). [Mean-
while STRASBURGER (1904^) has shown in the case of Alchemilla and JUEL (1904)
in the case of Taraxacum, both of which are parthenogenetic, that no reduction
takes place in the formation of the ovum. The frequency in change of view
on this subject and the contradictory nature of the observations make it neces-
sary to proceed with caution. The fact established by OSTENFELD (1904) that
certain species of Hieracium can produce embryos both sexually and parthenp-
genetically is of the greatest interest. Since it is unlikely that there are in
these plants two kinds of ova we are bound to assume that the normal ova
in the higher plants may develop parthenogenetically just as easily as those
of Marsilia, and that the number of the chromosomes has not that significance
which is attributed to it by the cytologists.] So far as the question that princi-
pally concerns us here is concerned these two plants are unimportant. Impor-
tant on account of their bearing on the present question are certain species of
Marsilia of which more than one shows a tendency to parthenogenesis. As
NATHANSOHN (1900 a) has shown, it is possible, by raising the temperature, to
increase very considerably the percentage of unfertilized eggs which develop.
In one investigation on Marsilia vestita, at 18° C. only 1-3 per cent, developed
parthenogenetically, at 35° C. 7-3 per cent, developed. The egg has the capacity,
whether fertilized or unfertilized, of developing into an embryo, and in the
latter case it is not the entry of a material substance but a rise in temperature
that supplies the requisite stimulus. We must realize therefore that the
stimulus may be supplied by a rise of temperature, just as by material contained
in the sperm. That this material need not necessarily be nuclein or an organ-
ized portion of the cell, such as a centrosome or chromosome, has been shown
by the experiments of LOEB (1899-1902) and WINKLER (1900) on animals.
The ovum of the sea-urchin, which in nature only develops after fertilization,
was treated by LOEB with magnesium chloride, and by WINKLER with a watery

JOST B b

370 METAMORPHOSIS

extract of sea-urchin sperms, containing no nuclein, and as a consequence of this
treatment the ovum developed.

We thus arrive at the conclusion that the egg needs a developmental
stimulus before commencing to divide and grow (compare SOLMS, 1900, and
the literature cited there), and such developmental stimuli we have already
found to be very widely distributed. Thus we have seen that buds whose
developmental activity has been temporarily inhibited may be made to unfold
by ether, that the spores of many mosses germinate in the dark only when the
temperature is raised, and that high temperatures are quite generally essential
in propagation by cuttings. It is also known that the pollen-tubes in many
cases induce development, apparently not by fertilization but by the secretion
of some soluble material ; thus the seed initials in Orchidaceae develop only if
pollen-grains germinate on the stigma. [The same is true of Fritillaria persica,
according to STRASBURGER (1886).] And this stimulus, doubtless chemical in
its nature, may, according to TREUB (1882), also be effected by certain insects
in one of the tropical orchids. In addition to this, the germination of the
pollen-tube has an exciting influence in the development of the fruit. This is
particularly noticeable in certain cultivated plants, which, as for example
currants and Sultana raisins, produce no seeds, the ovules having degenerated.
If the stigmas of these plants be not pollinated, the fruit fails to develop,
but pollination causes development without leading to any fertilization (MiiLLER-
THURGAU, 1898, compare also NOLL, 1902 [MASS ART, 1902]).

Sufficient examples of the renewal of development as a result of stimulus
action have now been given to show that the special effect of sperms is not
without analogies. This is probably the most appropriate place to discuss certain
phenomena which appear in the embryo-sac. We have comparatively recently
learned that in Phanerogams not only does one sperm-cell from the pollen-grain
fuse with the ovum, but that the second sperm-cell also passes into the embryo-
sac and fuses with the united polar nuclei before these give rise to the endo-
sperm (NAWASCHIN, 1898 ; GUIGNARD, 1899). As to the phylogenetic meaning
of this second act of fertilization we need not here speak. We are only concerned
at present with the fact that we appear here to be dealing with the removal
of a developmental check. Apparently all the nuclei of the embryo-sac are
incapable of development without some developmental stimulus, but the requisite
stimulus is not always a nuclear fusion. There is a whole series of plants in
which the embryo, as in apogamous ferns, arises not from the ovum but from
neighbouring cells (compare ERNST, 1901), it may be from the synergidae, the
antipodal cells, or from cells of the sporangium wall external to the spores — i. e.
from the nucellus. In the last case especially a fusion with a male cell is entirely
out of the question. But a definite external stimulus is in many cases necessary
for the formation of such adventitious embryos. In Nothoscordon fragrans, for
instance, the adventitious embryos first appear only after the ovum is normally
fertilized. After recent discoveries a reinvestigation of the question is much
to be desired, all the more as in certain other cases where adventitious embryos
occur, e. g. Coelobogyne ilicifolia (STRASBURGER, 1878) and probably also Eu-
phorbia dulcis (HEGELMAIER, 1901), they certainly arise without the previous
action of pollen-tubes or sperm-cells. All stages, from normal embryo forma-
tion to the complete parthenogenesis of Antennaria and Alchemilla, and the
adventitious embryo formation in Coelobogyne, occur in nature. In the last-
mentioned cases we must look for an internal stimulus which initiates the de-
velopment of the cells concerned.

The above examples of adventitious embryos are also of interest from
another point of view. It appears that all cells contained in the embryo-sac or
which come to lie therein, assume a form similar to the normal embryo ; the
embryo-sac cell must be able to exert a similar stimulus (}UEL, 1900 a ;
STRASBURGER, 1878).

We may now return to the phenomena of normal fertilization. We have

HYBRIDITY 37I

seen that the egg requires a developmental stimulus and we may suppose that
the sperm-cell by itself, perhaps only on account of its poverty in protoplasm,
perhaps also for other reasons, is incapable of development. Bearing on the
first of these possibilities is the phenomenon of ' merogony ', which has been
examined in both animals and plants (RosxAFiNSKi ; WINKLER, 1901). By
appropriate means, portions of the egg containing no nuclei were detached
and fertilized. The sperm-cell when embedded in a considerable mass of proto-
plasm commences to develop. This shows that the sperm-cell when enclosed
in protoplasm derived from the egg-cell becomes capable of development and
undergoes ' andro- ' or ' ephebo-genesis ' ; we may indeed give it another meaning
and say that the nucleus of the ovum is unnecessary to development, and that
it is sufficient if a sperm-nucleus is added to a non-nucleate egg.

Moreover, there is also a question of fundamental importance to be faced,
viz. why are the two sexual-cells incapable of development on their own account ?
Is the inhibition of development due to internal causes ? Is it a result of old age
and is the fusion to be regarded as a case of rejuvenescence? This latter hypothesis
has often been suggested, although not proved, though sound arguments
against it are not forthcoming. We must content ourselves therefore with a
a reference to what has been said already against the need for rejuvenescence,
adding only that the means employed is extremely peculiar. It is certainly
by no means clear how by the fusion of two senile units a rejuvenescence can
result — one might just as well expect that such a fusion should lead to increased
senility. If we can allot a more suitable meaning to the fusion, then the idea
of rejuvenescence as resulting therefrom may be readily given up.

In recent years another interpretation of the process has become more and
more prominent and has now received very general support, namely, that the
fusion of the two cells in the act of fertilization is of primary importance, inas-
much as it unites the characters of two organisms. In asexual reproduction,
i. e. by means of spores in Algae and Fungi, a cell is set free from the mother-
plant, and from this a new organism is produced with characters similar to
those of the parent. The spore thus transmits the characters of the ancestor
to the offspring, the latter inherits the ancestral peculiarities. If now in
two plants or in two branches of the same plant differences of some kind exist,
then these differences — at least under certain conditions — may be transferred
to the offspring. In asexual reproduction, which we may describe as monogeny,
individual peculiarities may thus remain unaltered. Let us now assume that in
sexual reproduction (digeny) each cell carries potentially certain individual differ-
ences derived from its parent, or, as it is usually put, has in it- initials of these
characters, then these initials are united and mixed in the fertilized egg. Hence
we may, with WEISMANN (1892 b), term fertilization 'amphimixis'. So far,
there is a general concurrence of opinion amongst authors that this union of
characters is the predominant feature in fertilization, but whether the signifi-
cance of the union be the balancing of individual characters or whether by a blend-
ing of the two organisms new characters originate is still a matter of discussion.

We must now attempt to discuss these two possibilities more closely, and
first of all it may be emphasized that in such an interpretation of fertilization
the observed inhibition in the development of the sexual-cells at once attains
a new significance. It is to be regarded as an adaptation, which in the first
instance renders fusion possible. For if the egg or sperm surrounded itself with
a cell- wall immediately after its formation and then began to grow, any fusion
of the protoplasms and nuclei of the two cells would be impossible.

We must not, however, overlook the fact that the above views do not
harmonize with all the facts we recognize under the term fertilization. If the
swarmspores formed by division in one cell of an alga copulate in pairs on
swarming free from the mother-cell (p. 354), the differences between these cells
can hardly be so great that a mixing has any very special significance. The

B b 2

372 METAMORPHOSIS

sexual-cells also which arise from stamens and carpels of one and the same
flower can scarcely possess any very marked individual characteristics in their
initials. Still, it is known that there are numberless arrangements to be observed
in flowers by means of which ' self-pollination ', that is to say, transference of
pollen to the stigma of the same flower, is prevented, and by which cross-
pollination between neighbouring branches or even between neighbouring
plants is facilitated (Cross-fertilization, DARWIN, 1876). Moreover the gametes
formed in one gametangium in certain Algae are said not to fuse with each
other (SxRASBURGER, 1900 b, 306).

If we now try to follow up more closely the complete combination of initials
in fertilization we at once come face to face with a great difficulty, for system-
atic experiments are wanting in which well-chosen examples with marked
individual characteristics have been crossed and the offspring thoroughly
studied. To carry out such researches would be very difficult, since individual
differences in plants are not, as a general rule, very well marked. For this
reason we must go further afield and study hybrids between different plant
relations (races, varieties, species) which show more easily recognizable differ-
ences between each other. As to such crosses there exists a voluminous litera-
ture. The existence of such races we take for granted — the next lecture will
deal with their mode of origin — in the present lecture we will deal only with the
results of crossing individuals belonging to two different races — that is to say,
with the production of hybrids (the older literature relating to these will be
found in FOCKE, 1881). [For more recent literature see DE VRIES, 1903.]

So far as we are aware, FAIRCHILD, in England, in the year 1717, was the
first gardener to raise a hybrid, inasmuch as he pollinated the stigma of Dianthus
caryophyllus with pollen for D. barbatus. Among botanists KOLREUTER (1761)
was the first to conduct experiments in hybridization for years on a large scale.
He was interested in hybrids from the point of view of their bearing on the
sexuality of plants, a subject which had been much disputed. His first hybrid,
Nicotiana rustica $xN. paniculata a*, flowered in the summer of 1761. Since
then innumerable hybrids have been produced for scientific and horticultural
purposes ; many of them have arisen naturally, and yet recent developments
in our science indicate that we have only just crossed the borders of the sub-
ject, and that in this province a wide and interesting field of research lies open
to us (compare DE VRIES, 1900 ; CORRENS, 1900 onwards ; H. TSCHERMAK,
1900 ; summarized by CORRENS, 1901 a and 1903, [1905]).

It is not possible to produce hybrids from any two plants selected at
random, because the capacity for forming hybrids is generally restricted to
nearly-allied plants. Rarely do we find species of different genera capable of
hybridizing ; less frequently still are hybrids themselves capable of crossing. The
more closely related the plants, the easier it is, as a rule, to produce a hybrid.
Still, the capacity for hybrid-production does not, in any sense, run parallel with
systematic relationship. It is very noteworthy that in certain cases the
hybrid A $ x B a* may occur while the reciprocal B $ x A c/7 is impossible. Thus
Mirabilis jalapay is easily crossed with M.longi flora a*9 whilst it is impossible to
fertilize M . longiflora <j> by M . jalapa a*. Such a fact at first sight seems almost
incomprehensible, but it is suggested that the success of the cross depends not
only on the capacity of the sexual-cells to fuse with each other but on the possi-
bility of the approximation of these cells. It is usually assumed that the
pollen-tube of M . jalapa is too short to grow through the much longer style of
M. longiflora. This may be true, but there is also another possibility, recently
drawn attention to by BURCK (1900). BURCK found that many stigmas contain
substances which are capable of stimulating the pollen-grains only of some, but
not of all, species of the same genus to develop. It must be assumed that the
failure of many experiments in crossing is due only to the fact that the pollen-
grains of the male parent are incapable of germinating on the stigma of the

HYBRID1TY 373

other plant. It is possible that by transferring a drop of the substance secreted
by the stigma of the pollen-bearing parent to the stigma of the other plant
germination of the pollen-grains may be induced.

When the hybridization is successful seeds are formed which resemble in
form, colour, and size the normal seeds of the mother-plant ; the fruits also are
uninfluenced by the male parent. A variation can ensue only where the contents
of the pollen-grain come immediately into action ; that is to say, where the two
generative nuclei have fused with the essential nuclei of the embryo-sac, i. e., the
endosperm on the one hand, and the embryo and the plant arising from it on
the other. At present we must omit any consideration of the endosperm of the
hybrid, and deal only with the hybrid proper. No general rule can be laid down
as to the appearance of the hybrid. Many interspecific hybrids show a struc-
ture exactly intermediate between that of the two parents, as KOLREUTER
described it in the case of the first recorded ' botanical cross ' — the hybrid
Nicotiana rustica$ x N. paniculata o*: — 'I was gratified to find that the hybrid
took a median place between the two parents not only in the arrangement of the
branches, and in the position and colour of the flowers, but also in all the parts
of the flower (the stamens alone excepted), which exhibited an almost geo-
metrical mean/

Horticulturists, in their researches on hybrids, usually start with the
assumption that the hybrids will show characters intermediate between those
of the parents, but it is now becoming more and more evident that this is only
one of many possibilities. In hybrids between members of closely-related
races intermediate characters are often wanting ; for instance, the hybrid
obtained by crossing a red with a white-flowered pea is not light red, •; but red,
like one of the parents. Although the white-flowered pea has yellow cotyledons
and the red-flowered one has green, the hybrid has yellow cotyledons. From
this example it is clear that in the hybrid the one parent does not simply pre-
dominate over the other, but that the separate characters of the two parents
struggle with each other, one parent being eventually victorious with regard to
one character the other in another. In this connexion we may, with CORRENS,
speak of dominant and recessive characters, and we may designate hybrids with
such strongly-marked characters as heterodynamic, in contradistinction to homo-
dynamic forms, which exhibit characters more or less exactly intermediate. A
further complication not infrequently arises, viz. in certain hybrids it cannot
be determined once and for all which character is dominant and which reces-
sive ; the result may be different in each individual case where the characters
are brought into conjunction. Individuals resulting from a single bastard cross-
pollination may differ, and in one or in all characters may resemble at one time
the mother at other times the father (many species of Hieracium, MENDEL, 1870),
or some of the hybrids may partly resemble the mother only or partly the father
only (strawberry, MILLARDET, 1894). Finally, in other cases still, the decision
as to which character shall dominate, or how vigorously it shall show itself is by
no means determined by fertilization itself, for individual branches, tissues,
cells, or even cell-parts belonging to one and the same hybrid may show varia-
tions. Examples of such ' mosaic hybrids ' were found by NAUDIN (1862) and
in the hybrid Datura laevis $ x D. stramonium a*, which produced, in addition to
fruits with small spines, intermediate, therefore, between the large-spined
D. stramonium and the spineless D. laevis, fruits which were smooth on one
side and spiny on the other.

Although the hybrid at first sight often appears as a new creation, more
careful study shows that it is only the combination of characters in it that is
new ; absolutely new characters dp not seem to arise in hybrids, although there
are certainly exceptions. The hybrid between the green-stemmed, white-flowered
species Datura ferox and D. laevis has brown stems and violet flowers ; it would
appear in this case as if a new character, the formation of a new pigment, had

374 METAMORPHOSIS

arisen. In reality, however (WEISMANN, 1892 a, p. 421), the phenomenon may be
otherwise explained. The white-flowered species may have arisen from a violet-
flowered stock and may, so to speak, have the capacity for forming this colour in a
latent state (compare p. 376), but which in the hybrid becomes actual. Whether
all the unexpected colours which appear in hybrids are to be explained in the
same way must remain at present an open question. A much more frequent
variation of the hybrid from the parents lies in this, that the hybrid, as a rule,
differs in its growth-energy. This energy may be feebler than that of the parent
plants when the parents are not closely related forms. In this case the seeds ger-
minate badly and the seedlings are difficult to rear. Or — and this is particularly
applicable to hybridization between nearly-related races — ' they are remarkable
for their size, rapidity of growth, early blooming, free flowering, longer period of
life, great capacity for multiplication, abnormal size of individual organs, and
similar characters' (FocKE, 1881, p. 475). If, for instance, the hybrid Datum
tatula $ x D. stramonium a* attains a height of two metres, while the parents only
attain a height of about one metre, we may say that the hybrid has acquired
a new character, nevertheless it is only a quantitative and not a qualitative
variation such as we might obtain otherwise, e.g., by over-nutrition in seed-
formation or good manuring in germination. One may look just as little
upon the increased growth-energy of the hybrid as on the other, at all events,
frequent characteristic, its diminished fertility, as a serious objection to the
view that hybrids show no new characters. This diminished fertility is usually
manifested in partially or completely unfertile pollen, more rarely in immature
ovules. For this reason it is often possible to rear fruits and seeds only by
pollinating from the parental line, although there is a class of hybrids whose
ovules are quite fertile with their own pollen (Salix, Hieracium). At the ex-
treme limit of sterility are many species of Rhododendron, Epilobium, &c.,
hybrids of which, in general, do not even form flowers (compare p. 376).

Let us now inquire what the hybrids of the second generation look like,
that is to say, the plants arising from the seeds produced by the first hybrid
generation. In this case, even less than the other, no general rule can be laid
down, even leaving out of consideration self -sterile hybrids. Among fertile
hybrids there are doubtless those in which the offspring after self-fertilization
are quite similar to the parents (MENDEL'S Hieracium hybrids, CORRENS, 1901 a,
pp. 75, 80) and, in contrast to these forms, those in which the offspring are entirely
different. [Owing to the discovery of parthenogenesis in Hieracium the uni-
formity of their offspring appears in an entirely different light ; there are hybrids,
however, which are perfectly constant (DE VRIES, 1903, 66).] Much work has
recently been carried out on the latter class of hybrids, and the numerical relation-
ships of the individual variations have been worked out on MENDEL'S principles.
In experiments of this kind, all, or at least very many, of the seeds produced
must be sown, but in the earlier investigations on this subject only a few plants
were raised, and hence no conclusions as to the constancy or inconstancy of
hybrid progenies could be arrived at. We cannot do more than give one
example from the very voluminous literature on the subject. Let us consider
a pea-hybrid that has arisen from two races differing in one character only, say
the colour of the flowers, which in race A are red, in race B white. As we have
already seen red is the dominant character. All the hybrids of the first genera-
tion have red flowers. If the seeds produced by self-pollination from these
hybrids be raised, the majority of the offspring will be found to produce red
flowers, but a certain proportion will be white-flowered. An enumeration shows
that 25 per cent, are white-flowered and 75 per cent, red-flowered. All the off-
spring of these white-flowered plants remain white-flowered, while of those of the
red-flowered forms one-third remain unaltered in colour (red) while of the
other two-thirds 25 per cent, are white and 75 per cent. red. In order to
explain this extremely peculiar result MENDEL (1866) assumed that the

HYBRID1TY

375

hybrid possessed two sets of reproductive cells, initials of the red-flowered form
and initials of the white-flowered form, in equal number. Fusion between two
sexual-cells with the same initials might then be as frequent as fusion between
sexual-cells with different initials. In 100 fertilizations we shall get on an average
fifty cases of fusion of similar initials — twenty-five fusions of cells bearing the
initials of white with white, twenty-five fusions of red with red — while fifty
fusions of dissimilar initials might also take place. Whether the red fuses with
red, or red with white is immaterial ; since red is dominant, 75 per cent, of the
second generation will show red flowers, and only the 25 per cent, in which
white has united with white will exhibit white flowers. But these 25 per cent,
have for ever lost the power of producing red flowers, and in this fact lies
the evidence for the support of the theory that the sexual-cells of the hybrid
contain only one kind of initial, while the vegetative-cells contain both types
of initial. There is thus in the formation of sex-cells a segregation of the
initials. Amongst the 75 per cent, of red-flowered forms only 25 per cent,
have red initials, and these are marked off from the fifty others which carry
white also, when sexual-cells are first formed. The twenty-five again form
only one kind of sex-cell, while the remaining fifty again segregate. Let us
imagine a case in which each plant produced only four offspring. We may then
construct the following scheme of the numerical relationships between the forms
with white, red, and mixed (red + white) initials in five generations.

Generation I.

i red -i- white

Thus we see that the ' white-flowered ' character, lost apparently in the first
generation, reappears in the second generation in 25 per cent, of the forms, and
rapidly increases, until by the fifth generation there is but little difference in
number between the red and the white-flowered individuals.

MENDEL'S law of segregation is, however, not of universal application.
There are hybrids which do not segregate and also others which segregate in
different proportions. Segregation may occur in one character while another
does not segregate. The segregation or non-segregation of characters does not
depend on whether the characters in the formation of hybrids are homodynamous
or heterodynamous. Notwithstanding the great interest attached to the
behaviour of races differing in two or more characters we must content our-
selves with what has been already said, and in conclusion raise the question as
to how far these facts suggested by the study of hybrids bear on the general
problem of fertilization.

Bearing in mind the increased power of growth of many hybrids and DAR-
WIN'S statement that seeds derived from cross-fertilization produced more
vigorous seedlings than those resulting from self-fertilization, it may be said
that cross-fertilization brings about a rejuvenescence or renewal of youth in the
protoplasm. Further, we may assume that protoplasm without such rejuve-
nescence may finally become, by continued vegetative reproduction, senile.
Although it is certain that the hybrid frequently exhibits more vigorous powers
of growth still any conclusion founded on that fact is insecure. It has not been
shown that the increased vigour depends on the fusion of the two kinds of
initials from which the hybrid results, and it seems almost more probable that
the very stimulus which removes from the egg its inability to develop may be also

Generation II. Generation III. Generation IV.

Generation V.

i white — > 4 white — > 16 white — >

64 white

2 white — > 8 white — >

32 white

4 white — >

1 6 white

( 8 white

2 red + white -

4 red + white

8 red + white

I 16 red + white

1 8 red

4 red — >

16 red

2 red — > 8 red — >

32 red

i red — > 4 red — > 16 red — >

64 red

376 METAMORPHOSIS

the cause of the more vigorous growth of the hybrid. This releasing stimulus
may reside in a soluble chemical compound. We have already often spoken of
stimulation of growth by ' poisons ', and we may compare the poisonous
effects of the action of copper-sulphate and other chemicals with the effect
of hybridization, for there, as in hybrids, we may often have, side by side, an
increase in vegetative growth and an inhibition of the formation of reproduc-
tive organs (compare pp. 88 and 374). Such facts as these, viz. that widely
separated races produce weak-growing hybrids, that more widely separated forms
do not hybridize at all, and that the pollen of a race still more distantly
related may even injure the stigma of another form, recall at once the action
of poisons [comp. DARWIN, 1868, II, 180].

In addition to the incentive to growth, fertilization leads to the fusion of
two different protoplasts, each having their individual initials. What
do hybrids teach us in this relation ? It is perfectly obvious that such a
combination occurs but we have nothing to go upon to enable us to determine
what will be the primary result of this union. On account of the varied be-
haviour of the hybrid we can say nothing as to whether a form will arise inter-
mediate between the two individuals, whether the differences will be neu-
tralized and the species remain constant or whether on the contrary new types
will appear and maintain themselves and the species become polymorphic.

Although hybrids dp not at present throw light on the problems as to the
significance of fertilization, they are nevertheless of primary importance in
another important question, i.e. heredity, the phenomenon of the handing on of
the parental characters to the offspring. Heredity is a peculiarity of organisms
which is shown both in the simplest form of reproduction, i.e. fission, and in
the most complicated sexual process, but although in the first case it may be re-
garded as a matter of course, in the latter it is an extremely wonderful pheno-
menon. Considering, e.g., division in the cell oiSpirogyra, the protoplasm, the
nucleus, and the chloroplasts divide, and these products of division have the
power of growth. Now if new characters appeared in these two halves which
were not present in the parent it would be a much more wonderful fact and one
much more difficult of explanation than if each half had the peculiarities of the
whole as is, as a matter of fact, the case. If we consider now a complicated
plant, such as a mushroom or a moss, we find there single cells — the spores —
capable of giving rise to a new organism similar in all respects to the parent.
In each spore the initials of the whole organism must be present and there
must also be arrangements for the development of these initials in the way
characteristic of the organism in question. Looking at sexual reproduction
only we have seen clearly from our study of hybrids that in both sex-cells the
initials of an entire organism are present, and hence that the fertilized ovum
contains the initials of two organisms — although only 'one results. Since in
certain cases it may be clearly proved that these initials do not unite later on,
we arrive at the conviction that in each sexually-produced plant there are
many initials which do not develop but remain latent.

It is impossible for us at present to deal with the problem of heredity in all
its bearings, all we can do is to treat of it briefly in so far as it is elucidated by
fertilization and hybridization. Let us direct our attention in the first place to
the question of the material basis of heredity.

In the division of Spirogyra no special theory need be referred to. All the
organs of the daughter-cell are parts of corresponding organs of the mother-cell ;
the nucleus inherits qualities of the nucleus, the chloroplast those of the chloro-
plast, and so on. Let us compare with this case the development of the flowering
plant from the ovum, and confine ourselves at first to the individual cells which
arise from it. The protoplasm and the nucleus of each of these are also deriva-
tives of the plasma and the nucleus of the egg, and we may therefore admit at once
that all the peculiarities of protoplasm, and all the peculiarities of the nucleus

HEREDITY 377

are inherited from the parent protoplasm and nucleus respectively. In the
chromatophores, however, we meet with difficulties at the very outset. So far
as we know, only the egg possesses these chromatophores, the sperm-cell has
none. Should a more thorough examination lead to another result — which is
quite possible — we should expect the chromatophores to fuse during fertilization
just as the nucleus and protoplasm do. According to the present point of view,
however, either the peculiarities of the chromatophores of the father are incapable
of being inherited, or they must be passed on by another portion of the cell, e. g.
the protoplasm or nucleus or both. The assumption that the inheritance of a
character by part of a cell which was not itself the natural carrier of this feature
cannot be avoided, since all the peculiarities of the cell are not associated with
those organs of the cell which are capable of division and which increase
by that means only. The wall of the vacuole with its contents and the external
protoplasmic layer with its product, the cell- wall, are, for example, parts of the
cell which are not directly transmitted from generation to generation. The
capacity of forming such parts, which undoubtedly is possessed by the egg, is
what is meant when we say that such a cell contains the initials of such organs.
We may now inquire whether there is any evidence for the definite localization
of these ' initials '. Although we do not really know what the ' initials ' are, we
must nevertheless assume that they are inherent in the protoplasm. It is not
probable that any selected part of the protoplasm functions as the bearer of the
initials, and yet since the time of NAGELI (1884) it has been customary to
designate as ' idioplasm ' that part of the protoplasm which contained the
initials and induces the rest to go through a certain development, and to
describe the remainder of the protoplasm as trophoplasm. NAGELI considered
the idioplasm as a network which extended throughout the entire cell, although
in recent times greater localization has been given to it.

On the zoological side (HACKEL, 1866) the nucleus is very generally claimed
to be the bearer of hereditary characters, and of the idioplasm (compare
HACKER, 1902), and on the botanical side, investigators, such as STRASBURGER
(1884) and DE VRIES (1889), have supported this view or endeavoured to prove
it independently. It may be pointed out, in the first place, that in the process
of fertilization the male element consists of a nucleus with little or no protoplasm.
That the idioplasm is localized in the nucleus has not satisfied every one, and
other hypotheses have been advanced, such as, for example, that the chromosomes
of the nucleus are the agents specially concerned in heredity. In this connexion
the changes preceding division of cells and fertilization must be kept in view. In
every normal cell division the characters of the mother-cell are equally divided
between the two resulting daughter-cells, and so also the units which act as
the bearers of the initials must be divided into exactly similar halves. This
tallies with the well-known division of the chromosomes, where the division
allots a longitudinal half to each daughter-cell. The bearers of individual initials
are believed to be serially arranged in the chromosome, and to correspond to
a certain degree with the chromatin-granules, which in certain cases may be
seen with the microscope, and are separated from each other by linin- threads.
These chromatin-granules, arranged longitudinally in the chromosome, are
necessarily present in the resting nucleus, although they cannot be seen
in it. The conclusion has been drawn, therefore, that the chromosomes
are persistent organs of the nucleus which can only multiply by division, and
moreover by longitudinal division, but which can never be created afresh. It
is very remarkable that at every division the chromosomes appear in the same
number, and their behaviour during fertilization in the animal kingdom is even
more remarkable still. There the so-called reduction division (p. 367) comes into
operation, ovum and sperm have only half the number of chromosomes found in
the somatic cells, and so the doubling of the number of chromosomes at each
fertilization is prevented. The fact that the sperm-nucleus contains the same

378 METAMORPHOSIS

number of chromosomes as the nucleus of the ovum is regarded as a proof that
the chromosomes are really the transporters of hereditary characters, for in
general the offspring inherit in equal degree from father and mother.

We have hinted at the basis on which certain modern theories of heredity
are founded, and more especially WEISMANN'S germ plasm theory (1892 and
1902), although their characteristics have by no means been treated exhaustively.
WEISMANN'S theory is at least the first theory of heredity to be worked out in
detail, and leads to results of the greatest consequence. It is for this reason
worthy of consideration and its scientific importance is greatly enhanced when
we reflect how many specific researches it has given rise to. If we do not deal
with it here in extenso it is not merely from considerations of space, but also on
its merits, for botanists must, in our opinion, reject this theory. Detailed reasons
for rejecting WEISMANN'S theory we cannot give here, all we need say is that to
attribute to the chromosomes, or indeed to the nucleus at all, the exclusive
possession of the initials is a view which has in no sense been justified.

Let us first of all consider the quest ion of the individuality of the chromosomes.
[The principal supporter of the individuality of the chromosomes of late years
has been BOVERI (1904), for STRASBURGER, who also held that view in the past,
appears to have abandoned it (1905). The parts of the chromosomes, the chrb-
matin-granules or ' ids ', are still to be considered as organs which may increase
in number by dividing, but which cannot be created afresh.] In the majority of
nuclear divisions an unprejudiced observer will conclude that the chromosomes
are formed in the so-called prophase stage of the division and will disappear in the
anaphase. The constant return to the same number of chromosomes results from
the fact that before each division the mass of chromatin is approximately of the
same amount. There are statements enough available which suggest that the con-
stancy of the number of chromosomes is often more a pious wish on the part of the
observer than an actual scientific fact. Very frequently the number of chromo-
somes cannot be accurately counted at all, and the observer contents himself
with ascertaining whether they correspond approximately to the normal number,
or to its half, or its double. GUIGNARD (1891) adduces an example of a very
remarkable anomaly in the number of the chromosomes. The primary embryo-sac
nucleus of Lilium has in its first division twelve chromosomes ; one of the resulting
daughter-cells always exhibits in the two following divisions twelve chromo-
somes, the other and lower one, however, shows, in the first division, sixteen or
more, and in the second twenty to twenty-four. This, at all events, proves that
the chromosomes can reproduce themselves otherwise than by longitudinal
division. DIXON (1894) has made similar observations in the prothallium of
Pinus, where the large nuclei of the archegonium-wall cells possess more than
double the number of chromosomes found in the nuclei of the first prothallial
cells. Finally, we may draw attention to the increase in the number of chromo-
somes in apogampus ferns (compare p. 368). In spore formation also, as above
explained, a diminution in the number of the chromosomes arises, not, as one
might assume, because, as in the case of animals, the longitudinal division fails to
appear, but because the chromatin of the nucleus breaks up into only half the
number of chromosomes. We conclude from all this that the chromosomes are
not definite organs of the cell as are the chromatophores and the nucleus ; they
are reformed at each division, and hence the chief basis for believing them
to be transmitters of hereditary characters disappears.

Starting from the fact that the internodal cells of the Characeae are
incapable of regeneration, it has been claimed that only by normal nuclear
division can two cells with equal hereditary properties arise. The above-named
internodal cells show later, as a matter of fact, the so-called direct cell division,
in which no chromosomes and consequently no longitudinal division of the
chromosomes can be found. When first produced, however, the nucleus divides in
a thoroughly typical manner, and the sister cells at the nodes possess fully the

HEREDITY 379

quality of regeneration. The later indirect nuclear divisions in the internodal
cells may possibly be the result of the loss of capacity for regeneration, but could
never be its cause. Furthermore, NATHANSOHN (1900 b) has shown for Spirogyra
and WASILIEWSKI (1903) for Faba that by taking certain measures the typical
nuclear division may be transformed into the direct method without the cells
suffering any loss of function (compare p. 270). Besides this, there are plenty of
cells which are unable to exercise this or any function, e. g. regeneration, but
whose nuclei have nevertheless arisen in the normal manner. In a word, con-
clusive proof that the idioplasm is localized in the chromosome does not exist.

But the chromosomes do not constitute the whole of the nucleus, and
perhaps the hereditary capacity lies in some other nuclear substance. The
supposition that the nucleus serves only as a receptacle for reserve material
connected with the hereditary substance must be entirely rejected, for in such
cases as Spirogyra it would be quite superfluous. If, however, other functions than
heredity be attributed to the nucleus, but if heredity is to be associated with it
only and not with the plasma, we have before us a view to which we must take
exception. This fact alone would appear conclusive, viz. that every male cell
bears protoplasm in addition to the nucleus, but it does not seem permissible to
placea value on its amount. Further, in the Phanerogams, STRASBURGER(i90ob),
himself an advocate of the ' nuclear theory ', has quite lately conceded the pas-
sage of protoplasm along with the male nucleus into the embryo-sac, although,
it is true, he points out that it is difficult to recognize it there. GUIGNARD
(1900, 373) has also seen protoplasm pass from the male cell to the ovum.

If BOVERI'S experiments were beyond criticism they would give extremely
strong support to the nuclear theory. BOVERI fertilized the non-nucleate ova of
one sea-urchin with the sperms of another, and obtained hybrids showing
paternal qualities only. Unfortunately, grave exception has been taken to these
experiments (compare A. MEYER, 1902, p. 173). [BOVERI, 1904, 105.] [GoD-
LEWSKI (1905) has advanced experimental proof that a sea-urchin egg deprived
of its nucleus may be fertilized by the sperm of Antedon (one of the Crinoideae),
and that the organism resulting has the characters of the mother, whence it may
be concluded that cytoplasm without a nucleus is capable of transmitting
hereditary characters.] The question of the localization of the bearer of the
hereditary characters must for the present therefore be left undecided, or else
we must assume that this is to be sought for just as much in the nucleus as in
the protoplasm, perhaps also in the chromatophores. Moreover, we are not only
ignorant where this substance is to be found, but we are equally in the dark as to
the way in which it operates.

Having examined the distribution of the idioplasm in the cell, let us now
turn to its distribution in the plant as a whole. Here we find two sharply op-
posed views. One claims for the germ-cells (sex-cells and cells of growing points)
a very special role, they alone are said to be the bearers of the whole idioplasm ;
the other theory assumes the same potentialities to exist in any and every cell.
The first bases itself on the specialized phenomena in the animal world, the
other on similar phenomena in the vegetable world. In many animals the egg
by the first division is divided into two essentially different cells, one of which
is devoted to the construction of the whole soma, the other to the formation of
the sex-cells. The germ-cell, according to WEISMANN, contains the initials of
many organisms ; the somatic-cell of one only. By the further division of the
somatic-cells there arise the rudiments of individual somatic organs which only
develop further in this one direction. WEISMANN, therefore, assumes that
unequally inheritable divisions occur, so that, for instance, in one cell there are
only initials for the ectoderm, in another only those for the endoderm. Against
this view we may urge the behaviour of certain plants, notably Begonia and
Marchantia, in which undoubtedly from every cell, even when advanced in growth,
the whole organism can be reproduced by regeneration (p. 330), and in which,

380 METAMORPHOSIS

therefore, the vegetative-cell evidently contains the same initials as the germ-
cell. Now there are, on the one hand, animals which are like typical plants in
possessing extensive powers of regeneration, and there are also plants which
are comparable with the higher animals in their slight powers of regeneration.
How are such plants to be regarded ? Are the root initials absent in a twig which
cannot strike root, and does a leaf which forms roots but not shoots, possess
only root initials ? We can give no final answer to this inquiry ; but when we
reflect how much regeneration depends on external conditions (e. g. high tempera-
ture) we shall be inclined to believe that only a favourable environment is
wanting and not initials. Moreover, initials originally present may easily be
lost or so very much attenuated in efficiency — sit venia verbo — by the growth
of the cell that they become inoperative.

Regeneration can but rarely proceed, as in the case of Marchantia, from small
fractions of the body of the plant ; isolated cells of fully-developed tissue are
mostly only just capable of growth (HABERLANDT, 1902), or on the application of
stimuli only produce a few divisions (WINKLER, 1902), but they never regenerate
a complete organism. The possibility is not excluded that suitable nutriment
is wanting in these cells such as would render possible an increase of the small
quantity of idioplasm which they_contain. If regeneration taking place in
a tissue
1903)) t
supposes

the formation of organs is always preceded by an ample division of cells, which
WINKLER regards in a similar way, but nothing more definite is known. In any
case the botanist may, in opposition to WEISMANN, maintain that all cells have
the same initials, and that there is no qualitative division of hereditary characters.
The germ-cells are distinguished from the vegetative-cells only by the fact that
in them the idioplasm is predominant, while in the latter, in accordance with
their function, trophoplasm prevails.

But how is this to be reconciled with the segregation of characters which
appears in certain hybrids and which seems to be characteristic of the germ-cell ?
Is not this segregation in itself a new argument for the chromosome hypothesis ?
If every chromosome were the bearer of one quality, then the reduction division
must be an excellent means of effecting qualitative division of hereditary charac-
ters, thus leading to segregation. The small number of chromosomes, however,
renders it impossible to allot one quality only to each of them, and the theories
which allot to the chromosome the hereditary substance are obliged to assume
that each chromatin-granule is the bearer of several qualities. The reduction
division as understood by the zoologist does not explain the segregation of the
hybrids ; on the other hand, CORRENS (1902) has formulated a view which
renders it possible to understand this process on the grounds of the chromosome
theory. We will not pursue further these purely hypothetical questions, but
confine ourselves to the facts, regarding it as established that the supposed
distinction between somatic- and germ-cells does not in reality exist at all.

We have already referred to segregations which take place in the vegetative
region, but one of the most instructive examples is furnished by Cytisus adami,
a hybrid of C. laburnum and C. purpureus. This hybrid is approximately
intermediate between its parents so far as its organs of vegetation and repro-
duction are concerned ; thus, for example, it produced flesh-coloured flowers.
But reversions to the parental forms also occur, for on individual branches we
frequently find the yellow blossom of C. laburnum, and also, but more rarely, the
red flowers of C. purpureus. If a branch has assumed the character of one of
the parents it never returns to the intermediate form. The segregation need
not involve the whole branch ; it can also be accomplished in the longitudinal
half of a bud, so that the branch arising from it has on one side the character of
C. laburnum, and on the other that of C. adami ; and in this case the line sepa-

HEREDITY 381

rating the two often passes through a flower or through a leaf (BRAUN, 1851).
These segregations also claim our interest in that they are not confined to the
derivatives of individual cells, but present themselves at the same time in many
cells (BEIJERINCK, 1901). The segregation in this case would therefore not be
directly connected with the division of the cells, and there must evidently be a
subsequent destruction of initials in the fully-formed cells.

If initials, however, can disappear in a fully-formed cell it is possible that
they may also arise in them. We have every reason for treating this question
in connexion with C. adami. This plant, which passes for a hybrid, is said to
arise from the grafting of C. purpureus on C. laburnum. As, however, an ex-
perimental production of this graft hybrid, after its first fortuitous occurrence,
no longer succeeded, and as there appeared little probability of the formation of
graft hybrids in any case, their existence has latterly been denied (VoCHTiNG,
1892). The very definite statements of KOHNE (1902) about a hybrid arising
from a grafting of Mespilus on Crataegus would appear to render this sceptical
point of view no longer quite justified. [According to NOLL'S (1905) most
recent statements it is scarcely possible to doubt that in this case we are dealing
with a genuine graft hybrid.] The hybrid twigs arise in this case at a certain
distance from the point of grafting, and it therefore appears quite impossible
that they could have arisen from cells which have grown together in the
process of grafting. [NOLL considers that such a fusion of nuclei is self -apparent.]
It must rather be an effect produced by the Mespilus cells on distant Crataegus
cells. As STRASBURGER (1901 a) has found plasma bridges between graft and
scion, a migration of plasma particles, therefore, also of idioplasm, from the graft
into the scion is not impossible. No one, however, will wish to maintain that
cell-nuclei wander through these plasma bridges and unite with the nuclei of
other far distant cells, for the observations we owe to MIEHE (1901) and KOR-
NICKE (1901) regarding the wandering of nuclei from closed cells refer evidently
to pathological processes or artificial products. [In any case we must take
exception to NEMEC'S (1904) and FARMER'S (10,03) statements that a nucleus
which has migrated from one cell can fuse with another in a neighbouring
cell.] The graft hybrids pronounce distinctly against the exclusive allotment
of the idioplasm to either the nuclei or the chromosomes (compare, DE VRIES,
1903, for another interpretation of graft-hybrids [and NOLL, 1905]), and we
may expect interesting lights on the question of heredity from a more exact
study of this kind of hybrid.

Bibliography to Lecture XXIX.

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BOVERI. 1902. Verhdl. d. Gesell. d. Naturforscher. Hamburg, 1901, p. 44.

[BOVERI. 1904. Konstitution d. chromatischen Substanzen. Jena.]

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BURCK, W. 1900. Proc. Kon. Akad. v. Wetensch. Amsterdam.

CORRENS. 1899. Ber. d. bot. Gesell. 17, 410.

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CORRENS. 1901 a. Ber. d. bot. Gesell. 19, 71.

CORRENS. 1901 b. Bibliotheca botan. 63.

CORRENS. 1902. Bot. Ztg. 60, 65.

CORRENS. 1903. Ibid. 61,113.

[CORRENS. 1905. Ueber Vererbungsgesetze. Berlin.]

DARWIN. 1876. Effects of Cross and Self -fertilisation. London.

[DARWIN. 1868. Das Variieren d. Pflanzen u. Tiere. Vol. II. 1 80. Leipzig.]

[DE VRIES. 1903. Die Mutationstheorie. II. Leipzig.]

[DiGBY. 1905. Proc. Roy. Soc., B. 76, 463.]

DIXON. 1 894. Annals of Botany, 8.

ERNST. 1901. Flora, 88, 37.

382 METAMORPHOSIS

[FARMER, MOORE, DIGBY. 1903. Proc. Roy. Soc. 71, 453.]

FOCKE. 1 88 1. Die Pflanzenmischlinge. Berlin.

[GoDLEWSKi. 1905. Anzeig. d. Krakauer Akad. 501.]

GUIGNARD. 1891. Annales Sc. nat. VII, 14, 163.

GUIGNARD. 1899. Revuegen.deBot.il.

GUIGNARD. 1900. Annales Sc. nat. VIII, n, 367.

GUIGNARD. 1901. Journal de Bot. 15, 37, 205, 394.

HABERLANDT. 1902. Sitzungsber. Wiener Akad., Math.-Nat. Kl. 1 1 1, Abt. I.

HACKEL. 1 866. Generelle Morphologic.

HACKER. 1902. Ueb. d. Schicksal d. elterl. u. grosselterl. Kernanteile. Jena.

HANSEN. 1881. Abh. d. Senkenbergschen Gesell. 12.

HANSTEIN. 1877. Bot. Abhand. 3, Heft 3.

HEGELMAIER. 1901. Ber. d. bot. Gesell. 19, 488; ibid. 1903, 21, 6.

JUEL. 1900 a. Svenska Akad. Handlingar, 33.

JUEL. 1900 b. Jahrb. f. wiss. Bot. 35, 626.

[JUEL. 1904. Review in Bot. Central. 95, 361.]

KLEBS. 1899. Biol. Centrbl. 19, 209.

KOHNE. 1901. Gartenflora, 50, 628.

KOLREUTER. 1761-66. Vorl. Nachricht von e. das Geschlecht der Pflanzen betr.

Versuchen u. Beobachtungen. Leipzig. (Ostwald's Klassiker, Nr. 41, 1893.)
KORNICKE. 1901. Sitzungsber. niederrhein. Gesell.
[KORNICKE. 1904. Bot. Ztg. 62, II, 305.]
LOEB. 1899. Amer. Journ. of Physiol. 3, 135.
LOEB. 1902. Archiv f. Entw.-Mechanik, 13, 481.
[M ASSART. 1902. Bull. Jard. bot. d. Bruxelles.]
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MILLARDET. 1894. Mem. Soc. sc. phys. et nat. Bordeaux, 4.
MULLER-THURGAU. 1898. Landw. Jahrb. d. Schweiz. (Bot. Centrbl. 1899, 77, 135.)
MURBECK. 1901. Lunds Univ. Arsskr. 36.

NAGELI. 1884. Theorie d. Abstammungslehre. Munich and Leipzig.
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NAUDIN. 1 862. Nouv. Rech. sur I'hybridite d. Veget. Paris.
NAWASCHIN. 1898. Bullet. Acad. St-Petersbourg.

NOLL. 1902. Sitzungsber. niederrhein. Gesell., 10 Nov. 1902 [and 1905 do. do.]
[NIsMEC. 1904. Sitzungsber. Kgl. Gesell. Prag, 13.]
[OsxENFELD. 1904. Ber. d. bot. Gesell. 22, 537.]
ROSTAFINSKI, cf. GIARD. loxn. C. rend. Soc. Biol.
SOLMS-LAUBACH, GRAF zu. 1900. Bot. Ztg. 58, II. Abt. 376.
STRASBURGER. 1878. Ueb. Polyembryonie. Jena. Comp. also BRAUN (1857-59)

and HANSTEIN (1877).

STRASBURGER. 1884. N. Unters. iiber d. Befruchtungsvorgang. Jena.
[STRASBURGER. 1886. Jahrb. f. wiss. Bot. 17, 52.]
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STRASBURGER. 1901 a. Jahrb. f. wiss. Bot. 36, 493.
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[STRASBURGER. 1904 a. Sitzungsber. k. preuss. Akad.]
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[STRASBURGER. 1905 b. Die stofflichen Grundlagen d. Vererbung. Jena.]
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VARIATION. ADAPTATION. ORIGIN OF SPECIES 383

WINKLER. 1902. Bot. Ztg. 60, II. Abt., 264.
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ZACHARIAS. 1901. Ber. d. bot. Gesell. 19, 377.

LECTURE XXX
' VARIATION. ADAPTATION. ORIGIN OF SPECIES

(See note beneath the title of Lecture XXIX.)

IN the last lecture we considered the subject of hybridity with the view of
gaining some acquaintance with the significance of fertilization. The facts
brought forward introduced, however, another problem of even greater interest,
viz. that of the transmission of characters in reproduction. It maybe shown that
when hybridization is effected there is a hereditary transmission of the pecu-
liarities of two organisms, so that the offspring differs in appearance from both
parents. Although the variations so induced are often by no means permanent,
either because segregation occurs or because many hybrids are sterile and hence
produce no offspring at all, there are, on the other hand, hybrids, both natural
and artificial, which are completely fertile, and in which the deviations from
the parental types are permanent, or, in other words, new plant forms may arise by
hybridization. Two questions now suggest themselves ; in the first place, are
the multitudinous forms which we meet with in nature the result of crossing
of a few original types ? And, again, is crossing the only cause of variation in
nature, or, conversely, were it not for this crossing, would the offspring produced
sexually or asexually always resemble their parents in all essentials ?

If for the term ' types ' we read ' species ', we have stated the problem,
par excellence, which has exercised the minds of biologists since the middle
of the last century, viz. the problem of the origin of species. In the treat-
ment of this problem biologists have collected an overwhelming amount of evi-
dence, zoological, botanical, and palaeontological, all tending to show that the
species which now inhabit the surface of the earth have been derived from other
different types which previously existed on it. This view has been briefly
termed the Theory of Descent, a theory which, as is well known, was brought
into prominent notice by the immortal CHARLES DARWIN, who succeeded where
previous supporters of the theory were less 'fortunate. The facts on which the
Theory of Descent is based are essentially morphological and systematic, but
the experiences of the gardener and the agriculturist have also proved of immense
value as corroborative evidence. Plant physiology, however, has in the past
been only to a limited extent concerned with this problem, and it is only of
recent years that it has been recognized that experimental physiology might
aid in the solution of such problems. It is obvious that physiology alone is
able to shed light on the modifications which species undergo, and to elucidate
the more immediate conditions of such changes — to answer the questions how ?
and why ? Of course experiment cannot tell us how the present vegetation came
into existence — that is an historical question whose solution cannot be reached
even in the most elementary form, since the necessary documents are available
only to the most meagre extent. The task of physiology is to study the
changes which go on in plants at present living, to measure their extent, and
to investigate their causes. From that study one may draw conclusions as to
the phenomena which took place in earlier periods of the earth's history, and
so it is possible for physiology and morphology to work hand in hand in this
field of inquiry.

384 METAMORPHOSIS

Although, even now, physiological data on this subject are extremely
limited, still it is of the greatest importance that a brief summary of these
should be presented in treating of the physiology of form changes. Owing to
the great part which CHARLES DARWIN played in the founding of the Theory of
Descent it will be most convenient to start with a very brief consideration of
his conception of the origin of species, more especially as his views are now very
generally accepted.

DARWIN (1860) compared the origin of species in nature to the origin of
races in cultivation. The formation of different breeds starts with individual
variation, that is to say, with the fact that the offspring of the same two parents
are not similar in all respects ; of this variation there can be no doubt. The
breeder selects for propagation only such organisms as exhibit a certain
desired peculiarity, and he expects this character to be transmitted to their
offspring in turn. The appearance of variations and their inheritance is entirely
a natural phenomenon ; what the breeder does is merely to select (artificial selec-
tion) for propagation certain definite individuals. DARWIN believed that a
process, similar in all respects to that carried out by the breeder, could be recog-
nized in nature. From each animal or plant so many offspring originate that
only a fraction of them are able to find the necessaries of life ; the rest ' succumb
in the struggle for existence'. If it be asked how this struggle for existence
brings about natural selection we believe DARWIN to be correct in his assumption
that all the better equipped individuals will have a better chance of remaining
in existence and producing progeny than those less well adapted. Whether
the organism is well or less well adapted depends on its capacity for making
use of all that is favourable in its surroundings and for guarding itself against
what is injurious. Let us imagine two seeds of the same species germinating in
close proximity to each other ; the one seedling produces its root system rather
more rapidly than the other and seizes on the water and nutritive salts in the
soil before the other succeeds in doing so. The result will obviously be that the
former plant will be successful in the struggle for existence, while the other will
be dwarfed. The same result will come about if differences arise in the rate of
growth of aerial organs, and a rapidly-growing seedling will deprive the other of
the essential light rays. If a seedling contains some poisonous materials which
may act as protective agents, and if it possesses some mechanical protective struc-
tures, such as raphides or thorns, it will be better protected from the attacks of
animals, and have a greater chance of handing on its characters to offspring
than the plant that is destitute of such. In nature, as in artificial cultivation,
only some individuals reach the propagative stage, and if they transmit their
characters then these will be gradually emphasized, and species must alter as they
become more and more adapted to their conditions of life.

Considering now the Darwinian theory more in detail, it is obvious that we
must give careful attention to the three factors which are specially concerned,
viz. variation, heredity, selection. Let us begin with the last, and ask ourselves
how it affects species formation. In attempting to answer this question we
cannot avoid discussing briefly the significance of the term ' species '. The
idea of species is a purely abstract one ; in nature there are no species — only
individuals. By a species we mean the totality of individuals which belong to
the same line and which preserve their same characters for successive genera-
tions. In nature, however, we know nothing of the genealogy of each individual,
and regard as a species all those plants which agree in all essential features and
live under as nearly as possible similar external conditions. Since it is obvious
that different botanists may hold entirely different views as to what constitutes an
essential and what a non-essential character, there arise allsorts of discrepancies in
the identification and naming of forms . These discrepancies are aggravated accord-
ing to the varying accuracy of the investigations carried out and the tendency of
the investigator to lay emphasis on general characters possessed in common by

VARIATION. ADAPTATION. ORIGIN OF SPECIES

385

the greatest possible number of individuals, or, conversely, the greatest possible
differences between individuals. The first type of investigator tends to widen
the conception of species, the second to restrict it ; LINNAEUS' s species may be
taken as representative of the former, JORDAN'S of the latter. Looked at from
LINNAEUS'S point of view JORDAN'S species are to be regarded as 'petitesespeces,'
sub-species or as varieties, from JORDAN'S point of view LINNAEUS'S species are
to be considered as collections of species or sub-genera. According to the end
the author has in view the limits of the species will sometimes be wide, some-
times narrow. Obviously for our purpose the more restricted the limits of
species are the better, for if only their origin can be cleared up, the application
of the Theory of Descent to the origin of higher groups (sub-genera, genera,
families) presents no essential difficulty whatever.

Let us take as an example the numerous forms of LINNAEUS'S species,
Draba (Erophila) verna, studied by JORDAN (1873), DE BARY and ROSEN (1889).
JORDAN distinguished more than 200 forms, each of which preserved its own
special characters for many generations with complete constancy. There can
be no doubt that more extended investigations would have resulted in the dis-
covery of an even greater number of
forms, distinguished by minuter differ-
ences, so that, in short, there would ap-
pear to be no limits to species-mongering.
How are the individual sub-species of
Erophila verna distinguished ? In ad-
dition to the general characters, which
are difficult to analyse, there are the
differential characteristics of form (con-
tour, margin) more especially of the
leaves of the radical rosette, the form
and number of the hairs, the appearance
of the floral leaves and the fruits. With-
out further description we may refer to
Fig. 113 from one of ROSEN'S illustra-
tions.

It is very difficult in most cases to
attribute a use to a specific character,
and, again, it is just as difficult to furnish evidence that it can be of no service and
cannot arise in the struggle for existence. Still it must be admitted, as DARWIN'S
own investigations and later those of STAHL and HABERLANDT show, that many
characters previously believed to be of no service to the organism have distinct
functions and hence may have arisen by selection. It is always open to the
zealous Darwinian to assert that many a feature that is now of no import may
have been of service when it first appeared, for indifferent characters, if non-
injurious, may also be transmitted. But we have good grounds for taking
exception to the view that the species of Draba are all adapted forms which have
arisen in the struggle for existence. For instance, we very frequently find a
certain habitat of very limited extent occupied by several species forming an
association. It is highly improbable that each individual species arose as an
adaptation to a certain environment and then that these adapted types should
be collected together in a new habitat ; but even if this assumption be made,
it is very remarkable how three forms, let us say, a, b, c9 which have originated
in three different places, each in complete harmony with its surroundings, should
collect in a new surrounding, and all be equally well adapted to the new conditions,
so that no one of them drives out the other. In addition to several species occur-
ring gregariously in one region, one and the same species occurs in quite different
habitats which have so little in common that they have been unable to affect

Fig. 113. Erophila verna. After ROSEN (1889,
pi. 8). /, 'leaf rosette of Erophila spec. (/), of E.
graminifolia(2\ ofE. subtilis ( ?), of E. procerula (4\.
77, hairs of E. oblongata (i\ of E. procerula (2), of E.
graminea (j), of E. obconica (4). Ill, Flowers of
E. violacea (/). of E. subnitens (2), of E. scabra (j),
of E. majuscula (4).

JOST

c c

386 METAMORPHOSIS

these species. In a word, look at it as we will, we are compelled to believe that
the specific characters of Draba verna cannot have been evolved in the struggle
for existence, that they are not adaptive characters, and that they are in them-
selves useless. Many sub-species in addition to those of Draba verna are
similarly gregarious, others, however, such as those studied by WETTSTEIN,
exclude their near relations from their habitats (WETTSTEIN, 1898, Grundzuge
d. geographisch-morphologischen Methode der Pflanzensystematik. Jena). What
we have said as to sub-species applies to species also of other plants. In all
large genera we find species which live in the same habitats, but whose specific
characters are in no ways adaptive. Examine the differences, for instance, which
exist between Avena elatior and A. pubescens which grow beside each other in
the same field ; it is impossible to regard these as adaptive characters ; still less
can we regard as adaptive the characters which distinguish genera, families,
and higher groups. We are inclined to hold that in most cases (p. 395) where
two forms are distinguished only by dissimilar adaptive characters, either we are
not dealing with genuine species or that their real differences have not yet been
discovered. In the former case we should be dealing with modifications due to
habitat only, such as we so often meet with in water and land, light and shade,
mountain and lowland forms ; they differ from genuine species in this, that their
characters are not hereditary, but disappear again when, or soon after, the in-
ducing factors are removed.

At the same time we do not mean to affirm that adaptive characters cannot
be inherited. We shall return to that point later on, but meanwhile it may be
noted that there are obviously two quite distinct kinds of characters possessed
by species, adaptive characters and organic characters, a distinction which has
been emphasized with special clearness by NAGELI (1884). This distinction may
be made out at each stage in the classificatory system. There are aquatic and
terrestrial Algae just as there are aquatic and terrestrial Phanerogams, and
both Angiosperms and ferns include both xerophytic and hygrophytic types.
In smaller groups also adaptive and organic characters may be distinguished.
The problem as to the origin of species is primarily the problem as to the origin
of definite organic characters; however, since all species exhibit adaptive
characters as well, we must study their origin also.

From what has been already said it must be admitted that DARWIN'S
principle of natural selection cannot explain the origin of species. Let us
see whether it is sufficient to explain adaptive characters. Here again, how-
ever, we meet with great difficulties. According to DARWIN, the differences
between competing individuals are not great but they become gradually inten-
sified by summation in the course of an indefinite number of generations. Con-
sider, for example, some peculiarity of a plant obviously of service to it, such as
the prickles of a rose which aid it in climbing, or the spines of a thistle which
protect it from the attacks of animals ; according to DARWIN'S theory these
prickles and spines began as excrescences of minimum height on a previously
smooth plant and attained their present structure and dimensions gradually.
Only after the organ had become sufficiently prominent, however, could it have
been of any use to the plant ; in a word, it is easy to understand how DARWIN'S
theory may explain the improvement of an organ already in existence, but it
does not make clear how it first arose.

Having seen that the mode of operation of natural selection as defined by
DARWIN does not afford a satisfactory explanation of adaptive, let alone specific,
characters, let us next inquire as to DARWIN'S interpretation of variation and of
heredity. It will be necessary to study these two questions in conjunction,
since the important point for consideration is whether or not variations are
inherited. DARWIN assumed that every new character, however it arose, was
capable of being inherited, but this assumption has yet to be proved, and
especially in reference to each type of variation, of which we may distinguish

VARIATION. ADAPTATION. ORIGIN OF SPECIES 387

three (DE VRIES, 1901 a) (compare also KLEBS, 1903, as to the types of varia-
tions and as to the origin of species), fluctuating variations, adaptive variations,
and mutations). To these may be added a fourth type, variations which arise
from hybridization ; into these, however, we cannot enter here.

Fluctuating variations may be termed also individual, because they show
themselves in single members of the species, arising apart altogether from
crossing or extraneous influences of that sort. If the seeds from a single
capitulum of a member of the Compositae be planted in a garden the plants
which result vary extremely in weight and size, and, later, the different organs
of each plant also vary in number, size, and weight from the quantitative aspect.
If statistics be collected of a larger number of individuals a certain average
may be determined for each character, and the deviations from that mean occur
less and less frequently the greater they are. We may, in fact, from the data
so established construct a curve (Galtonian curve) which corresponds more or
less exactly to the law of probabilities (QUETELET). The curve has one crest
decreasing rapidly to zero on either side. These individual variations may be
illustrated first by a few examples taken from observations on wild plants
selected at random.

Number of rays in the archegoniophore of Marchantia (LuDWiG, 1900,
p. 22) :—

No. 7 8 9 10 ii 12 13
Frequency i 14 joj 152 44 3 i Total 522.

Number of petals in Linaria spuria (VocnxiNG, 1898) : —

No. 234 5 6789

Frequency in actinomorphic flowers i 2 43 810 52 2 i i Total 912.
» » zygomorphic ,, — 4 240 60250 169 7 i Total 60671.

Length of seeds of Phaseolus vulgaris (DE VRIES, 1901 a, p. 34): —

Mm. 8 9 10 ii 12 13 14 15 16
Frequency i 2 23 108 167 106 33 7 i

Percentage of sugar in 40,000 beets from Naarden (DE VRIES, 1901 a, p. 74): —

% sugar 12 I2| 13 i3| 14 i4| 15 15!
Frequency 340 635 1192 2205 3597 5561 7178 7829

% sugar 16 i6| 17 17! 18 i8| 19
Frequency 6925 4458 2233 692 133 14 5

The data given in this last example are also expressed in the form of a
curve in Fig. 114. It should be noted that curves with seyeral maxima occur,
and also half curves as well, but these cannot be further discussed.

We have now to discuss what is the relationship of these individual varia-
tions to species formation. First of all, we must note that there is a very close
connexion between such variations and the mode of formation of agricultural
breeds or races by selection. Thus, in the sugar-beet industry, the selection
and employment for propagation of the seeds of those plants which are richest
in sugar leads to a marked rise in the general average of the sugar percentage in
the beet. Fifty years ago the percentage was 7-8 per cent., now it has been
raised to about 15 per cent. In the same way, by rigorous selection, races
may be produced which will exhibit especially large flowers or fruits, better
flavour, increased succulence, &c. So far as we know, no new characters arise
spontaneously, although those already existent may be added to or reduced.
The limits of such variations are usually reached in a few generations (3-5) ;
further selection merely serves to fix the character which has been acquired. It
must be specially noted, however, that such characters are liable to fade away
quite as rapidly as they appear ; after a few generations, if selection be dis-
continued, the original condition is reverted to. Herein lies a great distinction

c c 2

388

METAMORPHOSIS

between artificially produced races and natural species : the former are transi-
tory, the latter are constant.

It has been said above that the most important point about a variation is
whether it is hereditary or not. Are these individual variations hereditary or
not ? This is a question by no means easy to answer. The beetroots, from
which the graphic curve given at Fig. 114 was taken, arose from seed, which
had been obtained from plants possessing 16-18 per cent, of sugar, and yet by
far the majority of them have less than 16 per cent., and individuals them-
selves differ very greatly. We have to deal here only with a ' partial ' in-
heritance, since only some of the characters of the parents reappear in the
offspring. On the whole, this is probably not a case of heredity. Our posi-
tion in relation to this question will be more secure after we have successfully
determined the factors which induce individual variations. Meanwhile, we may
suggest a hypothesis which may be considered as highly probable. Fluctuating
variations must arise from irregularities in growth conditions (nutrition in the
widest sense of the word) which must arise even in the most carefully conducted
experiments. [Compare KLEBS, 1903.] Indeed, one cannot prevent one plant
from taking up more water or mineral constituents from the soil or absorbing
more light than another ; similar organs on the same axis must be differently

nourished according to their position,
for it may be readily proved that the
flow of nutritive materials to a terminal
organ cannot be equal to that which
reaches a similar organ in a lateral
position. The lateral inflorescences of
the sunflower, for instance, become
considerably larger when the terminal
inflorescences are removed, and many
examples of like character might be
quoted in support of this view.
When one flower is better nourished
than another larger seeds will natur-
ally result, as also larger embryos

wou

7000
6000

sooa
woo

3000
2000

1000
500
250

/

\

\

1

1

V

1

1

1

\

1

5

1

1

5

\

.

\

Percentage
of Sttgar.

12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
12-5 13-5 14-5 15-5 16-5 17-5 1S'5

Fig. 114. Percenta
pressed graphically.

e of sugar in 40,000 beets, ex-
)EVRIES (Mutationstheorie, I).

and more abundant reserves. Again,
a plant which has arisen from a large
seed will in turn tend to grow more
vigorously, produce a greater number

of seeds, and develop greater quantities of sugar. If individual variations
be really due to nutritive influences only, we cannot speak of an inheritance
of such peculiarities in a single individual, and, at the same time, readily
account for the rapid disappearance of these characters when selection is dis-
continued. Further, it is conceivable that selection may be aided or even re-
placed by good manuring. [As the result of JOHANNSEN'S (1903) researches it is
clear that even races which breed quite true are always the result of the mingling
of several forms differing from each other in minute but constant characters.
If we start from the offspring of a self-fertilized plant such offspring also exhibit
differences among themselves, which, when plotted out, give a graphic curve
such as that shown in Fig. 114. If from among such forms it be desired to
produce a breed with a certain definite character, and if we select for that
purpose seeds for a third generation from plants standing at the very beginning
or the very end of the curve, it will be found that the two sets of plants result-
ing present no differences ; in fact, selection, in this case also, is quite useless.
If selection appears to have a different effect in the other experiments already
spoken of, it must be assumed that we are dealing in these cases not with indi-
vidual variants but with mutation phenomena. Compare also CORRENS, 1904.]

VARIATION. ADAPTATION. ORIGIN OF SPECIES 389

Just as the first type of variations may be referred to the influence of
external factors, so also the second type, viz. adaptive variations, may be re-
garded as closely related [DETTO, 1904]. By adaptive characters we understand
variations, many of which (p. 391, under 4) are induced by external factors,
changes which are characterized by some quite special feature, and which may
be considered as a purposeful reaction. Examples of such reactions are to be met
with everywhere, and many instances have already been given of these in Lectures
XXIV and XXV. The external factor in these cases acts as a stimulus, and
the reaction consists in a morphological or anatomical alteration, calculated to
render the plant more capable of making the best use possible of its surroundings,
or of protecting itself from injurious influences in its environment — as we may
term the sum total of the external factors which affect it. The capacity for so
adapting themselves is possessed by different plants in very varying degree,
and hence some are able to thrive in the most varied situations, whilst others
are injured, in some cases fatally, by extremely slight deviations from optimal
conditions. Even the most adaptive of plants have, however, their limitations,
for although amphibious plants can live in water as well as on land, there is
usually in the long run a certain minimum and a certain maximum degree of
dampness which may not be exceeded; in other words, amphibious plants cannot
on the one hand become aquatics nor on the other xerophytes. According to
MASSART (i902),however,Po/ygowww amphibium may assume axerophilousform
and structure. It is impossible to say how this widely distributed capacity to react
adaptively to external stimuli has arisen, but it must be assumed that external in-
fluences have been operating on the plant world in the same way and in no greater
variety ,f or thousands of years, and that, in thestruggle for existence, those plants
which failed to react by adapting themselves in this manner succumbed. In other
words, the external stimuli at first resulted in reactions, some of which were
adaptive and some not, and by natural selection only those which responded
in a suitable manner remained in existence ; the offspring of these plants would
inherit such peculiarities and so gradually the power of adaptation would become
fixed by heredity. These are hypotheses, however, into which we need not go
any further. But we must emphasize the fact that by no means all stimuli
induce purposeful reactions. The gall, for example, is of service only to the
insect, but is highly disadvantageous to the plant ; we must assume indeed,
by way of explanation, that the insect succeeded in deluding the plant, so that
instead of treating the insect as an enemy and an intruder it behaved towards it
as if it were a bit of itself. Under other conditions, also, we meet with non-
purposeful reactions, such as those which result from the application of an un-
wonted stimulus to which the plant is not subjected in a state of nature, and to
which it has had no opportunity of adapting itself. We know of no cases of
alteration in form which would serve as examples of such reactions, but illustra-
tions frequently occur, especially in the phenomena of movement, as when a
bacterium is attracted by ether, which is of no service to it, and is not repelled
by corrosive sublimate, which is fatal to it ; or when a root bends towards
light, and a tendril refuses to curve round a stick smeared with gelatine. We
may well believe — to select the last case — that the tendril would grasp such a
support if it had often the opportunity in nature of meeting with supports
possessing a gelatinous surface.

As to the causes inducing this adaptive capacity perhaps the best sources
of information are studies on unusual and artificial stimuli, especially as they
have been systematically investigated, and, further, since it is very doubtful
whether it is possible to study a ' natural ' stimulus of any kind which the plant
is not already acquainted with. GOEBEL (1898) has drawn attention to the
response given by Cardamine pratensis, which, according to SCHENCK (1884),
can produce a typically aquatic form, although it usually occurs on land ; but

390 METAMORPHOSIS

in the case of meadow plants, which are not infrequently subject to inunda-
tion, the special adaptation to aquatic life may have arisen a long time before
the capacity for adaptation had come to be inherited.

Hitherto we have dealt with the so-called ' active ' adaptations only, but
there are also other adaptations which may be termed ' passive '. There are not
only plants which are able to adapt themselves, but also others which are adapted,
which exhibit a series of peculiarities which permit them to live under certain
environmental extremes. In addition to the genuine aquatics we have also
hygrophilous types (such as the Hymenophyllaceae), the numerous xerophytes,
and halpphytes (or salt plants), and a general consideration of such passive
adaptations leads us to the conclusion that these have arisen by a hereditary
fixing of active adaptations. In many liverworts we find that the shape
of the thallus is dependent on light, for it remains narrow in light of limited in-
tensity and broadens as the degree of illumination increases, thus exposing a
maximum surface at right angles to the incident ray. The flattened form of
the green assimilatory organ has, as we know, a definite purpose. In the lower
plants this is a case of active adaptation, whilst in the leaf -blades of the higher
plants the adaptation has become fixed by heredity. The same is true of the
roots of many epiphytic orchids, where in many species these organs become
flattened when exposed to light, although in other cases the flattening is always
present, even when the roots are grown in the dark (GoEBEL, 1898). Numerous
examples of the same kind might be quoted, especially in relation to the phe-
nomena of dorsiventrality and polarity, phenomena which — although, perhaps,
they should not, strictly speaking, be classed among adaptations — may be re-
ferred to here because they can in many cases be readily shown to be due to
external factors such as light and gravity, although in other cases they have
arisen without such stimuli. Thus VOCHTING (1886) showed that the flowers of
Epilobium angustifolium and of Hemerocallis fulya owe their dorsiventrality to
gravity and that they become radial when this unilateral stimulus is with-
drawn. Dorsiventrality appears in Amaryllis formosissima under all conditions,
however, and gravity merely renders the dorsiventrality more intense.

As has been already said, a general comparison of examples compels us to
believe in the derivation of the passive from the active adaptations, hence
these active adaptations must be capable of transmission, and yet experiment
does not confirm this conclusion. Plants which have lived in the high Alps for
thousands of years, and which have adapted themselves to their surroundings
by taking on very characteristic forms, lose all these peculiarities when they are
cultivated in the plains below. Conversely, lowland plants transplanted to an
alpine habitat take on an alpine form but lose the adaptations which they thus
acquire when once more brought back to their original home (BONNIER, 1895).
In the same way, in cases where adaptations have been induced experimentally,
it is found that these are in no sense permanent, and that the seeds of plants
which have been cultivated for a long time under exactly similar external con-
ditions still retain complete power of adapting themselves. The gap between
practice and theory can at present be bridged by hypotheses only. Perhaps an
active adaptation induces a certain effect on the idioplasm, so that it disposes
it to repeat more readily this adaptation than any other. The initiation and
disappearance of adaptations take place often not in one generation but in the
course of several; the influences which have been operative in the first generation
obviously have still some effect in the second, and, if that after-effect be com-
bined with the new influences, at first there will be only a partial inheritance, and
a complete transmission only after several generations. We are also acquainted
with changes in the plant which outlast the stimuli which induced them, and
we have, in our discussion of periodicity, recognized after-effects, which may be
compared to a certain extent with the partial transmission postulated above.

VARIATION. ADAPTATION. ORIGIN OF SPECIES 391

But we can scarcely rest satisfied with this hypothesis only ; we must also assume
a definite capacity in the plant of establishing certain adaptations by heredity, and
this capacity is certainly not universally possessed. To this view, however,
scarcely any exception can be taken, since it is a well-known fact that the
capacity for adaptation is especially pronounced in some circles of relationship
whilst it is absent from others. This is shown very strikingly by the way in
which some families among higher plants tend to a parasitic or carnivorous habit
while others exhibit no such leaning. Similarly, a tendency to inherit an adapta-
tion to an aquatic or to a xerophytic life may exist here and there, whilst other
plants, perhaps, have, on the contrary, lost that adaptive capacity. Two prin-
ciples, in a certain degree antagonistic, make themselves evident here, the one
aiming at making the plant as many-sided as possible and so permitting it to
find a footing in numerous different situations, the other making it one-sided
but also gifting it with the capacity of adapting itself to one extreme condition.
If all organisms made exactly the same demands on the environment their
continued existence would be a mere matter of days. The correctness of this
conclusion has already been emphasized elsewhere (Metabiosis, Lecture XIX).
Weighty objections to the inheritance of acquired characters have been
advanced in the animal world, especially by WEISMANN. He (1892) regards
adaptations as acquired characters, and tries to show that inheritance of these is
theoretically impossible and has never been established practically. This view
of acquired characters is, in the first instance, based on a study of the animal
world, where there is frequently a sharply marked demarcation between germ and
somatic cells from the very commencement of the divisions in the egg-cell. The
peculiarities which occur in the somatic regions induced by external influences
or functional stimuli are regarded as acquired, or originating during the life of
the individual, in contrast to those which are inherent, that is, whose initials
were already present in the ovum. We may distinguish four types of acquired
characters: (i) mutilations; (2) diseases; (3) adaptations to external conditions ;
(4) functional adaptations. Botanists and zoologists are agreed as to the
non -inheritance of changes which are induced by mutilation and disease. That
there is no direct evidence oi the inheritance of adaptations due to external factors,
was admitted above. The same is true of functional adaptations ; VOCHTING
(1899) has made many experiments on the subject and has established beyond
all question that such adaptations are not hereditary. The question comes to be,
whether any fundamental considerations can be advanced against the assumption
of their transmissibility. That would be the case if we conceived of the origin of
the adaptations in the individual as the zoologists often appear to do ; for they
assume that any alterations in the somatic-cells must be appreciated by the germ-
cells, but that can scarcely occur unless by a transference of idioplasm from the
somatic to the germ-cells. Such an assumption (Pangenesis, DARWIN, 1868)
verges too near to empiricism, and it would appear to us that such an idea is
not only unessential for the explanation of the phenomena as presented by the
vegetable kingdom, but is in itself quite incorrect. Let us study a single example
of adaptation in the plant. If we place a land plant in water we do not find
that leaves already present change their shape and structure, but die off
just because they no longer possess the power of adapting themselves to
their new surroundings ; on the other hand, adaptations appear in the quite
embryonic leaf-initials, close to the growing point, where germ-plasm or idio-
plasm is much more abundant than in the full-grown parts. We find, that is to
say (and this is of general significance), that the adaptation does not take place
in the soma proper, but in the growing point. It is from the growing point,
however, that the reproductive cells are also derived, and they are able to receive
adaptive impressions without the inexplicable transference of a material basis
from the soma. Certainly we must assume the transference of a stimulus, inas-

392

METAMORPHOSIS

much as the growing point is not itself in contact with the water and can only be
indirectly effected by it (p. 339). Similarly, in other cases, as when leaves take
on forms adapted to light or to shade, the relationship of light to the growing
point enclosed in its scales must be the same in both cases, although a quite
different type of leaf is differentiated in the bud of the shaded branch from that
of the illuminated one. Experimental investigations on this subject are, how-
ever, much required. As NORDHAUSEN (1903) has shown, the characteristic
anatomical differences between light- and shade-leaves of the beech are already
established in the bud, and the light relations concerned in the unfolding of the
bud play only a limited part in the process. If we arrange that the initials of
the light-leaf are allowed to develop in the dark, the typical double row of pali-
sade-cells (Fig. 115, /) is retained, whilst the shade-leaf retains its own characters,
although it be developed in bright sunlight (Fig. 115, //). The growing point,
in this case, undergoes adaptation, and the effect outlasts the stimulus, so that
one may readily conceive how hereditary races may in this way come into exist-
ence. As a matter of fact, this is not the case in the beech ; further research is
needed to show how far the after-effect is continued, whether the branches
which have been exposed to light for ten years form
leaves of the light-type, after shading, longer than those
which have been exposed to intense light for one year.

Let us ask finally how functional adaptations be-
have in regard to inheritance. These naturally cannot
be clearly demarcated from those we have just spoken of,
and so our knowledge with regard to them is even more
limited. The functional adaptations with which we are
acquainted are, for the most part, the result of long
past influences, and we may well assume that they also
first made themselves apparent at the growing point.
Where that is not the case, as in VOCHTING'S experi-
ments (stem tubers of Boussingaultia, leaf tubers of
Oxalis), no transmission can be determined. WINKLER
(1902) has recently drawn attention to a case of great
interest in this connexion. He observed floral-leaves
and styles of a chrysanthemum become green after
flowering was over, and exhibit all sorts of anatomical
alterations, which were functional adaptations per-
mitting assimilation to be carried out. We do not know whether these
malformations were transmissible and we are ignorant as to their causes, nor
can we assert that they were first operative in the full-grown organ.

The ideas which we have formed as to the inheritances of acquired charac-
ters may be more briefly expressed. We may say that there are no acquired
characters in the sense indicated above. The characters do not appear in the
soma generally (G6TTE, 1898) but at the growing point, and so far the possibility
of their inheritance is granted. In lower organisms, which, as a rule, show no
differentiation of somatic and germ plasma, it is quite obvious how the effect
of external influences may be inherited (and several observations have been made
on the subject), but perhaps no special adaptations may occur in such cases, since
the effects produced are of no service to the organism (for literature, see PFEFFER,
Phys. II, 242). We have succeeded, by special means, in developing certain
races of Bacteria which have permanently lost the power of producing colouring
bodies or certain special poisons. We have also been successful, by prolonged
culture at high temperatures, in destroying the power of forming spores in
various Saccharomycetes. The point which is characteristic in this process is
the gradual fixation of the loss ; at a definite high temperature the formation of
spores ceases, but the capacity for forming them returns when the temperature is

Fip. 115. Transverse sec-
tions through leaves of a copper
beech, /, pnmordium of a light-
leaf developed in the dark, II,
of a shade-leaf developed in a
bright light. After NORD-
HAUSEN (1903).

VARIATION. ADAPTATION. ORIGIN OF SPECIES 393

again reduced, and it is only after long-continued exposure to high temperatures
that a constantly asporogenous race is formed.

Let us now turn to the third type of variation, viz. mutations. These have
come into special prominence of late years owing to the labours of DE VRIES
(1901) and KORSCHINSKY (1901). By mutations we understand variations
which, once initiated, are thereafter quite constant. Mutations are variations
which appear spasmodically, one or more characters of a species suddenly
exhibit alteration or appear ab initio, and these may show themselves for the
first time in a plant which has arisen from a seed or a single bud. Let
us consider first of all a few examples of malformations which appear to be of
the nature of mutations. Look at the plants which possess branched leaves.
The oldest example is Chelidonium laciniatum, a form of Chelidonium majus
which suddenly made its appearance at Heidelberg in 1590, a form possessed
of branched leaves, both floral and vegetative, and which has constantly
reproduced these characters by seed from that day to this. The double flowers
which occur in many plants have arisen by mutations, as also cases of
fasciation, best known in the commonly cultivated plant Celosia cristata.
Further, very characteristic examples are seen in the rayless Compositae
e.g. Matricaria discoidea, the thornless varieties of plants which usually
possess thorns, e.g. on the fruit (Ranunculus arvensis inermis, Datura tatula, &c.).
Again, we have the notable case of Capsella heegeri, which was found growing
wild near Landau, and which has been shown (SoLMS-LAUBACH, 1900) to be
a mutation of Capsella bursa-pastoris. As an example of bud variation equally
well known we may select the case of Sedum reftexum, recorded by WETTSTEIN
(1900) as having been found near Prag, which showed fasciation on a single
lateral branch. This branch, when cultivated, flowered, and from the seed
arose again magnificently fasciated examples.

The reason why mutations are of so much importance for the theory of the
origin of species is especially because, in addition to the constancy of the new
character, the innovation is in no sense an adaptation but entirely a mark of
organization. The mutations cited, however, differ certainly in one charac-
teristic only from the parent species, such as we find in those natural forms
termed by many authors varieties (e.g. white-flowered varieties), but not in ' ele-
mentary ' or sub-species, which usually exhibit differences in all characters. The
direct observation of a new sub-species by mutation is to be considered as a great
advance in our knowledge, and this observation we owe to DE VRIES (1901 a).
He cultivated many specimens of Oenothera lamarckiana, an American immi-
grant into Europe, and now partly naturalized there, and was able to establish
the occurrence of many mutations which differed from the type in many or in all
characters. The offspring arising from one seed were constant in exhibiting the
new character or characters. We shall limit ourselves here to the consideration
of one example only, viz. Oenothera gigas. Let us hear what DE* VRIES (1901 b)
has to say on the subject : — ' It is of the same height as the parent species, but the
stern is thicker, the leaves are more numerous, the corolla is more widely opened,
and the buds are much thicker (compare Figs. 116 and 117), the fruits are only
half as long as the fruits of the parent, and they contain fewer seeds. The seeds
are larger, rounder, and heavier. This type arose in my garden in the year
1895 as a solitary case amongst 14,000.' By preventing any crossing, pure
seeds of this type were gathered in 1896. 'These were planted in 1897. As
soon as the third and fourth leaves had unfolded, the differences appeared. All the
young seedlings were stronger and more fully clothed with leaves and darker in colour
than the parent. There were several hundred such, but obviously all of one type
only, and as in the course of the summer, first the stem and then in succession
the leaves, the buds, the flowers, and, finally, the fruits showed themselves, all
doubt was removed that a new and constant species had made its appearance.

394

METAMORPHOSIS

Taking origin from a solitary example, 0. gigas forthwith appears with constant
and pure seed ; arising at a bound from the parent form, in a moment a
new species sprung into independent existence. In this way also the other
species I have described arose, suddenly and without any transitional forms/

There are a whole series of questions which are closely connected with this
observation of DE VRIES, and of these we can touch on only a few of the more
important. First of all, how can a sub-species arising from a single specimen main-
tain itself in nature ? If we assume that crossing with the type takes place, the
species would soon disappear, unless its characters were absolutely predominant ;
further, no subsequent segregation is possible, as in the case of the hybrid peas
quoted previously. That something of this kind may happen is shown by
the experiences of the American farmer with the Ancon sheep. There was born

Fig. 116. Oenofhtra lamarckiana. Apex of
a shoot at the commencement of flowering; b,
withered flower lying on the subtended leaf. After
DE VRIES (1901).

Fig. 117. Oenothera gigas ; derived from O. la-
marckiana in 1895. Apex of the shoot at the com-
mencement of flowering; at a, a petal has been re-
moved ; *, withering flower. After DE VRIES (1001).

in Massachusetts in 1891 a lamb with the form of a dachshund. In spite of
cross-breeding with the parental stock the Ancon type still predominated, and
the race increased without any selection at all, artificial or natural (DARWIN,
1868). If we assume self-fertilization, a maintenance of the new species can
only take place if it be more dominant than the parent species, and gradually
drives it out. The gregarious occurrence of closely-related species would, under
these conditions, be difficult to explain. Such being the case, another possibility
must be admitted. DE VRIES observed, in the case of the new species of
Oenothera, that they arose not once but several times from the parent stock.
It seems probable that the old species produced the new ones in ever-increasing
numbers. We certainly cannot advance any proof of this assumption, but it
may be supposed that the originating of a mutation induced besides an internal

VARIATION. ADAPTATION. ORIGIN OF SPECIES 395

alteration in the mutating organism. The new characters must, as NAGELI
(1884) acutely pointed out, be firmly fixed and ready as initials which are at first
latent, and later on unfold themselves. When an initial once finds the way to
complete development then it may go on elaborating in ever-increasing degree.
In this way also certain new characters would behave just like certain ancestral
characters which make their appearance as so-called ' reversions ' whenever op-
portunity offers, and which generally are transmitted in the latent form. We
know as little about the causes which lead to their appearance as we do of the
origin of mutations. All we can say is that the causes appear to be internal,
which is equivalent to saying that we know nothing about them.

If then, as DE VRIES thinks, the mutation, and not the individual variation,
is the factor concerned in the origin of new species, then selection must have
a significance quite different from that attributed to it by DARWIN. According
to DARWIN, individuals are for ever engaged in a struggle for existence, and new
species arise by selection of the varieties best equipped for the struggle. Accord-
ing to DE VRIES, however, fully developed species come into conflict, but the
origin of these species by mutation is not thereby explained. We have certainly,
if we accept DE VRIES' s view, a ' Mutationstheorie ' but we have absolutely no
theory of mutation itself.

The formation of species farther apart out of the fundamental species is,
according to DE VRIES, easily understood, since
many 'petites especes' disappear in the struggle £"x
for existence. Of course, the mutation may be \\^
so great that a new genus or a new family vsf
may come into existence. Capsella heegeri, m

for example, would scarcely have been placed v

in the genus Capsella were it not that its origin
was known. It is also quite possible that whole
genera and families may be referred back in their ng z hic flowers of

chief features to monstrosities. HILDEBRAND Fuchsia. /, lateral view. //, ground
(1899) has found Fuchsia to produce zygomor- Plan- After HILDEBRAND (1899).
phic flowers arising by mutation (Fig. 118), so

that the allied genus Lopezia may have arisen from a malformation (Compare
SACHS, 1893).

HOFMEISTER long ago (1868, p. 564) ascribed to mutation a very prominent
part in the formation of species. He said : ' New forms do not come into
existence by the summation in successive generations of small differences from
the customary form, all tending in the same direction ; they appear suddenly,
and are widely different from the parent type.' It may well be that, as our know-
ledge increases, the distinction between the different types of variation may be
broken down ; still it is certain that mutability will remain the chief if not the
only factor in species formation. That species may also owe their origin to
individual variation, in the extreme way suggested by WALLACE, appears to us
practically out of the question, but that they may rise by hybridization is, in
individual cases, quite established, and that they may owe their origin to adapta-
tion is especially true of those species (biological species) recently recognized
among parasitic Fungi. Unfortunately, space will not permit of our discussing
these forms. [Compare KLEBAHN, 1904, and DETTO, 1904.]

In this lecture we have not been able to give more than the briefest sketch
of the theory of the origin of species, and have been compelled to omit considera-
tion of very many most important observations and views. We must, therefore,
refer to the special literature on the subject, in which, it is true, some one * theory '
frequently predominates. The task of the future is less to establish new theories
than carry out systematic observations and experiments. General summaries
of the Theory of Descent will be found in the following works : — WALLACE, 1891 ;

396 METAMORPHOSIS

WEISMANN, 1902 ; REINKE, 1901 ; WETTSTEIN, 1903 ; [PLATE, 1903 ; ERRERA,
1904 ; LOTSY, 1906].

Bibliography to Lecture XXX.

BONNIER. 1895. Annales Sc. nat. VII, 20, 217.

[CoRRENS. 1904. Archiv f. Rassen- u. Gesell-Biologie, i, 2.]

DARWIN. 1860. Die Entstehung der Arten (German ed. by BRONN from the

2nd Engl. ed.)

DARWIN. 1868. Das Variieren d. Tiere u. Pflanzen. German ed. by CARUS.
[DETTO. 1904. D. Theorie d. direkten Anpassung. Jena.]
[ERRERA. 1904. Une Ie9on elem. s. 1. Darwinisme. Bmxelles.]
GOEBEL. 1 898. Studium u. Auf f assung der Anpassungserscheinungen bei Pflanzen.

Munich.

GOTTE. 1898. Vererbung u. Anpassung. Rectorial Address, Strassburg.
HILDEBRAND. 1899. Bot. Centrbl. 77, 177.

HOFMEISTER. 1 868. Allgem. Morphologic d. Gewachse. Leipzig.
[JOHANNSEN. 1903. Erblichkeit in Populationen. Jena.]
JORDAN. 1873. Rem. sur le fait de 1'existence en societe a 1'etat sauvage des

especes vegetales affines. Lyon.

[KLEBAHN. 1904. Die wirtwechselnden Rostpilze. Berlin.]
KLEBS. 1903. Willkurliche Entwicklungsanderungen. Jena.
KORSCHINSKY. 1 90 1. Flora, 89, 240.

[LoxsY. 1906. Vorles. iiber Deszendenztheorien. I. Jena.]
LUDWIG. 1900. Bot. Centrbl., Beihefte, 9, 89.
[MASSART. 1902. Bull. d. Jardin botan. d. Bruxelles.]
NAGELI. 1884. Theorie der Abstammungslehre. Munich and Leipzig.
NORDHAUSEN. 1903. Ber. d. bot. Gesell. 21, 30.

[PLATE. 1903. Ueber die Bedeutung des Darwinschen Selectionsprinzips.]
REINKE. 1901. Einleitung in die theoretische Biologic. Berlin.
ROSEN. 1889. Bot. Ztg. 47, 565.
SACHS. 1893. Flora, 77, 234 Note.
SCHENCK. 1884. Ber/d. bot. Gesell. 2, 479.
SOLMS-LAUBACH. 1900. Bot. Ztg. 58, 167.
V6CHTING. 1886. Jahrb. f. wiss. Bot. 17, 297.
VftcHTiNG. 1898. Ibid. 31, 391.
V5CHTING. 1899. Ibid. 34, i.

DE VRIES. 1901 a. Die Mutationstheorie, i . Leipzig.
DEVRIES. 1901 b. Die Mutationen u. die Mutationsperioden. Leipzig.
WALLACE. 1891. Der Darwinismus. German ed. by BRAUN. Braunschweig.
WEISMANN. 1892. Ges. Aufsatze iiber Vererbung. Jena.
WEISMANN. 1902. Vortrage iiber Deszendenztheorie. Jena.
WETTSTEIN, R. v. 1900. Ber. d. bot. Gesell. 18, (184).
WETTSTEIN, R. v. 1904. Der Neu-Lamarckismus. Jena.
WINKLER, H. 1902. Ber. d. bot. Gesell. 20, 494.

PART III
TRANSFORMATION OF ENERGY

LECTURE XXXI

FORMS OF ENERGY IN THE PLANT

SIDE by side with the law of conservation of matter, one aspect of which we
have learned to recognize in the circulation of eleihents in the organic world,
we place the law of conservation of energy. According to this law the sum of
the energies of the universe is a constant quantity ; energy cannot be created or
destroyed, it can only be transformed. Thus, for example, heat energy can be
transformed into mechanical energy, or electrical energy can be transformed into
radiant energy. No special evidence need be adduced in proof of the fact that the
laws of conservation of matter and of energy are applicable alike to the living
and to the non-living world. The task before us now is to follow the transforma-
tion of energy, just as, in Part I, we studied the transformation of matter ; that
is to say, we must attempt to answer this fundamental question, viz. ' Whence
does the plant obtain its energy, and what does it do with it ? ' Of course, this
is not the first time we have encountered this problem, for in speaking of meta-
bolism it was impossible to avoid mentioning the question of the transformation
of energy which is inseparable from it. Separation of the treatment of the
subject of transformation of matter from that of transformation of energy is
merely a matter of practical convenience ; in nature they are bound together in
the most intimate manner.

Thus we had to draw attention to the fact that light was essential to the
assimilation of carbon in the green plant, or, more exactly, that light energy
becomes transformed into chemical energy, which latter reappears as potential
energy in the products of assimilation. It has also been noted that sunlight
is the most important source of energy for all living things, since those plants
and animals which are unable to make use of sunlight directly with the aid of
chlorophyll, are compelled to absorb the products of assimilation of green plants,
and thus acquire the energy of sunlight indirectly. Further, it has been already
shown that not only light energy but also, in many cases, chemical energy,
obtained by the absorption of materials (nutriment), is a primary source of
energy in the plant. Nutritive substances are employed, as we have already
seen, only to a small extent in the actual manufacture of the organism ; the
greater part is again broken down in the course of respiration or in other related
processes, and thus the chemical energy released in the decomposition of more
complex compounds is turned to advantage in the general vital phenomena of
plant life. In the process of nutrition, however, the plant acquires other forms
of energy as well, which operate quite independently of the energy evolved in
metabolism. Thus electric energy, osmotic and surface-tension energy or energy
of crystallization of the included substances must be recognized, and these, with
the exception of electricity, play, as everyone knows, important parts in the plant
economy. It is not at all likely that the plant is able to make any use of
electrical or mechanical energy, absorbed as such from the environment and
not associated with actual materials, at least we are quite ignorant of such
cases ; on the contrary, we know that the plant thrives perfectly well without

398 TRANSFORMATION OF ENERGY

such supplies of energy. It is quite otherwise with heat. It has akeady been
shown that all plant activity is conditioned by certain definite limits of tempera-
ture, but it does not follow that the heat of the external medium forms a
source of energy to the plant. There can be no doubt that the plant does
absorb heat from the exterior in cases where it exhibits a lower temperature
than the outer world, a condition of things by no means rare ; and if the plant
absorbs heat under such conditions obviously its energy must be increased.
We need not dwell on this fact, for it is self-evident ; what we must inquire
into is whether the plant requires such an addition of heat energy from without,
but to this question we can give no conclusive answer ; the probability is that
it does not require it.

Probably, all the energy required by the plant is obtained in the form of
light and nutriment. The law of causality compels us to believe that all the
energy of the plant has entered it from without, for we cannot conceive of the
plant, any more than of non-living matter, creating energy within its own body,
so that what we have to investigate is the way in which the energy which
enters the plant becomes altered within it. Just as chemical compounds occur
in the organism which are manufactured in it only, so there may exist in it
forms of energy which occur nowhere else. Such specifically organic forms of
energy are, however, unknown. It must be confessed, however, that, we know
nothing of energies peculiar to the organism. Even as to the changes which
occur in the energy entering the plant we know but little. Since only the
final stages in the process which manifest themselves externally are accessible
for investigation, we are confined to guesswork as to the transformations
taking place within. Among the final stages visible to us the most important
is mechanical energy. The movements which the organism, in whole or in
part, exhibits are obviously those most prominent and hence also most studied.
The production of heat must also be noted as a phenomenon of very wide oc-
currence, as to the meaning of which, however, we know very little, though we
are more conversant with its causes. In addition to heat, the production of
electric currents and of light in plants must be referred to, phenomena which,
as yet, can only be said to play a subordinate part in plant physiology.

This third section of our lectures must deal almost exclusively with the
phenomena of movement, but before we commence their study we may briefly
refer to the other manifestations of energy met with in the plant, viz. heat, light,
and electricity.

The temperature of the plant conforms, generally speaking, with that of the
exterior medium, sometimes it gives off heat to the environment, sometimes it
absorbs heat from it ; it is destitute of those special contrivances which are
found in warm-blooded animals for maintaining a constant temperature inde-
pendent of variations in the temperature of the surroundings. The temperature
of the plant may be lowered beneath that of the surroundings by radiation and
also by transpiration. If we desire to demonstrate the production of heat in the
plant it will be necessary for us to retard radiation and transpiration more
especially, also to prevent loss of heat by conduction. It will also be necessary
to prevent gain of heat by the plant, especially by insolation. Merely by
repressing transpiration one may often cause a plant organ, previously exhibiting
a temperature below that of the air, to attain a temperature considerably above
it. This may be accomplished most easily by selecting for experiment a massive
organ with relatively small superficial area, or by heaping together smaller parts
and surrounding them with a bad conductor. Thus the temperature of many
inflorescences exceeds that of the air very considerably, often as much as from
5° to 10° C. If germinating seeds, growing points, or flower-buds be collected
in a flask, and surrounded by a bad conductor, and if special care be taken that
oxygen gas can enter in sufficient quantity, considerable rises in temperature

FORMS OF ENERGY IN THE PLANT 399

may easily be obtained ; on the other hand, if these structures be dead, and if
micro-organisms be excluded, no such rise of temperature is observable even
though all the other conditions be maintained. If it be desired to measure
minute differences of temperature in individual organs the best method to
employ is the thermo-electric pile, viz. copper and iron wires in the form of
needles, soldered together, and varnished over, the one stuck into the part to be
investigated, the other being in the air or in another organ for comparison ; the
apparatus is connected with a galvanometer, from the movements of whose
indicator the difference in temperature between the needles may be readily
determined (DUTROCHET, 1840).

This method is merely qualitative and shows whether heat is developed or not ;
the amount of heat produced can be estimated only by minute calorimetric
investigations, the carrying out of which presents many difficulties. According to
G. BONNIER (1893) i kg. of germinating seeds or young seedlings can evolve twenty,
fifty, or even a hundred calories or more per minute. This is a quite considerable
amount, for one calorie is the amount of heat necessary to raise I g. of water
from o° C. to i° C. Both calorimetric and thermometric measurements show
that not only the conditions of the plant itself but also those of the surroundings
play a great part in heat production. We must base our statements on thermo-
metric calculations, for as yet there are very few calorimetric measurements
forthcoming.

The production of heat depends to a remarkable degree on the state of
development of the organ of the plant under investigation ; generally speaking,
growing points and young members produce more heat than the same parts
when mature. Still DUTROCHET (1840) was able in many plants to establish that
an excess of 0-1° to 0-3° C. over the temperature of the air existed in the mature
stem if transpiration were prevented. A similar phenomenon is observable in
mushrooms, whilst in leaves and fruits evolution of heat is generally more re-
stricted. There are, however, organs which when mature exhibit a maximum
production of heat and the highest temperatures of all have been observed in
fully-developed parts of flowers or inflorescences. Indeed, it is often sufficient
merely to feel such organs to be convinced that an evolution of heat is taking
place. By means of a thermometer it has been shown that the inflorescences of
Palmaceae and Cycadaceae and certain parts of the flower of Victoria regia not
infrequently possess a temperature 10° or more above that of the air, while in
the Araceae much higher temperatures have been obtained. Thus KRAUS (1894)
found that a thermometer placed in the large spa dices of Arum italicum gave
a maximum temperature of from 49-2° to 51-3° C., or 33-2° to 35-9° higher than
that of the air. In nature transpiration brings about a marked cooling effect,
since the plant could not for long have maintained such temperatures as were
found. Special adaptations may certainly arise quite generally, for we know
of Bacteria which are characterized by their high maximum temperature and
such forms produce a considerable amount of heat (CoHN, 1893).

Amongst external factors temperature itself is entitled to receive special
attention, since the production of heat by the plant is not entirely independent
of external heat. As in the combustion of carbon, &c., production of heat begins
to take place only after a sufficiently high preliminary temperature has been
attained. At 5° to 6° C. the buds of Aesculus show no development of heat,
but at about 20° C. an excess of 0*63° may be obtained. Germinating wheat,
which at 11° C. gave an excess of 1-1°, at 15° C. showed a rise of 1-4° C. Syste-
matic research on this question is still required, and detailed investigations are
especially needed to determine whether a rise in external temperature above a
certain point produces once more a diminution of heat production in the plant.

Not infrequently the evolution of heat exhibits a certain regular periodicity.
Thus the young inflorescences of Arum italicum show at first about the same

400 TRANSFORMATION OF ENERGY

excess of heat as any other plant organ. The great evolution of heat takes place
on the opening of the spathe towards evening ; this increases rapidly in intensity,
reaching a maximum just before midnight. Next morning the temperature
has again become equal to that of the air and remains so. In Victoria regia
the rise in temperature commences about nine hours before the opening of the
flower, and rapidly increases to a maximum after the flower opens (i. e. towards
evening). Then in the night ensues a reduction in temperature, followed by the
attainment of a secondlesser maximum on the evening of the second day (KNOCK,
1899). Periodicities such as these have been observed in all cases where the pro-
duction of heat continues for some time. The maxima do not always occur ex-
actly at the same hour from day to day, but they occur during the day hours,
before or after noon, apparently never at night. The immediate cause of this
periodicity is naturally closely connected with periodic variations in the en-
vironment, but in what respect temperature operates has as yet not been exactly
ascertained.

The relation existing between respiration and the production of heat is a
close one. The experiment with germinating seeds, referred to above, is successful
only if a sufficient amount of oxygen is permitted to enter the vessel. It has
also been long known that the consumption of oxygen increases pari passu with
the rise in temperature, and that it is excessive in flowers and inflorescences,
which develop large amounts of heat. GARREAU (1857) carried out accurate
experiments on Arum italicum, and demonstrated an almost perfect correspon-
dence between the absorption of oxygen and the rise in temperature. On the
other hand, ERIKSON (1881) showed that on the exclusion of oxygen, and after
in tra-molecular respiration had begun, the temperature scarcely exceeded that of
the air outside. In Arum, for example, he was able to demonstrate in intra-mole-
cular respiration, an excess of only 0-3° over the normal respiratory tempera-
ture of 16-5°, and in Raphanus seedlings 0-2° C. over the normal 5-7° C.
Again, in cases of fermentation conducted under anaerobic conditions, a very
obvious increase in temperature occurs in the fermenting substance. ERIKSON
found that fermenting yeast, under definite experimental conditions, showed a
rise of temperature of almost 4°, while the same yeast gave an increase of only
o'2° when milk-sugar was provided instead of grape sugar ; moreover, no fer-
mentation could be induced. This result agrees with that previously established,
viz. that growth (and we may assert the same of movement also) cannot be
carried on in ordinary plants by intra-molecular respiration only, although, by
fermentation, it is quite possible in anaerobes. When we finally remember that
increase in respiration (p. 202) has also been observed as a result of the action
of traumatic factors (RICHARDS, 1896) we may consider the relation between
respiration and heat production as sufficiently well established.

These relationships can be most readily explained by assuming that respira-
tion and the related process, fermentation, are the sources of the heat evolved. It
has been already clearly pointed out that chemical energy must be released when
organic substances are oxidized or other chemical decompositions are effected,
and this release of energy affords an explanation of the processes under con-
sideration. It needs no proof to establish the view that the energy so released
must in whole or in part appear as heat, since in everyday life we employ this
method for obtaining it. Still, it may be asked whether respiration is sufficient
to account for the amount of heat production which has been observed. This
question has been answered only by BONNIER (1893), who compared the actual
amounts of heat produced with the theoretical amounts arrived at by calculating
the oxygen absorbed or the carbon-dioxide given off. Should BONNIER' s results
prove correct — and it is advisable that they should be confirmed — it would
appear that more heat is actually produced in the germination of many seeds
than can be accounted for by respiration. There are other processes in the plant
which might aid in the production of heat, such as the solution of solids, the

FORMS OF ENERGY IN THE PLANT 401

and friction, e. g. the friction of water against the walls of the vessels. What
part these processes severally play in heat production we do not know, but we
cannot be far wrong in assuming that they are only of secondary importance,
and that respiration is actually the chief factor in the evolution of heat energy
by the plant.

If, however, the total amount of energy in the materials oxidized in respira-
tion, i. e. chemical energy, is released in the form of heat, we must revise our
previous conception of the significance of respiration. Should respiration be
the source of the energy required to maintain vitality, chemical energy cannot
be entirely transformed into heat, otherwise it would be possible to replace the
energy released in respiration by heat energy obtained from external sources,
which, as already seen, is not the case. Other forms of energy must obviously
be produced by respiration, which cannot originate in any other way. Heat is
only a by-product, it might almost be termed a loss of available energy. This
holds at least for the majority of plant organs but cannot be true of all ; mani-
festly, it is not in agreement with the great evolution of heat from many flowers.
In Arum italicum, for example, the spadix is the special organ in which heat
is produced. This spadix, before the flowering-period, consists, according to the
researches of G. KRAUS (1894-5), of about three-fifths water and two-fifths
dry substance, 80 per cent, of which latter consists of carbohydrate. The
carbohydrate is, in the course of a few hours, completely used up and evidently
decomposed into water and carbon dioxide, while the nitrogenous constituents
remain intact. The spadix is, however, a mature organ whose function disap-
pears soon after the flowering-period is over, and which exhibits certain special
activities during this rapid combustion, which we cannot conceive of as
occurring in growing organs. In this case the conclusion cannot be avoided
that the carbohydrate is exclusively employed for the production of heat.
But that all such collections of material are employed for the production of heat,
to the exclusion of all other uses, is a proposition not to be entertained. Hence
one is compelled to believe that the heat evolved in Arum italicum and in
such flowers as produce large amounts of heat is not a useless by-product but
a special adaptation for attracting insects, as DELPINO (1870), and KRAUS
(1894-5) have suggested. We may at least consider heat production in flowers
as a phenomenon sui generis, having nothing to do biologically with heat
production in other organs, although, looked at from a purely physiological
standpoint, the heat is produced in the same way as it is in ordinary respiration
in all plants and all organisms.

Although at present we are, generally speaking, unable to present a com-
plete and comprehensive explanation of the phenomenon of heat production
in plants, still, the way to reach such an explanation is clearly indicated. For
that purpose it is necessary to make comprehensive calorimetric investiga-
tions which alone would afford a basis for conclusions as to the amount of heat
as compared with the total of energy released in the process of respiration.

Imperfect as our knowledge is of the phenomena connected with the
evolution of heat, it is extensive as compared with what we know as to the
production of light by plants. It is true that luminosity is of much rarer
occurrence and hence of less general interest, limited as it is to certain groups of
Fungi and Bacteria [MoLiscn, 1904]. (For literature see VERWORN, 1901).
The phenomenon is closely connected with vital processes, although we have not
as yet been able to isolate from the organism any substance which emits light
rays, although, certain non-living compounds are known to be luminous.
Luminosity cannot be accounted for by a previous storage of light, for it is
quite independent of precedent illumination ; luminous Bacteria and rhizo-
morphs emit light rays in continuous darkness. There can be no doubt that
luminosity bears the same relation to respiration as heat does, for it is manifested
only when oxygen is supplied in abundance. We further know that luminosity

JOST D d

402 TRANSFORMATION OF ENERGY

is associated with low temperatures, and that luminous organisms lose their
power of emitting light when the temperature is raised, eventually losing
it permanently. BEIJERINCK (1890) showed that, in the case of Bacteria,
luminosity was dependent on the presence of certain food-stuffs. Since
these, however, differed in the case of different organisms, it was impossible
to draw any general conclusions on the subject. As to the uses of lumin-
osity in the organisms concerned we know less even than as to its causes ; it
is not therefore worth our while to discuss more fully the detailed observations
which have been made on the subject but at once turn our attention to
the third form of energy which exhibits itself in plant life, viz. electricity.

It has long been known that it is possible with the aid of an accurate galva-
nometer or a capillary electrometer to demonstrate electrical currents in uninjured
plant-organs. If we place non-polarizable electrodes on the leaf of a suitable
dicotyledonous plant, so that one electrode rests on the mesophyll and the
other on the mid-rib, a positive current will, as a rule, be generated which
passes from the mid-rib to the blade of the leaf. The mid-rib is positive
to the leaf -surf ace, as also to the weaker lateral veins. If two points, symmetri-
cally situated so far as the mid-rib is concerned, be connected, no current is
demonstrable, nor does it appear when two corresponding points on a stem are
connected. Even if no current at all be observable in the uninjured plant,
such a current is at once established if the plant be cut or bruised, the electrode
nearest to the wound becoming positive to the one farther away. If the intact
epidermis be connected with the transverse section of the leaf a current is set
up in the direction of the section. If, however, the epidermis be removed and
the exposed surface (or a longitudinal section) be connected with the transverse
the current flows from the latter to the former.

In 1878 KUNKEL attempted to prove experimentally that all electric
currents in the plant were traceable to one cause, viz. the movement of water.
It is quite true that disturbances of electric equilibrium may be occasioned by
streaming of water, and,according to KUNKEL' s theory the interesting phenomena
just described in the uninjured plant are to be explained by the facts that veins
and leaf surfaces are unequally wet, and that when wet electrodes are placed on
such regions different water currents are set up. According to this theory the
electric phenomena observed would have nothing to do with the plant's vitality
but might equally well be manifested by a dead leaf.

KUNKEL' s views have not, however, been able to withstand criticism, and
more recent investigations, especially those of 0. HAACKE (1892) have demon-
strated clearly that the evolution of electric currents in plants is by no means
such a simple phenomenon as KUNKEL would make out. The movements of water
can undoubtedly cause electric disturbances, but they are not the only, or even
the chief, agents in the process. It is possible, as HAACKE shows, to demonstrate
electric currents in leaves of aquatic plants, equally wet all over, and, on the other
hand, the very active transpiration currents are unaccompanied by any electrical
manifestations. On the other hand, electric currents are inseparably bound
up with vital activities, for dead leaves do not show normal electric currents at
all. Moreover, the electric phenomena are intimately connected with respira-
tion, for the currents at once come to an end when oxygen is excluded, while
they are especially prominent in actively respiring organs, such as the inflorescence
of Arum above alluded to. Differences in electric potential are also related to
carbon-assimilation. In non-green organs darkness produces no change on the
current, whilst in green organs the current ceases at once when these organs are
brought into the dark or when carbon-assimilation ceases (compare KLEIN, 1898).
Finally, it may be noted that in plants like Mimosa and Dionaea, which exhibit
active movement as a result of stimulus, the movement is accompanied by
electric currents which are quite remarkable and regular in their character
(MUNK, 1876 ; BURDON-SANDERSON, 1888).

FORMS OF ENERGY IN THE PLANT 403

From the facts cited we must conclude that differences in electric potential
appear in all organs of the plant, wherever adjoining parts exhibit chemical or
physical differences, and such differences may exist between parts of the same
cell or between individual cells or complete tissues.

Detailed information as to the causes of these electric phenomena is not
forthcoming, and we will content ourselves with these scanty remarks, since
not even guesses as to the significance of such electric currents can be made
(compare BIEDERMANN, 1895).

Amongst the varied activities of the plant the production of mechanical
energy is, as we have already remarked, by far the most prominent, and the
movements which are the expressions of the expenditure of mechanical energy
have been much more accurately studied than electric, thermic, or photic
phenomena. The rest of the present course of lectures will be devoted to a dis-
cussion of these movements. We are already familiar with some of these
movements, for when we dealt with the absorption and distribution of materials
we incidentally studied the movements of these substances in the plant, a subject
we dealt with in several lectures in the first section of this work. We have now
to study other movements, e. g. the free locomotion of the lower plants, proto-
plasmic streaming which takes place in a cell and which is analogous to these
movements, and, finally, the innumerable varieties of movement seen in fixed
organs. In all these movements the plant has to overcome resistance, internal
and external and to do work. Without inquiring more intimately at present into
the nature of the various movements we may here appropriately summarize the
information we possess as to the source of the energy used by the plant in carry-
ing out these movements.

We must first of all make inquiry with regard to the chemical energy which
undoubtedly plays an important part in these movements. It is true that the
role is partly an indirect one, in so far as without the chemical energy set free in
respiration it would be impossible to construct the plant or renovate the
apparatus which carries on the movement. But we cannot doubt that the
energy released during the decomposition of food material co-operates directly,
since the endless manifestations of movement are most intimately connected
with respiration and stop short at once when intra-molecular respiration begins
in ordinary plants. Having established that fact we need only add that respira-
tion is an indispensable condition of protoplasmic movement and that it fur-
nishes the energy necessary for it (PFEFFER, 1892). Again, we have learned else-
where to recognize the existence of necessary factors which act as stimuli only. It
is a fact, however, that the majority of stimuli also bring to the organism a cer-
tain amount of energy, but it is characteristic of these stimuli that their energy
stands in no relation to that of the effects they produce. The energy of the
stimulus may be much less or much greater than that of the movement released,
and the latter is certainly not produced from the stimulus but from the stores in
the plant itself. Respiration may also be only a releasing stimulus and it cer-
tainly is so in many cases, though it is probable that it frequently has a direct
energizing significance, or, in other words, that the chemical energy released may
be transformed directly into mechanical energy. It is impossible, however, to
demonstrate this view. In treating of respiration we are accustomed to compare
the energy evolved with that given off in other cases of combustion. In such
cases as, e. g. the burning of wood or coal in a steam-engine, we encounter at
first a transformation of chemical energy into heat, and it is the heat in the
first instance that does the work. In the plant, however, as we have seen, heat
as such, evolved in respiration, plays no great part, since it cannot be replaced
by heat produced by other means. But even though exact proof were forth-
coming that respiration had a purely energizing significance such proof would
still fail to satisfy us in the absence of information as to how the chemical
energy is transformed directly into mechanical.

D d 2

404 TRANSFORMATION OF ENERGY

Under these circumstances it is but natural that more interest centres round
those other forms of energy which may be considered as the immediate causes
of movement, simply because they are more clearly and more readily compre-
hended. It is PFEFFER'S (1892) special merit that he has most exhaustively
catalogued the forces which, apart from respiration, enable the plant to carry
on work. The following are the forces which operate independently of
chemical energy : —

1. The transformation of potential into kinetic energy which takes place
in the balance in tensions, to which may be referred so many of the slinging
movements in fruits, stamens, &c., and which manifest themselves in a variety
of ways, e.g. by unequal swelling of different wall layers, unequal osmotic
pressure of adjacent cells, &c.

2. By the action of osmotic energy which not only brings about movement of
food-stuffs but leads to vigorous pressures and tensions in the plant. The osmotic
energy of a substance is quite independent of its chemical energy, and hence
cannot be derived from its heat of combustion. An example quoted by
PFEFFER will make this clear. If we assume that the osmotic pressure in a
cell is due to glucose dissolved in the cell-sap then we are dealing with a
substance which not only possesses a high osmotic energy but also a significant
amount of chemical energy. Suppose, however, that the glucose be completely
respired into oxalic acid, in that oxidation the cell-sap loses the chemical
energy set free, but at the same time its osmotic energy is tripled, for highly
oxidized bodies possessing little chemical energy may yet exhibit large amounts
of osmotic energy.

3. Quite independently of chemical energy there are various forms of
surface energy, such as swelling and surface tension, to which will presently
be referred many conspicuous movements in plants.

4. ' Form energy,' e. g. cohesion, which should also be referred to here.

5. Finally, we may note the energy of crystallization and secretion, which
frequently plays an important part in the growth of the cell- wall.

The mechanical value of these various forms of energy may often be directly
measured in a variety of ways, and are, as we have already said, for that reason,
more readily comprehended than chemical energy, whose mechanical equivalent
is unknown. We must not forget, however, that although mechanical energy
plays an extremely important part in the plant, it is a great mistake to refer
all the best known cases of movement to the better known forces and to
ignore the others.

This bird's eye view of the forms of energy in the plant demonstrates to us
how far we are from having attained a comprehensive insight into the problem
of the transformation of energy. This is nothing more than what might have
been expected, since in the inorganic world also we have yet much to learn on
the subject. We are not, however, in a position to deny the validity of the law
of the conservation of energy in the organic world.

In the following pages we will attempt to give a detailed exposition of the
various movements manifested by the plant.

Bibliography to Lecture XXXI.

BEIJERINCK. 1890. Mededel. Akad. Amsterdam. Natuurk. II, 7.
BIEDERMANN, W. 1895. Elektrophysiologie. Jena.
BONNIER. 1893. Annal. Sc. nat. VII, 18, i.
BURDON-SANDERSON. 1888. Phil. Trans. 179, 417.
COHN. 1893. Ber. d. bot. Gesell. u (66).
DELPINO. 1870. Comp. HILDEBRAND, Bot. Ztg. 28, 590.
DUTROCHET. 1840. Annales Sc. nat. II, 13, 5.
ERIKSON. 1881. Unters. bot. Inst. Tubingen, i, 105.
GARREAU. 1851. Annales Sc. nat. Ill, 16, 250.

MOVEMENTS DUE TO SWELLING, ETC. 405

HAACKE. 1892. Flora, 75, 455.

KLEIN. 1898. Ber. d. bot. Gesell. 16, 335.

KNOCK. 1899. Bibliotheca botanica, Heft 47.

KRAUS. 1894-5. Abhandl. d. naturf. Gesell. Halle, 16, 35 and 257.

KUNKEL. 1878. Arb. bot. Inst. Wiirzburg, 2, i.

[MOLISCH. 1904. Leuchtende Pflanzen. Jena.]

MUNK. 1 876. Archiv f . Anat. u. Phys. p. 30.

PFEFFER. 1892. Energetik (Abh. kgl. Gesell. Leipzig, 18).

RICHARDS. 1896. Annals of Botany, 10, 531.

VERWORN. 1901. Allgem. Physiologic. 3rd ed. Jena.

LECTURE XXXII

MOVEMENTS RESULTING FROM SWELLING AND CONTRACTION
AND FROM COHESION OF IMBIBITION WATER

MANIFESTATIONS of movement in plants are everywhere apparent to an
attentive observer, but these movements are not all of equal interest to the
physiologist. Our native plants for the most part cast their leaves and even in
some cases their branches in autumn, and these deciduous parts may be carried
away for long distances by wind and water. Examples of a similar kind are seen
in the case of fruits and seeds, but the distribution of these structures differs
in this, that it is useful to the plant, and that it is facilitated by special con-
trivances, e.g. by special wings for distribution by air currents, floats for distri-
bution by water and by hooks for transport by animals. Such movements are,
however, effected without any expenditure of energy on the part of the plant,
they are purely passive movements, very important, it is true, from the bio-
logical point of view, but outside the domain of strict physiology. There are,
however, passive movements which do interest the physiologist, such as the
downward bending of branches by their own weight or, conversely, the straighten-
ing of the branches of submerged plants by water support. In the cell also
we have to take into account the passive movements of chlorophyll bodies, e. g.
in Vallisneria, consequent upon the rotation of the protoplasm, or, in other
cases, where protoplasmic movement distributes the chloroplasts in .definite
situations. Although we have to study in the following pages primarily the
active movements of plant organs, still we must not neglect the purely passive
ones, all the more so since no sharp line of demarcation can be drawn between
the two types. Observation of the gradual bending upwards of a shoot laid
horizontally teaches us that it is due to curvature taking place at a certain dis-
tance from the apex. This movement may certainly be termed active, since the
plant in this case actually does the work, but this work is performed only in a
definite spot, at the point of curvature ; the distal end of the shoot is lifted in
a purely passive manner.

Under active movements we may distinguish two main categories : — (i) loco-
motory movements of entire organisms, met with only among the lower plants ;
(2) movements exhibited by higher plants which grow in fixed positions. This
distinction is not a hard and fast one, since the protoplasm within the cells of
a higher plant moves in precisely the same way as the entire organism does, say
in the case of Amoebae or Myxomycetes ; indeed, in certain cells it may pass
out of the cell- wall and move through the water for a certain time, just as do
the cells of Flagellata during their entire life. Many analogies between these
two categories of movements are forced on our attention, and so, obviously, the
classification we have indicated is to be considered not as expressing a funda-
mental difference in nature, but rather a useful subdivision for teaching pur-
poses.

4o6

TRANSFORMATION OF ENERGY

Movements in fixed plants came under our notice in another relation, as for
example, when the root or the stem apex grows forwards in process of develop-
ment, and although such orthotropic movements are not those which, as a rule,
we have in mind when we speak of movements in plants, still it is impossible to
separate such movements as these from movements in general. The first type
of movement we naturally think of is the bending of an originally straight organ
or the alteration in curvature of an organ showing curvature to begin with.
We have now to study such curvatures both as regards their form and their
causes, and we may begin by considering such movements as appear in ripening
fruits or other drying parts, and whose cause lies in the loss of water from the
cell-membrane — in other words, hygroscopic movements.

The changes of shape which an organ thereby undergoes may be referred to
one or other of three fundamental types : thus we may speak of mere curvature,
when an originally straight organ becomes bent so that its axis lies in one plane ;

of twisting when the axis of an
organ retains its original direc-
tion, while the longitudinal lines
of growth from being straight
become spirally twisted ; and,
finally, of twining, when the
entire organ becomes altered into
the form of a spiral. Fig. 119
shows these three conditions in
the case of a quadrangular prism,
and might also represent to some
extent similar changes of form in
the case of a stem.

That the cause of the de-
formation in such organs lies in
the loss of water during desicca-
tion is shown by the fact that
they regain their original shape
when moistened, and that the one
or the other condition may be
induced at will according as water
is added or withdrawn. The
capacity for absorption of water is widely distributed in the vegetable kingdom
and may be due either to the osmotic activities of the cell-sap, or to the power
of imbibition possessed by the different parts of the cell. Hygroscopic move-
ments arise from the latter capacity, and especially from the imbibition of
water by the cell-wall ; indeed such movements take place even though the
parts under consideration consist of cell- walls only. We have several times re-
ferred to this phenomenon of imbibition, but now is the proper time to study it
in somewhat greater detail.

We must first ask ourselves wherein lies the essential essence of the imbi-
bition. Bodies which are capable of swelling are able to absorb a liquid, and
thereby to increase in volume ; but it is obvious that this imbibition of liquid
must, to a certain extent, be limited, and that it brings about for the most part
an alteration in the consistence of the swollen body. In plants, water is the
medium which especially induces swelling, a medium which can also produce
swelling in other bodies which do not occur in plants. If we take a piece of
gelatine or glue, and determine its weight, and then lay it in water at an ordinary
summer temperature, we can observe an absorption of water as well as an in-
crease in volume, and with the aid of a balance we may convince ourselves that
the absorption takes place with decreasing rapidity and finally ceases. If the

Fig. IIQ. Quadrangular prism.
Ill, twisted. IV) twined.

7, straight. 77, bent.

MOVEMENTS DUE TO SWELLING, ETC. 407

water be warmed, more will be absorbed, and we discover that the power of ab-
sorption possessed by gelatine depends directly on temperature. When the
temperature reaches a certain degree the absorption becomes unlimited, or, as
we are accustomed to put it, the gelatine dissolves in the water. The same thing
applies at ordinary temperatures to gum arabic — the absorption of water by
that substance suggesting at first a case of swelling, but becoming indefinite.
Hence we see that swelling may gradually merge into solution. It would be
quite incorrect, however, to assume that all bodies capable of swelling are also
capable at some definite temperature of dissolving in the medium employed.
Cell-walls especially, at the present moment of primary interest to us, remain
stationary when they reach a state of maximum imbibition. The changes there-
fore which gelatine undergoes in solution we need not discuss.

In order to obtain a closer insight into the phenomenon of water imbibition
we will compare substances capable of swelling, such as a cell- wall, or a piece
of gelatine with a finely porous body, such as a plate of plaster of Paris
saturated with water and afterwards air dried. If a plate of plaster of Paris be
placed in water, it absorbs a certain quantity of it and retains it firmly when with-
drawn from the water. The water is, however, retained in pre-existing spaces,
as may be seen from the fact that the air escapes in bubbles when the plaster
is placed under water ; in other words, the water forces its way into the plaster
by capillarity and replaces the air previously present in these spaces. In a cell-
wall or a piece of gelatine, on the other hand, capillary spaces containing air
cannot be seen even with the best lenses, and, further, impermeability of the
substance to air proves that such spaces do not exist. Even were such spaces
present there is yet another fundamental difference between finely porous
bodies and bodies capable of swelling. The plaster of Paris shows no increase
in volume after the absorption of water, such as substances do which are capable
of swelling. Further, when water forces its way into previously existing spaces
in the latter, these spaces must be enlarged by the water and the minute particles
separated from each other, a phenomenon which obviously does not take place
in a solid body which does not increase in volume. On the contrary, ASKENASY
(1900) has observed that in consequence of a capillary entrance of water a
diminution in the volume of a non-swelling body may take place, e. g. in deposits
on cover-glasses. Bodies capable of swelling must possess a special molecular
structure which may not be directly observable but which can only be deduced
from observation of their behaviour.

Among hypotheses of molecular structure, that advanced by NAGELI
has undoubtedly had the most lasting influence in Botany, more especially in
relation to the phenomena of imbibition, and even now it claims a certain
recognition. Since, however, some essential parts of the hypothesis have
been refuted, it will serve our purpose best if we deal here merely with
such parts as are vital to our discussion. NAGELI (1858) held that bodies
capable of swelling were composed of extremely minute particles, larger than
molecules, to which he gave the name of micellae. Since there is no longer any
ground for believing in the existence of micellae we need not lay any emphasis
on this conception. When the body was in the dry condition the micellae
were supposed to be close to each other without any air-spaces between them ;
the micellae were regarded as polyhedral in form, and were considered to be
held together by mutual attraction. Since the micellae had also an attraction
for water, each attempted to surround itself with a film of water. This, how-
ever, could not take place unless the force of attraction between water and
micella, was greater than that between the micellae themselves. The
addition of water thus induced a separation of the micellae, and explained at
once the increased volume of the swollen body. Should this be limited, the
resistance offered by the separation of the micellae to any further entry of

4o8 TRANSFORMATION OF ENERGY

water must rapidly increase. NAGELI suggested that the force of attraction
between the substance and the water decreased relatively more rapidly than
that between the micellae, the former being more difficult to separate in inverse
proportion to the latter. Thus all the water in the swollen body was not held
equally firmly. The particles nearest to the surface of the micella were held
most firmly, and as the distance from it increased the mobility of the water
also increased, and it was very probable that not all the water imbibed lay within
the spheres of attraction of the micellae, but was retained as capillary water in
minute spaces arising during the swelling. In fact, REINKE'S (1879) researches
on Laminaria have shown very clearly that imbibition water is retained with
varying tenacity. In these experiments certainly larger spaces, filled with
capillary water, e.g. the cell lumina, play a part. REINKE allowed a por-
tion of the blade of a Laminaria, which had absorbed i-026g. of water, to dry
in air, and found that it evaporated the following amounts in mg. in successive
hours : 148, 115, 105, 91, 74, 84, 68, 57, 51, 51, and, later on, still less. Further,
it is possible to express water from a completely swollen portion of Laminaria,
containing a large quantity of water, with only slight pressure, but great
pressure is necessary to extract water when it is present in small quantity. If
a Laminaria consists of 75 per cent, water and 25 per cent, solid, water may be
pressed out by a pressure of two atmospheres ; if the proportion of water to
solid be 43 per cent, to 57 per cent., a pressure of forty atmospheres is needed to
achieve the same result. Again, the swelling may be prevented by great pressure,
so that it is very obvious that a great deal of work is accomplished in the pro-
cess of swelling. RODEWALD (1895) showed that a pressure of twenty-five to
thirty-two atmospheres is needed to prevent dry starch from swelling, and it is
known that mechanical operations may be carried out by the swelling of certain
bodies, e. g. rocks may be split open by the swelling of wooden wedges, and a
skull may be separated into its constituent bones by filling it with peas and
allowing them to absorb water. Under these circumstances we may assume
that air-dry substances capable of swelling always retain demonstrable quantities
of water, and that they are able further to condense water vapour from damp air.
Simultaneously with the absorption of water a noticeable alteration takes
place in the mechanical characters of the swollen body. If the substance when
dry be brittle and only slightly extensible, when swollen it may become pliable
and very markedly extensible, and yet it loses its elasticity and its rigidity (under
tension and pressure). It is important to note how great are the quantities of
water which may be absorbed by a body without entirely losing its rigidity, and
without transforming it into a liquid. According to NAGELI the gelatinous cell-
walls of certain lower Algae contain only one-half per cent, of dry substance ;
but even that is far from being the extreme limit that may be reached, since,
according to GERICHTEN (1876), apiin, a glycoside obtained from parsley, begins
to lose the capacity for forming jelly only when one part of solid is dissolved
in more than 8,000 of water. It is not easy to understand how the characteristic
features of a solid are preserved when the individual molecules are separated so
far from each other as they are in the cell-walls of the Algae mentioned above,
where the molecules must be separated by a distance equal to many times the
diameter of the molecule. These considerations force us to accept some other
theory of structure than that hitherto held. What we need is a structure which
maintains its cohesion sufficiently well even when large quantities of water are
imbibed. We should have such a structure if we assume the substance capable
of swelling to be permeated by canals so that it consisted of minute particles
bound together in all directions, just like the meshes of a net in a plane, or,
better still, regard it as the honeycomb of soapsuds, where the walls are formed
of a substance capable of swelling while the alveoli are able to take in water.
Honeycomb constructions of this kind have been shown by BUTSCHLI (1892-

MOVEMENTS DUE TO SWELLING, ETC. 409

1900) to be of general occurrence in all bodies capable of swelling both in the
cell- membrane and in the protoplasm. He lays stress on the fact that the
diameter of the spaces is uniform, amounting to about one /A. Certainly, in
protoplasm these very minute but visible cavities show all transitions to the
large vacuoles. BUTSCHLI holds that the cavities contain a dilute solution of the
swellable body which is concentrated by loss of water and thus acts osmotically.
The expansion of the walls of the alveoli would thus be due to osmotic pressure.
In addition to other difficulties this conception is open to the criticism that cel-
lulose is quite insoluble in water. BUTSCHLI does not, however, hold that during
the process of swelling the stretching of the walls of the cavities is due only to
the pressure of their contents ; he expressly shows that water is also absorbed
by the walls of the cavities themselves, a point in which substances capable of
swelling differ from those incapable of doing so ; at the same time it must be
remembered that the latter also may possess a honeycomb structure. The
absorption of water by the walls of the cavities is considered by BUTSCHLI to
be a chemical phenomenon, a case of hydration in fact, and he thinks that this
water cannot be got rid of merely by pressure, and, further, that the water
expressed in REINKE'S experiments above described was only water from the
cavities of the honeycomb and from the larger spaces in the substance. We
may, however, quite well make use of NAGELI' s physical hypothesis for the
imbibition of water by the walls of these cavities and so combine his theory
with that of BUTSCHLI. Hence it is worth noting that (see p. 407) the existence
of intermicellar spaces, corresponding to BUTSCHLI' s alveoli, had already been
considered by NAGELI (1858, 342).

No matter which theory be the correct one, the walls of the cavities must
be increased by the imbibition, and the cavities must thus be able to hold more
water, itself out of reach of the attractive force of the micellae. BUTSCHLI' s
observation that in the process of drying the walls of the cavities collapse and
approach each other until the lumina entirely disappear is of the utmost im-
portance. The full significance of the disappearance of the alveolar structure in
drying and its reappearance on water being once more absorbed will become
evident later on when we have studied the phenomena of cohesion (p. 417).

The alterations in volume associated with swelling and shrivelling permits
of the execution of movements on the part of such bodies, and this leads us
back to the hygroscopic movements we started with. If the object under
consideration is capable of swelling equally in all directions, then it or its
parts will be able to exhibit movements only in straight lines, but such
movements are of no further interest. The bending, twining, and twisting
of hygroscopic organs can obviously be produced only if the capacity for
swelling varies in different directions, when layers with greater powers of imbibi-
tion stand in antagonism to those with less capacity for swelling. We distinguish
the layer which contracts most as the ' contractile ' or ' dynamical ' layer, and
that which does so least as the 'resistant' layer. Variations in the capacity for
swelling are due, in the first instance, to the varied nature of the material, in
the present instance the cell-wall, generally put down to chemical differences
but assumed by NAGELI to be physical, and especially dependent on the vary-
ing size of the micellae. On the other hand, the structure of the membrane
may render possible differences in capacity for swelling in different directions.
NAGELI' smicellar theory, as also BUTSCHLI' s alveolar theory, equally well explain
such unequal swelling. We will consider the observations themselves without
going into theories with regard to them, and we find, speaking quite generally,
that a cell which is not isodiametric is unequally extensible in its three chief
space dimensions. The greatest capacity for swelling in an elongated cell is in
a radial direction, i.e. at right angles to the concentric layers of which its wall is
composed ; it has less capacity for swelling tangentially, and least of all longi-

410 TRANSFORMATION OF ENERGY

tudinally. Its behaviour during contraction naturally corresponds. If we think
of a point in the interior of the swollen cell- wall as on the surface of a sphere,
this surface changes during drying to an ellipsoid form ('contraction-ellipsoid ')
whose shortest axis is at right angles to the lamellation, while both the other axes
come to lie in a tangential direction. But it is not necessary, as has been pointed
out, that the longest axis should be coincident with the greatest length of the
cell, it may lie obliquely or transversely. In most cases it would be very difficult
to determine directly by measurement the lie of the axis of contraction during
desiccation, and hence it is of importance to be acquainted with indirect methods
of doing so. In the first place, we may draw attention to polariscopic research,
which lends itself to the determination of optical elasticity-ellipsoids. Experience
teaches us that this almost always corresponds with the lie of the axis of the
contraction-ellipsoid. In the second place, the direction of the thickening bands,
of the striations, and pits must be noted, since that corresponds to the position
of the longest axis of the ellipsoid, and conforms to the line of least contraction
in desiccation. The position of the longest axis may, however, be different in
the different walls of the same cell ; for example, it may run longitudinally on
the outer wall and transversely on the inner, and may alter in successive layers.

It will not be necessary for us to discuss hygroscopic movements either
generally or in detail, for that would involve us in difficulties, due to the fact
that all authors do not agree as to the interpretation of the more complex cases.
Only a few examples need be cited here as illustrative of the chief types (for the
older literature see KRAUS, 1866 ; HILDEBRAND, 1873). We begin with the
consideration of simple bendings such as we meet with in Anastatica hierochun-
tica, the Rose of Jericho, a member of the Cruciferae from the Steppes of the
south-eastern Mediterranean area. When the fruit is ripe the numerous divergent
branches dry up, and in doing so contract much more on the upper than on the
under sides, thus bending inwards and causing the plant to take on a spherical
form. When moistened they again open out, and this performance may be
repeated again and again. Movements of the fruits take place at the same time,
but into these we need not go. It is easy to show that the bending is due to
the wood only, and anatomical investigation of a twig shows (VOLKENS, 1884)
that this consists especially of excentric xylem fibres, much more thickened
and lignified on the under (convex) than the upper sides. The strongly ligni-
fied fibres are much less capable of absorbing water than are the feebly lignified
ones, and hence on desiccation the upper side of the branch contracts much
more markedly than the under side. The bending in this case is due to the
differential capacities for swelling of antagonistic tissues. In the same way
(STEINBRINCK, 1878), each of the five mericarps in the fruit of the geranium
bends outwards after drying, releasing itself with a jerk from the central carpo-
phore, and so aiding in the dispersal of the seeds.

The bursting of many forms of capsule is brought about in a similar manner.
Some part of the fruit-wall endeavours to bend outwards and the tensions set
up finally cause a rupture in the region of least resistance, frequently at places
where there are special anatomical structures, differentiating lines of dehiscence.
The cause of the tension, for the most part, does not lie in the different intensity of
imbibition of antagonistic zones but in the layering of the cells, or, in other
words, in the direction of the lamination or striation of the wall. The various
possible arrangements may be illustrated by a few examples.

I. Differential contraction due to arrangement of cells. In the walls of the
segments of the capsule of Syringa we find a lignified layer which is the sole cause
of the bending, and which consists of six rows of elongated thick-walled
cells ; the innermost of these are arranged longitudinally, while the outer
layers are deposited obliquely and transversely. Since these cells are all alike,,
in so far as their capacity for absorbing water is concerned, bending when

MOVEMENTS DUE TO SWELLING, ETC. 411

desiccation sets in, must be due to the differences in their mode of deposition.
The cells contract relatively less in a longitudinal direction than trans-
versely, hence the outer sides of the segments contract more vigorously than
the inner sides, and the segments become concave outwardly. It is not,
however, essential that the elements of both the antagonistic layers should be,
as in Syringa, elongated and cross each other at 90° or less ; it is sufficient for
the purpose if one cell layer be composed of fibres while the other is formed of
isodiametric cells. Thus in the wall of the fruit of Veronica we find the
epidermal cells of the interior are thick-walled fibres, whilst externally there
lies a layer of parenchyma capable of contracting equally in all directions
(STEINBRINCK, 1878). Differential contraction between this layer and the inner
epidermis is greatest in the long axes of the latter elements, and must, therefore,
cause a bending outwards at right angles to the course of these elements.

2. Differential contraction due to lamination of the cell-wall. As an example
of this type we may select the capsular teeth of Linaria (STEINBRINCK, 1891).
Fig. 120, /, shows a part of a median longitudinal section of that portion of the
tooth specially concerned in the contractile movement, that is, through the
inner epidermis and the sclerotic layer abutting on it. The cells figured are,
it is true, by no means isodiametric, nevertheless the important factor in the
bending is not the arrangement of the cells but the lamination of the cell-mem-
branes. The two cell-layers differ essentially in the way in

which their cell-membranes are deposited. The lamina-

tion of the inner epidermis is almost perfectly parallel to

the long axes of the capsular teeth, and the same is true

of the inner walls of the sclerotic layer. In the remaining

portion of the latter one would expect a similar arrange-

ment, in accordance with the usual principles of

unilateral thickening, that is to say, a deposition of

secondary thickening in layers tangential to the outer

surface as in Fig. 120, 2. As a matter of fact, however,

a glance at Fig. 120, /, shows that all the lamellae are laid

down parallel to the horizontal walls . Since the maximum

contraction, as we have already pointed out, occurs at

right angles to the lamination, the greater part of the (1891).

sclerotic layer contracts much more markedly in the long

axis of the capsule than the inner epidermis and the inner wall of the sclerotic

layer. Measurements made on the isolated layer exhibited a shortening in the

former of 10 per cent., while in the latter the contraction was scarcely observable.

3. Differential contraction due to striation in the cell-wall. We may briefly
characterize the last case we took by saying that in it the difference between the
shortest and a longer axis of the contraction-ellipsoid is made the most of.
In contrast to this we find cases in which the difference between the longest and
the average axis comes into play, the cases in which the bending is due to stria-
tion of the membrane, (a) The capsules of Campanula (STEINBRINCK, 1895)
open in the same way as those of Linaria, viz. by valves, but the histological
structure and the mechanism of opening is quite different in the two plants.
The sclerenchyma is absent from Campanula altogether, and the bending
is brought about by the parenchyma, and is due in part to the form of the cells,
the external layers being composed of short cells gradually increasing in length
inwards. Any bending must therefore take place so that the concavity is
external, in accordance with the principles already laid down. A second factor
is the striation of the cell- wall which expresses itself in the position of the pits.
The outer cells have their pits arranged transversely, while on the walls of the
cells of the succeeding layers the pits are laid down obliquely to the left and finally
longitudinally. Since, as we have seen, the long axis of the contraction-ellipsoid

section through a valve of

412 TRANSFORMATION OF ENERGY

lies parallel with the long axes of the pits, the average axis of contraction is made
the most of in the dynamical external cells, and it operates in opposition to the
longest axis in the innermost cells, forming the resistant layer, (b) A difference
in the striation of the wall of a single cell may also occur, as in Saponaria, where
in the dehiscence of the capsule we have merely to deal with the external epi-
dermis, whose greatly thickened outer walls act as the contractile layer, whilst
the radial and inner walls serve the purpose of the passive layer. But, contrary
to one's expectation, the pits on the outer walls are not transverse, nor are those on
the inner wall longitudinal ; the difference is of quite another
character. According to STEINBRINCK (1891) the inner wall
bears numerous clearly marked, narrow, elliptical pores, trans-
versely placed, while on the outer wall the pits gradually
become less distinct, less numerous and more elongated and,
finally, in this region of maximum curvature, fade away
into dark narrow streaks running transversely from one
radial wall to the other, alternating with clear striae. It
is easily seen how, by this arrangement, the differential con-
traction is effected, for the inner wall with its short trans-
verse pores contracts far less than the outer wall. It should
be noted that STEINBRINCK (1891) found an extreme case in
Dianthus prolifer ; here the most external layer of the outer
walls of the epidermal cells acts dynamically, while the inner-

, _ * 1 11 -.~/ . ^ . ,

2, p. 773). most layers of the same cell- walls are the resistant ones, that

is to say, the antagonistic units are parts of the same cell- wall.
The way in which we have treated this subject might lead to the supposition
that in each individual case of hygroscopic bending only one or other of these three
principles came into operation, i. e. qualitative differences between the imbibitory
capacities of the cell-wall, differences between different layers, or differences in
striation. That is, however, not the case ; as a rule, combinations of these pos-
sibilities occur in nature, and it is only for brevity's sake that we have avoided
treating of such in individual cases.

Let us now turn from curvatures in one plane to the more complex pheno-
mena of twinings and torsions (compare NAGELI and SCHWENDENER, 1877). In
these cases also the same structural principles are applicable, but we must resist
the temptation to discuss these in as great detail as we have the
simple bending movements. We meet with very noticeable
twistings in the two segments into which the pods of Leguminosae
divide on ripening. The general nature of this twisting is illus-
trated in Fig. 121, where the inner surface of the fruit wall main-
tains its internal position during twisting. Anatomical investi-
gation shows that the inner epidermis abuts on a sclerotic layer,
which alone is the factor concerned in the twisting (ZIMMERMANN,
1881, p. 25). All the cells of this layer are elongated, but the
innermost have great powers of contraction (15 per cent.) while

x

X

Fig. 122. the outermost have none at all. It is possible to determine
anatomical differences between these fibres, but probably it is
not to these differences but to certain chemical differences which have not as
yet been elucidated that the varied behaviour of the cells is to be attributed.
If these fibres lay parallel to the long axis of the pod both segments would
simply bend inwards in a concave manner, but, as a matter of fact, they lie at
a sharp angle with the long axis of the legume, and thus the curvature which is
transverse to the direction of the fibres is also oblique to the long axis of the pod.
Let us assume a long narrow piece of paper folded obliquely, as represented by
the dotted lines in Fig. 122, it will take the form of a spiral when pulled out.
Although, as ZIMMERMANN has shown, the sclerotic layer is alone sufficient to

MOVEMENTS DUE TO SWELLING, ETC.

4'3

bend the pod, the external epidermis is able to aid in the process, as STEINBRINCK
(1873, p. 17) has suggested. The epidermal cells are elongated and traverse the
fibres, so that it is obvious that a differential contraction must occur between
them. In relation to what follows it is necessary to emphasize the fact that
twistings must result from the reactions between epidermis and fibres, since
differences such as those found by ZIMMERMANN do not occur among the fibres
themselves, all the fibres behaving exactly alike.

Of even greater interest is the spiral coiling exhibited by the lower parts
of the awns of Erodium (Fig. 123, A), for there the twisting takes place
obliquely to the long axis of the fibres of which the awn is composed — for
the epidermis and parenchyma need not be taken into account, having no power
of hygroscopic movement. The thick-walled
fibres are differentiated into four layers which
gradually merge into each other from without
inwards (STEINBRINCK, 1895) : —

1. A layer of fibres with transversely
placed pits, which when isolated bend out-
wards only slightly on drying.

2. Fibres provided with pits which lie
transversely or are slightly tilted upwards to
the right on the outer walls and with pits on
the inner walls tilted upwards considerably to
the left. The whole layer and each individual
fibre when separated from the other layers
twists on drying just as the whole awn does.

3. Fibres with pores arranged longitudi-
nally; these fibres on dry ing do not bend at all.

4. A layer of fibres which when isolated
and dried twists to the right, i. e. in the oppo-
site direction to that of the awn as a whole.

It will be obvious from what has been
said that to the second layer only or to that
in opposition to the other layers may be
attributed the twisting of the awn. Every
individual cell in it endeavours to twist on its
own account, and it is not difficult to see how
the twisting as a whole may be explained if we
compare the individual fibres of this layer with
the pods of the Papilionaceae. The transverse
or feebly oblique pores directed to the right on
the outer walls of these cells indicate to us
which is the long axis of the contraction-ellip-
soid ; they correspond in their deposition to the epidermal cells of the legume.
As regards the inner wall, the axis of the ellipsoid, as in the legume, lies almost
transversely to that of the outer walls. It is easily intelligible not only how
the second layer as a whole twists, owing to the efforts to twist of the individual
cells, but also how the first and third layers merely intensify this twisting. As a
matter of fact, it has been observed that the second layer twists whether in
conjunction with the first or the third or with both, and, finally, the fourth layer
must be added whose endeavours to twist in the reverse direction are completely
neutralized by the others.

The last -mentioned layer behaves differently from the others for other
reasons. In layers 1-3 we have to deal with an antagonism between flat inter-
secting plates, i. e. between layers whose axes of contraction intersect each other
(STEINBRINCK, 1888). These plates are variously distributed : — (a) they occupy

Fig. 123. Portion of the fruit of Erodium,
A) in the dry state ; B, in the wet con-
dition. From the Bonn Textbook.

4H TRANSFORMATION OF ENERGY

the outer and inner walls of one cell (layer 2) ; (b) they occur in different cells ;
or (c) both conditions are united. Under all conditions the radial cell- walls have
transverse pores and play no part in the twisting. In the fourth layer the pores
are laid down in continuous spiral lines directed to the right, running from the
front wall, over the lateral walls, to the rear walls ; in other words, we have here
to deal not with two intersecting plates but with elements which are spirally
arranged. If the wall of such an element be equally capable of swelling, each
must, as ZIMMERMANN has shown, twist independently. How a complex of
such cells bound into a tissue may bring about a spiral twisting may be easily
shown by experiment. Cover a strip of paper thickly with portions of the
twisting awn of Stipa, gum these to the paper and to each other, and leave the
whole to dry ; the structure will then exhibit a close left-hand twining with the
paper external (STEINBRINCK, 1888, p. 392).

This last example illustrates the close relationship which exists between
twisting movements and the torsions which we have now to discuss. We
have seen that the individual cells must undergo torsion if they are provided
with pits arranged in a spiral manner. An entire organ can also undergo torsions
similar in all respects to those exhibited by its constituent elements. DARWIN
(1876) bound together wet awns of Stipa, and found that the bundle underwent
torsion when it was allowed to dry, but apparently this principle does not come
into play in nature (Anemone? EICHHOLZ, 1885, 554). Torsions of the whole
organ occur much more frequently in consequence of the tendency to twist on
the part of the individual elements when these are arranged in concentric layers.
Such torsions must arise from a relatively greater extension in the peripheral
than in the central region. If we twist a bundle of fibres laid parallel to each
other we shall find that each, with the exception of the central ones, describes
a spirally curved line, and conversely if twisting of the individual elements occurs
there arises torsion in the bundle as a whole. The twisting cells of Stipa, which
have the structure of those composing the fourth layer in Erodium, are the
chief agents in the movement, and one important condition only must be fulfilled,
viz. that the fibres should exhibit an increasing capacity for water absorption
longitudinally from without inwards, so that in drying the central ones should
contract more than those on the periphery.

A few words may be said in conclusion on the biological significance of the
movements which have been described. Almost all of them have to do with the
dispersal of seed. In the great majority of cases fruits burst open or dehisce
when dried, part of the fruit wall being thrown off, and the seeds escaping in
this way from the capsule. There are many plants, however, such as A nastatica,
Mesembryanthemum, &c., whose fruits close in dry weather and open in wet.
Fruits which have the power of ejecting their seeds form more perfect illustrations
of these hygroscopic movements ; examples of such are Geranium, the twisting
pods of Leguminosae and many others which have not been referred to above,
such as Viola, Oxalis, &c. Fruits and seeds which have long twisting awns,
such as Erodium, Stipa (and many other grasses), many species of Anemone,
&c., are able to force their way into the soil by the torsions which take place
in these appendages.

Let us now turn to the consideration of a series of phenomena which are
illustrated by the movements of anthers and sporangia, connected with the ejec-
tion of the pollen -grains or spores, and hitherto briefly spoken of as hygroscopic
movements. We shall consider first the sporangia of ferns, and especially those
of the Polypodiaceae. The sporangium is a stalked, lens-shaped body, en-
closing the spores within a multicellular unilamellar wall. Most of the cells
of the wall are polyhedral, thin-walled plates, but the edge of the lens is occupied
by a ring (annulus) of horseshoe-shaped thick-walled cells, which starts from the
stalk and more or less completely encircles the capsule (Fig. 124, a).

MOVEMENTS DUE TO SWELLING, ETC. 415

The inner tangential walls of the cells of the annulus are strongly thickened,
but the outer walls are unthickened, while on the radial walls the thickening is
gradually reduced from within outwards. When the sporangium is ripe, at
the region where the annulus ceases, or, to be more exact, where the cells of the
annulus cease to have thickened walls (Fig. 124, /, st), there occurs a rupture due
to the contraction of the annulus, so that the sporangium takes the form seen
in Fig. 124, 2, and the spores can thus escape. Obviously in consequence of the
loss of water from its constituent cells, the annulus contracts and bends slowly
outwards, and this contraction and bending may proceed so far that the
annulus again forms a circle, but now what was the inner face becomes
the outer. At this moment a new phenomenon appears, for with a sudden
snap the annulus springs back and once more assumes almost its original
position and form. It rebounds on its base with considerable force, and the
whole sporangium is thrown often several centimetres into the air, and in the
process the spores which still adhere to the capsule are ejected. Looking more
closely at the annulus during the opening of the sporangium we see that its
cells undergo a remarkable deformation. The water they contain evaporates,
and the cell cavities become smaller, the thin external walls become concave
inwards while the lateral walls approximate. The external outline of the
annulus thus becomes gradually shorter, and the inward curving of the ring is
thus simply explained. The curving in-
wards of the outer walls of the individual
cells of the annulus and the water they con-
tain proves clearly that this is not a case of
' shrivelling '.

Inquiry as to the factors which bring
about the ultimate deformation of the
cell whereby the outer wall approximates
to the base of the cell and the outer
corners of the radial walls come to touch
each other, shows us that we must in-
vestigate the cohesion of the imbibition

Water Of the Cell and itS adhesion tO Fig. 124. Closed (/) and open (*> sporangia

the membrane (STEINBRINCK, 1898, 1903). of the Wypodfce-e. */, iip-ceiis; «, annulus.
In speaking of the movements of water

in the plant we have already shown that the cohesive force of water is
very great. The adhesive force of water to the membrane is almost as great —
inasmuch as the tension of several atmospheres is required to tear the water
particles apart from each other or from the wall. When evaporation commences,
therefore, the water in the cells must be under tension and the effect of that
tension is to produce deformation of the cell. If the cell-wall could not be de-
formed the water cohesion would soon be overcome and an air space would
appear in the interior of the cell, or a rupture would occur between the water
and the membrane, and air would at once enter. It was previously thought
that such movements as those seen in the fern annulus (and which we may term
cohesion movements, in contradistinction to those due to swelling and con-
traction) could only take place if the cell membrane were impermeable to air.
If that were the case their occurrence would be very restricted, and they could
not in any case take place in the fern sporangia since the outer walls of the
annulus cells are, as a matter of fact, quite permeable to air. The entry of air
into the interior of the cell is, however, rendered impossible at first, inasmuch
as each minute bubble of air must first of all overcome the adhesion of the water
to the cell-wall.

The imbibition water in the annulus cells exercises in this way during
evaporation a vigorous pull on the walls, and stretches them elastically . Finally,

416 TRANSFORMATION OF ENERGY

this tension becomes so great that it overcomes the cohesive force of the water
particles, the water continuity in the interior of the cell is ruptured, and the
cell membranes assume once more their original form, and the annulus bends back
again with a sudden jerk into its original position of rest. Its cells now appear
dark, for they contain only a little water distributed on their walls, and for the
rest a space which may in general terms be said to be full of air. The entry of
air is not, however, essential to the execution of the elastic recoil, since this
recoil occurs when the sporangia are placed under low pressure in an air pump
(ScHRODT, 1897) ; in that case a vacuum appears in the centre of each cell of the
annulus after the recoil.

The mode of opening of the anthers of Phanerogams corresponds in all
essential points, according to STEINBRINCK'S researches (1898, 1899 a), with the
mode of dehiscence of fern sporangia. Each of the four lobes of the anther, filled
with pollen-grains, consists of a wall which in the ripe condition is often only two
cells thick. The pollen-grains are released in consequence of the power the wall
has of curling backwards. In this process the outer cell layer of the anther wall
takes no part ; the inner, usually known as the fibrous layer, holds the dyna-
mical elements. The thread-like thickenings on the inner walls of these cells are
laid down in a very characteristic manner (Fig. 125). They run almost parallel
with each other at regular distances apart, over more or less of the surface of the
lateral walls, uniting on the inner wall like the rays of a star ; the outer wall is

destitute of any thickening. The
comparison with the fern annulus
is perfectly obvious, but the fact
that the lateral walls are in this
case unequally thickened results
in a difference in behaviour be-
tween the anther and the fern spo-

Fig. ««. Anthers of Lilium candidum. isolated rangium. The fibrOUS Cells also
fibrous cell in the wet state. 77, the same as seen from Undergo deformation during the

x ST '" loss of their imbibition water on

drying. The deformation consists,

in the first instance, in the shortening of the diameter of the cell on
the external face of the anther, while that of the inner side remains rigid
owing to its secondary thickening. The change in form does not in this case
express itself in an inversion of the outer wall, but in a very noticeable contrac-
tion of the radial walls at right angles to the lines of thickening, so that these
bars approximate (Fig. 125, IV). This contraction, according to SCHWEN-
DENER (1899) and STEINBRINCK (1901) may amount to 50, 60, or 70 per cent.
of the original diameter. Were this the result of simple shrivelling the con-
traction would far and away exceed that shown by any other cell. But
STEINBRINCK has shown that the contraction begins while the cavity of the
cell is still full of water, and hence it is obvious that it cannot be due to mere
shrivelling.

As a matter of fact, the process must be explained in an entirely different
manner. Under the influence of the tension exerted by the imbibition water
the thin parts of the radial wall lying between the thickening bars are thrown
into folds, and hence the volume of the cell is reduced. Apart from these foldings,
which may be best observed in good tangential sections through the anther,
there is another difference to be noted between these cells and those of the fern
annulus. At the moment when the elasticity of the bent fibres overcomes the
cohesion of the imbibition water, when a bubble of air appears in the interior
of the cell, no jerk takes place as in the annulus, but the anther wall remains
in the outwardly bent concave condition. The reason for this is probably
that the thin-folded portions of the wall adhere to each other, and only become

MOVEMENTS DUE TO SWELLING, ETC. 417

smoothed out again when water is reabsorbed ; then only does a closing move-
ment of the lips of the anther take place.

It must be admitted that the mechanism of the anther has not as yet been
so completely explained on the basis of the cohesion hypothesis as that of the fern
sporangium. Indeed, this explanation has been combated by SCHWENDENER
(1902). It appears to us, however, that the cohesion hypothesis has more to
be said for it than any other of the numerous explanations hitherto given, but
into the discussion of which we cannot enter (compare STEINBRINCK, Ber. d.
bot. Gesell., 1898-1903). It has been shown also, both by KAMERLING (1898)
and STEINBRINCK (1899 b), that cohesion and not imbibition may play an impor-
tant part in many other phenomena of movement, such as those seen in the
sporangia and elaters of Hepaticae, in water- tissues, and in the pappus of certain
Compositae. Into the discussion of all these examples, however, we cannot enter
here, but we must point out in conclusion that the cohesion of water of imbibi-
tion may possibly co-operate with processes connected with absorption, so that
the contrast between these two sets of phenomena may not be so striking as
would at first sight appear. Here we come once more to a point which we
previously (p. 409) only hinted at. We saw that, according to BUTSCHLI, a
body capable of swelling has an alveolar structure, that the alveoli were filled
with water when the body was in the swollen state, and that the walls of the
alveoli collapsed when the body became desiccated. The forces which lead
to the deformation of the alveolar walls we have now no difficulty in recognizing
as those due to cohesion-tension in the evaporating liquid of the alveoli, and
we might compare a single alveolus with the fibrous cell of the anther-wall.

Bibliography to Lecture XXXII.

ASKENASY. 1900. Verhandl. naturhist. Vereins Heidelberg, N.F. 6.

BUTSCHLI. 1892. Uber die mikroskop. Schaume u. das Protoplasma. Leipzig.

BUTSCHLI. 1896. Abh. Kgl. Gesell. d. Wiss. Gottingen.

BUTSCHLI. 1898. Unters. iiber Strukturen etc. Leipzig.

BUTSCHLI. 1900. Zusammenfassender Bericht von Schuberg. Zool. Centrbl.

1900, 7.

DARWIN. 1876. Trans. Linn. Soc. II. Ser. i, 149.
EICHHOLZ. 1885. Jahrb. f. wiss. Bot. 17, 543.
VAN GERICHTEN. 1876. Ber. d. chem. Gesell. 9, 1121.
HILDEBRAND. 1873. Jahrb. f. wiss. Bot. 9, 235.
KAMERLING. 1898. Botan. Centrbl. 73, 369.
KRAUS, GR. 1866. Jahrb. f. wiss. Bot. 5, 83.
NAGELI, C. 1858. Pflanzenphys. Unters. 3. (Die Starkekorner.)
NAGELI and SCHWENDENER. 1877. Das Microskop, 2nd ed. Leipzig.
REINKE. 1879. Unters. iiber Quellung. (Hanstein's bot. Abhdlgn. 4.)
RODEWALD. 1895. Landw. Versuchsstationen, 45, 201.
SCHRODT. 1897. Ber. d. bot. Gesell. 15, 104.
SCHWENDENER. 1899. Sitzungsber. Berl. Akad. p. 101.
SCHWENDENER. 1902. Ibid. p. 1056.
STEINBRINCK. 1873. Untersuchungen iiber die anatomischen Ursachen des Auf-

springens der Friichte. Diss. Bonn.
STEINBRINCK. 1878. Bot. Ztg. 36, 561.
STEINBRINCK. 1888. Ber. d. bot. Gesell. 6, 385.
STEINBRINCK. 1891. Flora, 74, 193.
STEINBRINCK. 1895. Dodonaea, Bot. Jaarboek, 7. Gnindziige d. Oeffnungsme-

chanik von Bliitenstaub- u. a. Sporenbehaltern.
STEINBRINCK. 1898. Ber. d. bot. Gesell. 16, 97.
STEINBRINCK. 1899 a. Festschr. f. Schwendener, p. 165. Berlin.
STEINBRINCK. 1899 b. Ber. d. bot. Gesell. 17, 170.
STEINBRINCK. 1903. Ibid. 21, 217.
VOLKENS. 1884. Jahrb. d. Berliner bot. Gartens, 3, 6.
ZIMMERMANN. 1 88 1. Jahrb. f. wiss. Bot. 12, 542.
JOST E e

4i 8 TRANSFORMATION OF ENERGY

LECTURE XXXIII
MOVEMENTS DUE TO TURGOR AND GROWTH

So far as the movements referred to in the last lecture are concerned the
presence or absence of protoplasm is of no consequence, for they take place just
as well in dead as in living organs. As a rule, as soon as desiccation commences
in a tissue the protoplasm dies, and it is only in plants which can endure com-
plete desiccation that hygroscopic movements maybe repeated again and again,
the plant still remaining alive. This is true of most mosses, and, among higher
plants, in Selaginella lepidophylla, which undergoes alterations in form very
similar to those described as occurring in Anastatica. [The branches of many
forest trees also exhibit movements of this character, often due to periodic altera-
tions in the amount of water present in the cell- walls (GANONG, 1904).] We
have now to study movements which are possible only in the living plant, move-
ments which are not due to swelling or shrivelling of the cell-membranes nor
yet to tension in these membranes induced by the evaporation of imbibition
water. The causes of such movements, apart from locomotory movements.
(Lectures XLII and XLIII) lie rather in alterations in the cells, in which both the
walls and their living contents participate equally — alterations which are condi-
tioned either by osmotic pressure or growth in the cells, with both of which
phenomena we have already made acquaintance. It will be necessary for us,,
however, to consider these phenomena somewhat further in detail.

We have already seen how osmotic pressure is brought about ; we have also
seen that in plasmolysis we have a method of determining this pressure, which
has this great advantage that we do not need to know what the substances are
which are present in the cell-sap and produce this pressure. All we have to da
with here is the amount of osmotic pressure and how it acts on the cell-mem-
brane. If we assume to start with that the osmotic pressure is insufficient ta
stretch the cell-wall we may also conclude that the plasmolytic solution is of
the same concentration as the cell-sap. If a 2 per cent, solution of potassium
nitrate produces plasmolysis then we may conclude that the cell-sap has the same
osmotic value, although the cell-sap may consist of a mixture of all kinds of
substances such as various sugars and organic acids, &c. Strictly speaking,
the plasmolytic method gives us always rather too high a value, for if there be
an obvious retraction of the protoplasm from the cell-wall, the plasmolysing
liquid must have a somewhat higher value in terms of potassium nitrate
than the sap. When we have estimated the concentration of the cell-sap in
terms of potassium nitrate we can then calculate the osmotic pressure in the
cell, since it is known that a I per cent, solution of potassium nitrate
( =0-1 GM.) exerts a pressure of 3-5 atmospheres. With the aid of the table of
isosmotic coefficients we are able to calculate the osmotic pressure value of any
other solution we please. As a matter of fact, a potassium nitrate solution is
peculiarly convenient for plasmolytic experiments, and very many investigations,
have been carried out with its aid. The following data with reference to the
amount of osmotic pressure in different vegetable cells are taken from RYSSEL-
BERGHE (1899, p. 23) : —

Osmotic pressure Authority for the

in atmospheres. estimates given.

Peperomia (hypoderm of leaf) 3-4 WESTERMAIER

Plantago amplexicaule (peduncle) 6 DE VRIES

Phycomyces (hyphae) 7-8 LAURENT

Sorbus aucuparia 9 DE VRIES

Foeniculunt (peduncle) 9-12 AMBRONN

Helianthus (medulla) 13 DE VRIES

MOVEMENTS DUE TO TURGOR AND GROWTH 419

Osmotic pressures Authority for the

in atmospheres. estimates given.

Phaseolus (pulvinus) 10-12 HILBURG

Pinus (cambium) 13-16 WIELER

Populus (cambium) I4~I5 »

Pinus sylvestris (medullary rays) 13-21 ,,

Pinus nigra ,, ,, 16-21 ,,

Speaking generally, we may say then that in ordinary plant-cells the osmotic
pressure is equivalent to from five to ten atmospheres, but that variations
both above and below this average are not infrequent. The pressure in ordinary
parenchymatous cells, if they be in an extremely starved condition (STANCE,
1892, 391), does not fall below 3-5 atmospheres, and even in the cells of leaves
which have fallen off and are becoming yellow a quite obvious osmotic pressure
may still always be recognized. Whether or not a far less pressure occurs in
the tubers of artichoke than in other cells, as H. FISCHER (1898) and COPELAND
(1896) affirm, requires further confirmation. Examples of great osmotic pres-
sure, in addition to those quoted at the end of the table given above, are to
be found in the onion and beet, which possess large reserves of grape and cane
sugar, and in which pressures of fifteen to twenty-one atmospheres have been
registered, but the maximum pressure has been recorded in the nodal cells of
grasses, where PFEFFER has observed (1893,399) an osmotic pressure amounting
to as much as forty atmospheres. Greater pressures even than these, which
occur only under certain conditions, will be referred to afterwards.

It has already been pointed out that the osmotic pressure in a cell never
remains constant ; continual variations or adaptations are for ever taking place
in it. When the cell grows the absorption of water leads to a reduction in the
concentration of the cell-sap, and consequently to a reduction in osmotic pres-
sure, but if such a reduced pressure does not make its appearance, or appearing
does not continue, it may be assumed that a re-formation of osmotic substance
has taken place which rapidly leads to the re-establishment of the pressure
previously existing. Far more remarkable adaptations are obtainable by cultivat-
ing the cells in concentrated media, for we have already seen that the pressure
rapidly rises and may reach the enormous pressure of 150 atmospheres. An
internal pressure as great as that is, of course, possible only if the external liquid
be capable of exerting a strong osmotic pressure ; for instance, if we put a cell
which has been lying in a highly concentrated sugar solution into water the
internal pressure may operate unilaterally and be sufficiently great to burst the
cell. Such ruptures of cells, as we shall presently see, occur normally in some
cases during development, but, generally speaking, osmotic pressure is regulated
in such a way that the cell-wall is stretched only up to its limits of elasticity.

Let us now glance at the significance of osmotic pressure. In many cases,
e. g. in the beet and onion, great osmotic pressure is to be regarded as a secon-
dary and undesirable result of the accumulation of large quantities of reserves,
so much so that in most reserve stores an effort is made, by changing these
bodies into others which are insoluble and which have large molecules (e. g.
starch), to reduce the osmotic activity of the cell-sap. In other cases it is quite
likely that such high osmotic pressures are of some service to the plant.
Apart altogether from the fact that, generally speaking, osmotic pressure would
appear to be favourable to growth, one important function must be ascribed to
it, viz. that young cells attain by its means alone the necessary degree of rigidity.
In general, the rigidity of the elements which specially subserve mechanical ends
is attained by employing firm cell-walls, but CORRENS (1891) has shown that in
specific mechanical tissues osmotic pressure may play an important part, as in
the hairs of Aristolochia, whose articulating cells are thin- walled and maintain
the necessary rigidity only by marked turgor pressure (twenty-two atmospheres).

E e 2

420 TRANSFORMATION OF ENERGY

It is easily understood how far osmotic pressure has an effect on the
rigidity of the cell- wall. The pressure stretches the delicate cell-membrane until
its elasticity equals the pressure, then the wall resists all further attempts at
deformation, and as a result the cell becomes more rigid. Increase in rigidity
owing to stretching is well exemplified by the behaviour of a thin-walled caout-
chouc balloon, which, when blown out, maintains a constant shape but which
without such extension of its wall is by no means firm. Osmotic pressure as an
agent in the production of rigidity is, however, but little employed in the plant ;
it occurs in the lower forms which live in water or in moist air, but in the higher
plant only in young parts still capable of growth. Later on, secondary thicken-
ing in the cell- wall takes on the duty of maintaining rigidity ; such secondary
thickening would be distinctly disadvantageous in vigorously growing organs.
Certainly dependence on osmotic pressure for the maintenance of rigidity in such
organs has its dangers, for on a warm summer's day they become limp, that
is to say their rigidity has been destroyed by excessive evaporation.

It is of importance for us now to know to what extent the cell- wall may be
osmotically stretched. This is determined by the amount of contraction that
takes place when turgor is neutralized. Turgor may be arrested by wilting,
by killing the cells in hot water, or by plasmolysing them. It will then be seen
that all growing cell-walls are markedly stretched, so much so that, as a rule, a
contraction in length from 3 per cent, to 20 per cent, and about 10 per cent,
reduction in diameter take place when turgor is abolished (DE VRIES, 1877 ;
SCHWENDENER and KRABBE, 1898). If we stretch the walls of plasmolysed cells
by means of a weight until they have attained the same length that they had in the
turgid condition we are obviously able to determine the amount of the osmotic
pressure in the cells, apart from the values of the osmotic activity of cane sugar,
obtained by experimental apparatus, and apart from the use of the plasmolytic
method.

In full-grown cells the extensibility of the cell-wall is so limited that an
observable contraction after plasmolysis scarcely exists. Exceptional cases are
known, however, for fully grown cells exist which exhibit highly extensible walls.
Such cells occur in the leaf articulations, and we shall see later what an important
part they play in the movements of many leaves. At present it need only be
noted that such cells occur also in the stamens of such plants as the Cynareae.
Indeed, these cells are among the most extensible known in the whole vegetable
kingdom, for PFEFFER (1892, p. 234) found that when plasmolysed they con-
tracted to half their original length ; only in the seeds of Haemanthm perhaps
have cells with still more extensible cell- walls been observed (HILDEBRAND, 1900).

The determination of osmotic pressure, it may be said here, cannot be
carried out offhand by the plasmolytic method in cells which exhibit as great
extensibility as those of the Cynareae do. Plasmolysis indicates to us, indeed,
the occurrence of osmotic pressure in the cell that has contracted to half the
length it was in the turgescent condition, and in which consequently there is
relatively twice as much osmotic material as at first. The value for the osmotic
pressure so obtained must be in this case estimated at one half of that. It
would appear also that in cases where the contractions amount only to 10-20
per cent, of the original lengths, corresponding corrections must be made on
values determined by plasmolysis, corrections which can only be reached by
exact calculation of the decrease in the cell volume in each case.

From the action of osmotic pressure on the cell- wall just described there
arises the possibility of movements of the cell. Looking first of all at a single
cell we can see that a simple elongation, that is to say a rectilinear movement,
will take place either when osmotic pressure increases, or when the wall becomes
more extensible. Similarly, reduction in osmotic pressure, accompanied by con-
traction of the cell-wall, will also lead to shortening in a rectilinear direction.

MOVEMENTS DUE TO TURGOR AND GROWTH 421

If, however, the cell-wall be not of the same consistence all round, then an
alteration in osmotic pressure will always lead to an alteration in form. The best
known case of this kind is the movements of guard-cells, of which we have
already given an account. A glance at Fig. 8 (p. 39) will remind us that in the
guard-cell the convex side is thinner, and, therefore, more extensible than the
concave side. As osmotic pressure increases the curvature of the cell already
existing also increases, and it is easily seen that, by the appropriate distribution
of more resistant areas in the wall, a cylindrical cell may be made to exhibit not
only simple curvature but torsion and twining as well, such as are seen in Fig.
119 (p. 406). In nature, however, such torsions and twinings are due always
to phenomena of growth and scarcely to osmotic pressure.

Movements arising from variations in turgidity occur much more fre-
quently in multicellular tissues than in single cells. Inasmuch as in these cases
the individual cells are osmotically unequally stretched there arise widespread
tissue tensions such as those referred to in Lect. XXIII (p. 297). Tissue tensions
were referable, as we found, to unequal degrees of growth in the separate com-
ponents of these tissues, but it is obvious that the only condition necessary for
tissue tension is the unequal efforts to elongate of the different parts, and, further,
that it is immaterial whether that elongation be effected by osmotic pressure,
growth, or some other factor. The example we took on that occasion was a stem
or similar structure, whose central region had greater powers of extension than
the periphery ; as a consequence, we found that the peripheral regions were
in a state of tension, while the medulla was in a state of compression, and that
the total length of the organ was the resultant of these opposing factors.

So long as these antagonistic parts are distributed as they are in a normal
growing stem any alteration in the turgor conditions can only result in an altera-
tion in the length of the entire organ, and cannot induce any curvature, torsion,
&c. The significance of these tensions, which are of common occurrence, must
be purely mechanical ; for just as the single cell is rendered rigid by osmotic
pressure so a stem acquires rigidity from tissue tensions.

In the typical stem, &c., we find that the tissues which contract are uni-
formly distributed all round the compressed central cylinder, but as soon as
that uniformity of distribution is interfered with curvatures at once take place.
Such disturbances are of frequent occurrence in nature, according as one
of the longitudinal halves of the organ under consideration gains or loses in
turgidity. Experimentally, it is perfectly easy to demonstrate the curvature
resulting from tissue tensions ; all one need do is to split a growing shoot-axis
longitudinally, when the medulla will thereby be enabled to extend itself and will
become convex, while the cortex in its efforts to contract will become concave.

Movements due to variations in turgor are frequently reversible if the factors
be also reversible, for the cell-walls are both extensible and elastic. Movements
of this kind are known as variation movements and stand out in contrast to
growth or nutation movements. These latter movements also start with stretch-
ing of the cell- walls, and hence may, in their earlier stages, be reversed by plas-
molysis, but after a short time the osmotically extended membrane under-
goes growth and its elongation and the movement itself become permanent.
Growth movements, like those due to turgor, may be rectilinear, curved, or
spiral, &c. It is unnecessary to consider this in further detail, for the analogy
with variation movements is complete. (As to growth itself, see Lect. XXI.)
A few remarks of a more general character may, however, be added here.

In all movements, whether they be due to growth or turgor, a certain
amount of energy must be expended in the overcoming of external and internal
resistances. With internal resistances we are but slightly acquainted, but as to
the external, the elaborate experimental researches of PFEFFER (1893) have given
us very full particulars. These resistances may be very slight if the plant grows

422 TRANSFORMATION OF ENERGY

in a straight line in water or air, but they reach high values when, for example,
a root attempts to force its way into the ground or encounters stones in its
passage through the soil. A considerable amount of work must often be accom-
plished when curvatures take place, and the amount is all the greater the nearer
to the base of the organ the zone of curvature lies, for the weight of the passive
erect portion of the plant is so much the greater.

It would appear that the energy for carrying out these variation move-
ments can be obtained only from osmotic pressure, but since, as we previously
saw, far greater forces are developed in surface growth of the cell-wall by
excretion-energy than by osmotic pressure, we may assume that excretion-
energy also plays an important part in these external activities of the growing
plant. According to PFEFFER'S researches, although these external activities
are to be referred exclusively to turgor pressure, the plant is able, however, to
employ the whole of that pressure in the overcoming of resistance.

Without going into detail, we may merely note that PFEFFER imbedded
the part of the plant under investigation in plaster of Paris, forming a general
resistant layer, and, with the aid of appropriate apparatus, was able to measure
the pressure which the plant exerted to overcome the resistance thus given to
its expansion. External pressures of this kind often reached as much as twelve
atmospheres.

So long as the cell encounters no external obstacle to its expansion the
whole of the osmotic energy is devoted to the stretching of the cell-wall, but
after enclosure in plaster of Paris the wall is extended by growth, and, as the
extension progresses, the osmotic pressure is directed against the obstacle.
When extension is complete the whole internal pressure may in the long run be
devoted to the performance of external work, and in many cases, as the resist-
ance increases, not only does the extension of the cell- wall increase up to its
limits but the osmotic pressure also rises above the normal.

The first series of movements we have to study are those which are illus-
trated by ripe fruits and spores, and which may be termed ejaculatory move-
ments. All of these are characterized by their suddenness, and that sudden-
ness is rendered possible only by tensions induced between tissues, cells, or parts
of a cell, and which are equalized in a moment. A phenomenon such as this
we have already recognized as taking place in hygroscopic movements, where,
in addition to the slow oscillation due to gradual absorption and evaporation of
water, irregular movements also occur. At the moment when the dehiscence of
a capsule is effected by a sudden adjustment of tensions, not infrequently parts
of the fruit wall or the seeds are thereby thrown off. The ejaculatory movements
we have yet to study may be compared, from a biological point of view, with
these movements in dry fruits. So far as the mechanism is concerned they are
closely related to them, but the difference lies in this, that the tensions previously
mentioned are conditioned by the swelling of the membrane or cohesion of imbi-
bition water, while those we have now to speak of are due to osmotic pressure.

Let us commence with cases where the tensions are those which occur in
single cells, as, for example, in the spore cases (asci) of the Ascomycetes (DE
BARY, 1884), and let us select Ascobolus as our first example. The asci are
elongated cells, each containing eight young cells or spores in addition to the
normal cell constituents, viz. a peripheral protoplasmic layer — much reduced in
amount — and an osmotically active cell-sap. The asci are aggregated in
thousands into a single layer (hymenium), and are mixed with narrow sterile cells
(paraphyses). When the ascus becomes ripe a marked increase in the osmotic
pressure occurs, along with a subsequent increase in volume, which may easily
result in the doubling of the original length and diameter of the ascus. If the
ascus is cut off or plasmolysed it regains its original size, so that the increase in size
is not due to growth. At a certain moment in the natural course of develop-

MOVEMENTS DUE TO TURGOR AND GROWTH

423

ment a circular patch at the apex of the ascus can no longer withstand the

pressure inside, and it forthwith bursts. In consequence of the pressure

of the elastic and contractile membrane the entire contents of the cells are

at once ejected through the opening, in the case of Ascobolus to a height of

seven or more cm., and in Sordaria fimiseda to as much as 15 cm., whfld the

wall at once contracts to its original dimensions. In Ascobolus a large number

of asci burst at the same moment in the day, i. e. between one and three o'clock,

the extension and extrusion from the hymenial surface having begun the

evening before. The ejection occurs then after a very gentle

shaking, which probably acts so as to cause a slight bending

of the ascus and in this way an increased stretching of the

membrane beyond the capacity of the apical region to with-

stand ; at the same time it can scarcely be doubted that in

the long run ejection may occur without such oscillation.

This daily periodicity in the ejection of spores is obviously

dependent on light, but into these relationships we cannot

enter here. Nor can we discuss in detail the various

means whereby asci open, for the isolation of an apical lid

as in Ascobolus is by no means general.

A certain amount of interest, however, attaches to the
type of ascus which ejects its spores one at a time, as
takes place in many Pyrenomycetes, e.g. Sphaeria scirpi
(PRINGSHEIM, 1858). Before the spores begin to be ejected
a sudden extension of the ascus takes place, amounting to as
much as three times its original length. As a consequence,
the outer lamella of the wall of the ascus is ruptured, and rolls up, while the
inner lamella becomes stretched (Fig. 127) ; a tension thus arises between two
layers of the cell- wall not previously exhibiting any difference. By the vigorous
extension of the ascus the spores (which are in this case multicellular), are
pushed towards the apex, and very soon the uppermost spore is ejected through
the apical opening. As the ascus contracts it becomes slightly shorter and the
next spore at the same time closes the aperture, and so renders any further exit
of cell contents impossible. Thereupon ensues a fresh increase in osmotic ten-
sion in the ascus, increasing until the second spore is
ejected through the narrow aperture, and so on. When
all the spores are shed the ascus contracts markedly, and
at the same time its wall becomes much swollen, and
hence it is very apparent that the ejection is not occa-
sioned by osmotic pressure, or at least not by that only,
but that swelling of the cell-membrane, may take part
in the process. Similarly in many other lower organisms
the spores are pressed out of the mother-cell by swelling
of certain parts of the cell- wall.

Osmotic activity is, however, responsible for the
ejection of the spores of Empusa and its allies (e.g.
Basidiobolus, p. 249) and for the corresponding pheno-
menon in the sporangia of Pilobolus. Let us consider the

of Pilobolus crystallinus (compare DE BARY, 1884).

empty ascus. x 150.
After DE BARY (Morph.
d. Pilze. Leipzig, 1884).

Fig. is?. Sphaeria s
7, extended ascus, the outer
layer of the wall burst. 77,
the last spore has not as yet
been extruded. 777, empty
ascus. After PFEFFER (Pflan-
zenphysiologie, ist edition).

case

In this plant we have a sporangiophore (/, in Fig. 128)

which is much swollen and whose termination is inserted

into the base of the sporangium. When the pressure in the sporangiophore

has reached a certain height a circular rupture occurs in the membrane at r and

the contents of the cell are, owing to the contraction of the wall, ejected just as

in the case of the ascus of Ascobolus. The liquid which is ejected from the

sporangium in this case may be thrown into the air to a height of a metre.

424

TRANSFORMATION OF ENERGY

Fig. 128. Pilobolus, Dia-
grammatic longitudinal
section. /, upper end of
the sporangiophore ; r^
line of rupture ; sp,
After DK

There is another series of examples which may serve as an illustration of
ejaculatory movements, which are due not to the activity of a single cell but
to tensions in many. The method met with in the squirting cucumber (Ecballium
elaterium) (HILDEBRAND, 1873) reminds one of the phenomena of ejection as
illustrated by A scobolus. The elongated ovoid fruit (Fig. 129), owing to the
bending of the peduncle, turns its base upward. It consists of a wall formed of
several layers of cells, enclosing slimy contents enveloping the seeds. When
the fruit is ripe, as is indicated by its turning yellow, the part of the peduncle
nearest to the fruit wall becomes loose, and if the fruit be lightly touched it springs
out of the fruit- wall just like the cork out of a champagne
bottle (Fig. 129, //). At the same moment the mucilaginous
contents of the fruit, together with the seeds, are ejected
with great force to a considerable distance. It is obvious
that the fruit-wall contracts during this process and the
amount of contraction may readily be measured. Thus
a contraction in length from 100 to 86, and in diameter
from 100 to 84, has been observed in a fruit not perfectly
ripe, and in all probability a still greater contraction occurs
in the absolutely ripe condition, but owing to the readiness
with which such fruits explode it is by no means easy to
carry out measurements on them. The fruit-wall must,
therefore, have been in a stretched condition previous to
the bursting. Researches hitherto made on Ecballium
have not determined whether this stretching, as in the case of A scobolus, is due
to osmotic pressure or to pressure resulting from the swelling of the contents.
On the contrary it appears to us improbable that the pressure of the fruit wall
itself operates in the way HILDEBRAND suggests. According to this author
the external layers of the wall, which are composed of large, succulent, thin-
walled cells, are able to extend more than the inner layers, and must in conse-
quence exert pressure on the interior of the fruit and so bring about the
explosion. As a matter of fact, however, the layers of the fruit- wall including
the outer ones, as DUTROCHET (1837) long ago observed, shorten during the con-
traction of the fruit-wall.

In the majority of cases, certainly, stretching is induced in the fruit- wall in
the way ^ in which HILDEBRAND believed it takes place in
Ecballium, that is, by differential stretching of different cell
layers, by tissue expansion. As an example of this we may take
the case of Impatiens (EICHHOLZ, 1885). The fruit is composed
of five carpels, and the seeds arise from an axile placenta.
When the fruit is ripe the five thin lateral walls separate, not
only from the placenta but also from the outer wall, and this
latter splits into five valves, each corresponding to the region
between two lateral walls. When fully ripe the least touch
is sufficient to isolate the five valves from each other, and
each of these rolls itself up suddenly, like a watch-spring,
beginning at the base, striking against the seeds, and throwing
them out. If one attempts to bend the valve straight again,
it breaks across, but if it be plasmolysed all opposition to
the straightening is removed. Hence it may be concluded that
we are dealing here with an osmotic phenomenon, and closer
investigation demonstrates that a layer of parenchyma lying
under the external epidermis acts dynamically, swelling, and
to it the internal cell mass acts antagonistically. In the
complete fruit, this layer is positively stretched, it endeavours to expand, and
this at once results in the isolation of the valves, because its cells possess very

Fig. 129. Fruit
of Ecballium Ela-
terium. /, in lon-
gitudinal section.
If, open at the
top. After HILDE-
BRAND (1873).

MOVEMENTS DUE TO TURGOR AND GROWTH 425

elastic walls and present a very high osmotic pressure (7-5 atmospheres).
Whether the inner layers of the wall exhibit equal osmotic pressure but less
extensible membranes, or whether their osmotic pressure is less, does not appear
to have been determined ; a difference in the extensibility of the membranes
is quite sufficient, however, to account for the result.

Cyclanthera pedata (HILDEBRAND, 1873) may be compared in all respects
with Impatiens so far as its mechanism is concerned, but it differs entirely in
its form. The fruit of this plant consists of three carpels, forming a placenta,
however, on only one of the sutures, bearing two rows of obliquely-placed
seeds. The position of the placenta in the interior may readily be recognized
in the asymmetrical fruit (Fig. 130, /), for on this side there is less bulging,
and the spines, developed on the other side, are here absent.

When the fruit is ripe the wall bursts open, from the top downwards, into
two longitudinal halves, which curve outwards, so that the exterior of both
becomes concave, in consequence of tissue tensions which, however, operate in
the reverse way to those in Impatiens. Certain peculiar arrangements in the
interior of the fruit facilitate the ejection of the seeds, for the placenta becomes
released from that half of the fruit with which it was originally united, but
remains firmly attached to the apex of the other curving half, and hence, when
the sudden rupture occurs, it is slung backwards, and the seeds are in this way
released from their attachment and jerked with considerable force into the air.

Similar tensions, leading to movements, are found not merely in fruits but
in other regions of the plant, and are especially
frequent in flowers. The relation of the stamens
of many Leguminosae (e.g. Spartium) to the
carina is the only case that need be referred
to ; these are suddenly released when the flower
is visited by an insect, whereby the pollen is
ejected. These phenomena do not appear to
have been investigated from the physiological
standpoint, so that we need not discuss them
further. On the other hand, the ejaculatory
movements of the stamens of the Urticaceae j n

have been accurately studied, and a brief Fig iga Fruit of cycianthera ex.
reference may be made to them. Each stamen piodens. / general view, n, longi-

v ,-L n • j • j ji . tudmal section showing manner of

when the flower opens is curved inwards so that opening ; p, placenta. After HILDE-
the anther comes to touch the base of the fila- BRAND (,873).
ment. The concave side of the filament is thus

in a state of compression and attempts to straighten itself, but cannot
do so on account of certain obstacles. At first sight it would appear
as if the perianth on the one hand, and ovary on the other, between which
the anthers are pinched, were the cause, but, as ASKENASY (1879) showed,
it is possible to remove a stamen from the flower without any consequent
straightening of the filament. The anther is glued to the base of the
filament and it is only when this resistance is overcome that the concave side
straightens itself, while the anther opens with a sudden jerk, at the same time
ejecting the pollen. A touch or a slight heating may accelerate this movement,
but it takes place automatically when the osmotic pressure has become sufficiently
great. If the filaments be plasmolysed the tension is abolished, and hence we
may conclude that osmotic pressure alone is responsible for the movement. The
plasmolysed filaments, however, present a certain resistance to attempts to
straighten them on account of the fact that the convex sides are more fully
grown than the concave sides ; the rapidity of the releasing movement must
therefore overcome the resistance offered by the greater length of the convex side.
In the discussion of these examples of slinging movements it has been again

426

TRANSFORMATION OF ENERGY

and again remarked that equalizing of tensions is generally brought about after
the application of a stimulus, such as contact, shaking, &c., but that they may
take place to all appearance of their own accord when the tension has reached
a certain amount. To prove this is, however, by no means easy, for, obviously,
the nearer the organ approaches to a condition of ripeness, shocks, ever de-
creasing in intensity, are sufficient for the purpose, the application of which are
scarcely avoidable in the course of the observation. We have yet to discuss
the significance of these shocks.

In the preceding pages we have often spoken of such shocks, and have
seen that very frequently, by their means, energies stored up in the organisms
are released; they act as 'stimuli.' In speaking of plant formation, we saw
that contact especially might act in this way, and in Lect. XXXVIII we shall

learn to recognize a whole
series of movements which
were initiated by contact. The
question now before us is,
must we group such slinging
movements as we have just
been discussing alongside the
movements resulting from the
application of stimuli,such as we
have yet to consider,'or are they
to be placed in another category,
viz. 'autonomous' movements?
By autonomous move-
ments we mean such as are in-
duced by some internal factor
and not the result of the appli-
cation of an external stimulus
(Lect. XLI). From what has
been said it is evident that
the ejaculatory movements
manifested by spores and suc-
culent fruits may be autono-
mous. When such movements
follow a blow or shaking it is
obvious that the immediate
result consists only in a local
increase in tension which would
arise spontaneously in the
course of further ripening.

Seeing that the blow does not provide the force which brings about the
ejection we have here to deal with a releasing force only, or a stimulus. This
stimulus differs essentially from those usually met with in the plant, so that it
is doubtful whether the ejaculatory movements are to be counted as stimulus
movements or not. There are, however, slinging movements which are
undoubtedly typical stimulus-movements, and in order to show clearly the
difference between these two types it will be advisable to discuss one example.

The slinging movement in question is exemplified in the flower of the
orchid Catasetum, the structure of which will be made clear by a study of
Fig. 131 (DARWIN, 1877). In / the entire flower is shown after removal of five
of the perianth leaves ; the one left is the large labellum (/). In the middle
stands the column, from the face of which project two horn-like appendages,
the so-called antennae (an). Looking at the column in face view (//) it will be
seen that the antennae arise at the base of the massive anther (a). As in all

Fig. 131. Catasetum saccaium. 7, the flower after removal
of five, of the perianth leaves. 77, front view of the column. 777,
pollinium and cement disc. 7-777, after DARWIN. IV, longi-
tudinal section of the column (from nature), a, anther ; an,
antenna ; f, filament ; K, cement disc ; St, caudicle ; p, pollinium ;
7; boundary between the cement disc and rostellum ; A*, stig-
matic cavity ; /, labellum.

MOVEMENTS DUE TO TURGOR AND GROWTH 427

orchids the pollen-grains are here united into pollinia, and the pollinia are
connected by means of a stalk, the caudicle (st), with a cement disc (K). The space
relations of these three bodies may be best seen by an examination of the longi-
tudinal section of the column (Fig. 131, IV). The anther, it will be seen,
placed at the end of the long filament (/) is curved downwards, and it may be
noted that the caudicle uniting the pollinium and the cement disc is bent over a
small cushion of tissue, the rostellum ; perhaps it would be more accurate to say
that the caudicle is composed of the outermost cells of the rostellum and has
become differentiated from the deeper seated parts. The cement disc is also
a part of the rostellum, and only that side of it which faces backward is sticky.

If the flower be placed under appropriate external conditions, one of these
especially being a suitable temperature, and if one of the antennae be lightly
touched with a pencil or splinter of wood, the caudicle and cement disc are
released from the rostellum, the caudicle straightens itself, and throws the
cement disc forwards with considerable force. The movement is so vigorous that
the entire pollinium is jerked out of the flower and is shot through the air, cement
disc first. If it meets any obstacle in its passage the cement disc becomes
firmly attached to it. The biological significance of the movement is perfectly
clear, it is one of the extremely interesting adaptations one meets with in
Orchidaceae for bringing about cross-pollination by insect agency. It is
unnecessary to go into biological details ; what more immediately concerns us
is, in the first place, the nature of the tensions in the caudicle, which is obviously
the mechanical cause of the movement and, further, the significance of the
friction applied to the antennae.

As to the nature of the tensions in the caudicle there have been no investi-
gations, and absolutely nothing is known as to how the effort to elongate the
inside of the caudicle arises ; it may be due to swelling, osmotic pressure, or
growth. An investigation of the structure of the cells of the caudicle should
prove an acceptable subject for research at the hands of the physiological
anatomist. Moreover, the straightening of the caudicle longitudinally is by
no means the only movement which takes place ; at the same time, an inrolling
of its edges occur, so that the outer parts along the edge of the caudicle exhibit
stronger efforts to elongate than the inner parts.

The stipe may be made to spring loose not only by rubbing the antennae
but also by the application of pressure to the caudicle itself. Such a pressure
may be compared with the shaking or bending in the slinging movements
previously mentioned ; obviously it leads to increased tension and so to an
explosion when the resistance is overcome. The touching of the antennae
is another matter altogether ; there we have to deal with a genuine stimulus,
and this stimulus is applied at a considerable distance from the place where the
activity is manifested. It is quite out of the question that any contact between
a pencil point and the antenna can possibly lead to a mechanical deformation
of the caudicle, and so to an increased tension in that organ. What the prece-
dent phenomena are, how they are transmitted, and how they lead to an ejection
of the pollinia is as yet quite unknown.

Hitherto we have considered only such movements as take place in response
to an external stimulus applied to some part of the living plant. This stimulus
may be light, heat, electricity, gravity, or a mechanical or chemical action of
some body. We may indeed distinguish two types of action of such stimuli, viz.
general and special. The general stimuli or so-called formal conditions are
necessary in order that the plant may, first of all, be in the condition to react, in
order that growth and movement may be possible, and in order that special
stimuli may be able to induce movements in it. To the category of general
stimuli belong a certain degree of temperature, a certain amount of oxygen, and
all substances which we have termed nutrients. The special stimuli very fre •

428 TRANSFORMATION OF ENERGY

quently either affect different parts of the plants with different intensity or the
intensity itself alters from time to time. Every stimulus, as we have already
sufficiently shown, is a releasing stimulus only. That the formal conditions
(general vital conditions) operate also as stimuli has not always been clearly
appreciated ; but in the majority of cases it is impossible to regard them in any
other light, and it is often, indeed, extremely difficult to differentiate them from
special stimuli.

Given the general conditions the special stimuli lead to a number of internal
processes which we shall get to know about more exactly later on, and, finally,
to movement ; this we may term the stimulus reaction, or, more accurately, the
visible final reaction. We say that the stimulus induces a certain movement ;
the movement is therefore known as an induced or paratonic movement. Move-
ments, which are outwardly indistinguishable from paratonic movements, are
also frequently to be met with, which are not so induced ; these we speak of as
autonomous movements.

We must now attempt to formulate some suitable classification of stimulus
movements. We might group them either according to the nature of the stimulus,
that is to say, movements induced by heat, light, and so on, or we may base it
on the nature of the reaction, or, finally, on the biological significance of the move-
ment. We will select the nature of the reaction as the principle to follow, and
distinguish first of all the reactions exhibited by motile organisms, which we
shall discuss in the two final lectures, as opposed to the reactions manifested
by fixed forms. The latter may represent either alteration in length, or bendings,
twistings, or twinings, as illustrated on p. 406 in Fig. 119. Owing to these
alterations in form a part at least of the organ takes up a new relationship to
others, or occupies a new situation. When the new situation shows a relation to
the direction of application of the stimulus we speak of movement as a tropism.
When, however, the stimulus is not applied in any definite direction, or when
the orientation of the organ shows no relation to it but is determined by the
activity of the plant itself, we speak of the movements as nastic. We will
begin by considering directive movements or tropisms, and then deal with bending
or nastic movements, endeavouring in each case to determine whether they are
due to growth or to turgor.

Bibliography to Lecture XXXIII.

ASKENASY. 1879. Verhandl. naturw. Vereins Heidelberg, N. p. 2.

DE BARY. 1884. Morphologic u. Biologic d. Pilze etc. Leipzig.

COPELAND. 1 896. Einfl. d. Temperatur u. des Lichtes auf d. Turgor. Diss. Halle-

CORRENS. 1891. Jahrb. f. wiss. Bot. 22, 161.

DARWIN. 1877. Die Befruchtung d. Orchideen.

DUTROCHET. 1 837. Mem. pour servir a 1'histoire des vegetaux etc. Paris. 1,451-

EICHHOLZ. 1885. Jahrb. f. wiss. Bot. 17, 543.

FISCHER, H. 1898. Cohn's Beitr. z. Biologic, 8, 53.

[GANONG. 1904. Annals of Botany, 18, 631 (comp. Bot. Ztg. 1905).]

HILDEBRAND. 1873. Jahrb. f. wiss. Bot. 9, 235.

HILDEBRAND. loxx). Ber. d. bot. Gesell. 18, 376.

PFEFFER. 1892. Energetik. (Abh. K. Gesell. d. Wiss. Leipzig, 18.)

PFEFFER. 1893. Druck- u. Arbeitsleistung. (Ibid. 20.)

PRINGSHEIM. 1858. Jahrb. f. wiss. Bot. i, 189.

RYSSELBERGHE. ' 1899. Mem. couron. Acad. belg. in-8°, 28, i.

SCHWENDENER and KRABBE. 1893. Jahrb. f. wiss. Bot. 25, 323.

STANCE. 1892. Bot. Ztg. 50, 253.

DE VRIES. 1877. Unters. uber die mech. Ursachen d. Zellstreckung. Leipzig

GEOTROPISM. I 429

LECTURE XXXIV
GEOTROPISM. I

No special botanical knowledge is required to convince oneself of the fact,
which all practical experience teaches us, that plant organs assume certain
definite positions in space. The tree trunks in a fir-wood all stand perfectly
erect and are hence all parallel to each other ; their branches, large and small,
always conform to rule, still their lie cannot be stated only in terms of the
angle which they make with the perpendicular, since that obviously does
not comprise all the relations they bear to the chief axis for the time being.
Instead of a fir-tree let us examine a seedling, thus simplifying the problem,
since, in this latter case, the only organs present, at least at first, are those which
grow perpendicularly. At the same time we can observe here, much more
readily than in the case of a tree, the totally different behaviour of the root and
the stem. Both grow perpendicularly to the earth's surface, but the stem
grows upwards, while the root grows downwards. If we place the seedling
in an unnatural position, e.g. horizontally, we note immediately that both
organs begin to bend, the root downwards, the plumule upwards. Since
these curvatures take place, not at the place where stem and root meet, but
near the apices of both organs, a varying length of axis remains horizontal,
and only the two terminations resume the perpendicular position on bending,
growth being continued in that direction. Since almost every organ in the
plant has a certain definite position of rest and endeavours to regain it after it
has been interfered with, we must grant to the plant the capacity of orientating
itself in space, and the movements which its members exhibit in their endeavours
to assume their natural and appropriate positions, not by simple bending merely
but also by torsions and twinings, we term movements of orientation. Obviously
this orientation is the result of the action of certain external factors such as the
distribution of light, water, &c., and the plant must possess sense organs of
some kind by means of which it appreciates the influences thus brought to bear
upon it by the environment.

Very frequently the orientation of an organ is dependent on the combined
influence of several factors, but in the simple cases with which we will begin, as
the downward curving of the root and the upward curving of the shoot, ap-
parently one agent only is concerned, viz. gravity. That gravity is directly re-
sponsible for the perpendicular mode of growth of root and shoot may be shown
by direct observation, for these organs are orientated in the same way over the
whole surface of the globe, that is, parallel to the earth's radii, and we know of no
other force which acts universally in this direction. Still it is not on reflections
such as these but on the experiments of KNIGHT (1806), and SACHS (1874) that
our knowledge of the subject is actually founded. KNIGHT'S experiments rest
on the following basis : — Obviously gravity can cause the root to grow down
and the shoot to grow up only if the seed remain in a state of rest and in the
same relative position with reference to the direction of the earth's attraction ;
hence KNIGHT concluded that if the lie of the germinating seed were continuously
and rapidly changed by being subjected to the influence of another, say centri-
fugal force, the effect of gravity might be suspended.

He, therefore, fastened a number of germinating seeds to the rim of a wheel
in a variety of positions, so that the protruding radicles pointed outwards,
inwards, and tangentially, and rotated the wheel on a horizontal axis. As the
wheel was made to revolve at a very considerable speed, not only was the
unilateral influence of gravity neutralized, but at the same time a very consider-

430 TRANSFORMATION OF ENERGY

able centrifugal force was exerted which on its part affected the seedlings on
one side only.

' I soon had the satisfaction of seeing,' writes Knight, 'that the roots, in
whatever direction they stood in reference to the position of the seed, turned
their apices outwards from the rim of the wheel and in later growth formed
nearly a right angle with the axle. The young stems on the other hand
grew in the opposite direction, and in a few days all their apices met in
the centre of the wheel.'

In this experiment the seedlings are influenced by centrifugal force exactly
in the same way as they are by gravity when grown under natural conditions.

In another experiment KNIGHT allowed gravity and centrifugal force
to act at the same time but in different directions. The seedlings were fastened
to a horizontally rotating disc and the distance of the plants from the centre
and the speed of rotation of the disc were so arranged that the mechanical
effect of gravity and of the centrifugal force were equal. Under such circum-
stances the roots grow outwards and downwards at angle of 45°, while the
stem grew upwards and inwards at a similar angle. When the speed of

Fig. 132. PFEFFER'S Klinostat, as manufactured by ALBRECHT of Tubingen.

rotation was increased, the axes of the seedlings took up a position which
gradually approached the horizontal. From this it must be concluded that
the plant is unable to discriminate between centrifugal force and gravity and
that one force may be replaced by the other. Both forces have this in common,
however, that they act as accelerating forces on the plant body.

Long afterwards, SACHS'S experiments (1874) added very important facts
to the fundamental data established by KNIGHT. In SACHS'S as in KNIGHT'S
first experiments the seedlings were made to revolve round a horizontal axis
but the speed of the revolution was very low, viz. about one revolution in
10-20 minutes. This speed indeed is so low that no centrifugal force worth
mentioning is produced ; since, however, the unilateral action of gravity is
eliminated owing to the continuous revolving movement of the disc, the roots and
shoots go on growing in the directions in which they were originally orientated.
By the employment of this apparatus, the curving (/cAiVctv) of the plant is
inhibited, so that SACHS (1879) termed it a Klinostat. Fig. 132 illustrates
such an apparatus in operation. The horizontal axis is driven by clockwork
and to this axis the plant is attached ; to the mechanical arrangements for
altering the speed of rotation we need not pay any attention.

GEOTROPISM I. 431

The movements of orientation of the plant we term ' tropic ' curvatures,
and the capacity it possesses for producing such curvatures we term ' tropism '
(p. 428). According to the nature of the external cause we may distinguish
tropic movements due to gravity, light, &c., or, in other words, we may speak
of geo tropism, photo tropism, &c. In the present lecture we have to deal with
geotropism, and from what has been said it will be apparent that we may recognize
two varieties of geo tropism : positive geo tropism — such as are exhibited by roots
and all other organs whose direction of growth is towards the earth's centre, and
negative geotropism as manifested by shoots and such other parts of the plant
as grow away from the earth's centre, parallel to a radius from it. Although
root and shoot may be considered as characteristic organs illustrating the
two types of geotropism, it would be quite incorrect to assume that the geo-
tropic reaction was determined by the morphological nature of the organ.
The nature of the reaction is rather determined by the necessities of the plant,
and hence we meet with roots which are negatively geotropic, and grow out
of instead of into the soil (e. g. the pneumatophores of palms, &c., KARSTEN,
1890) and positively geotropic shoots which burrow into the soil or at least grow
in a downward direction (e.g., rhizomes of Yucca and Cordyline, and many
flower stalks after pollination, &c.). Nor is the type of geotropism always
constant for the same organ, for, as we shall see later, a normally positively
geotropic organ may become negatively geotropic and assume some other
relationships to the direction of the action of gravity, to which we have not as
yet made any reference. On the whole it may be said that geotropism is
a phenomenon of wide distribution in the plant world, for we meet with it not
only in the highest plants but also in mosses, Algae and Fungi ; it appears
both in multicellular structures and in unicellular organs (internodes of Nitella,
rhizoids of Chara), and in unicellular (coenocytic) plants such as Mucor and
Phy corny ces. On the other hand, some plants, such as the mistletoe and many
Algae, are not geotropic at all.

Our next task must be to examine more closely the precedent phenomena
of geotropic curvature. That this movement depends on the unequal elonga-
tion of opposing sides of an organ is self-evident, but how this arises cannot
be deduced from the actual curvature itself. Curvature, as we have seen in
the preceding lectures, may arise from turgor or from growth. Geotropic curva-
tures, as a matter of fact, arise in both ways, but curvature due to changes in
turgor occurs only in organs which we do not propose to discuss in this lecture.
We will confine ourselves at present to a consideration of those which are due to
growth, and endeavour to explain the principles of the subject by reference to
a few examples. We will select for that purpose multicellular organs, for there
are no exact researches available in unicellular organs. Geotropic curvatures
have served far more frequently as a means of demonstrating theories than as
the subject-matter of exact observation. Almost all that has been done in this
latter relation is due to SACHS. There is no reason to suppose, however, that
there are any differences in this respect between unicellular and multicellular
organisms.

Let us begin with geotropic curvature in the tap root, which we will imagine,
to begin with, is laid horizontally. Fig. 133 (SACHS, 1873) shows the different
stages in geotropic curvature taken up by the root of Vicia faba grown in very
loose soil at a temperature of 20° C. The growing region (p. 289) is divided into
five equal parts, each 2 mm. long, from the growing apex backwards ; these
may be indicated by the numerals I (from o to i), II, III, IV, V respectively.
A pointed paper index points to o (A). In B the same root is figured an hour
later ; the root is still straight, but it has already elongated about 1-6 mm., as
the change in the position of o shows. In C the root, after two hours' interval,
is seen to have developed still further and to have curved considerably. If the

432

TRANSFORMATION OF ENERGY

course of the root be watched through a transparent sheet of mica, on which circles
have been scratched, it will be found that it takes the form of an arc of about
15 mm. radius. D shows the same root seven hours after the commence-
ment of the experiment, and now it will be seen that the marks I and 2 have
already moved past the index, and that the root as a whole has elongated more
than 4 mm., the individual increments measured on the convex side being : —

Zone

Increase in mm.

V.

0-4

IV.

i-o

III.
1.8

II.
0-8

I. (apex) Total.

0-2 4-2

k

^f4-

B

*±i

k

1 *-

C

345

The curvature is further sharpened ; the radius of the convex arc, which was
15 mm. in C., is reduced to 10 mm. in D. The curve
corresponds to the arc of a circle, in the formation of
which all the growing parts take part up to mark 5, al-
though apparently the zones II and III are more sharply
bent than I, IV, and V. E illustrates the root after
twenty-three hours, and the curvature now exhibits
two changes ; in the first place, it is no longer repre-
sented by an arc of a circle, the curvature is much
greater between marks 2 and 3 than in the region in
front or behind ; in the second place the radius of
the curve between 2 and 3 is still further reduced,
viz. to about 8 mm. In stage D the apex of the root
lies at an angle of about 45° with the horizontal, in
E it is at right angles to the horizontal, and we can
see that the cause, but not the only cause, of the down-
ward direction taken by zones II and I is the bending
and growth of zone III (between 2 and 3). In zone II
curvature is still apparent, which decreases towards
mark i, while the bending in zone I is scarcely observ-
able at all. From mark 3 to the apex the form of the
root approaches a parabola whose apex lies somewhere
near 3 (SACHS, 1873 b, p. 440).

If we now inquire why it is that at stage D the
growing region does not show equal curvature in all
zones, we shall find that the reason lies in the dif-
ferent intensities of growth in the separate zones and
also the different positions assumed by them. Zone IV,
after seven hours, has increased markedly less than
III, and V is already full grown. The capacity for
bending has thus ceased in zone V, while in IV it is ob-
viously much less than in 1 1 1 . Zones I and 1 1 , however,
which in the end grow much more rapidly than III, in
a very short time attain the vertical, that is, succeed
in reaching a position where the geotropic stimulus
can no longer affect them. It would appear, however,
that another important condition must be taken into
account in considering the localization of the chief
region of curvature in zone III, a condition the value of

Fig. 133. Geotropic curvature
After SACHS, from
Smaller Practical
1903. (The index in
a little more to the

in a root.
DETMER'S

JDs
right).

lay be estimated with the greatest certainty in organs with longer growing
than roots have. The straightening taking place in zone I must, as

which ma^

regions

shown in the figure, be partly the result of elongation, for a curved organ must

become gradually flatter, as simple geometrical considerations show us, if it

grows equally both on the convex and concave sides.

From what we have now seen we may conclude that the bending of the
root is limited to the growing zone, but our observations have taught us nothing

GEOTROPISM. I 433

as to the more immediate course of growth. In this relation there are the
following possibilities to be taken into account :—

1. Growth proceeds on one side with uniform rapidity.

(a) This side is the concave side, but an increased growth must then
occur on the convex side.

(b) This side is the convex side, but the rate of growth must be reduced
on the concave side.

2. Growth alters on both sides, decreasing on the concave and increasing on
the convex.

In the second alternative the decrease of growth on one side may be as
great as the increase on the other, and then the rate of growth in the axis
of the root, which is equidistant from the convex and concave sides, does not
alter at all ; but in the former possibility growth of the axis must always alter,
showing an acceleration in (a) and a retardation in (b). In order to demonstrate
this point clearly SACHS (1873 b) calculated the increments of growth on
the convex and concave sides, and also in the axis of roots which had for
some hours undergone geotropic curvature, and, for the sake of comparison,
corresponding measurements were made on a root which was allowed to grow
straight. The following is a summary of the results obtained : —

Convex side. Concave side.

Axis.

Straight root.

Increase in 4 zones •

Root No. i

,, No. 2

10.8
8-7

6.1
5-3

8-4
7.0

10-5
8-5

Increase in 3 zones j

» No. 3
n No. 4

5-8
6.7

2-8

4.2

4-3
5-5

5-5
6-0

32-0 18-4 25.2 30-5

This table shows that the curvature both on the average and in each individual
case is due to a slight acceleration of growth on the convex side and a marked
retardation on the concave side ; axial growth is more restricted than in the
root allowed to grow normally. [According to LUXBURG'S (1905) measure-
ments SACHS'S results are not to be depended on. This author holds that
decrease of growth does not take place in the middle line.]

Negative geotropic curvature in a stem is illustrated at Fig. 134 (SACHS,
1888). The region in this example which is capable of growth is about 50 cm.
in length. It has been divided into five zones by indian-ink lines, the four lower
(5-2) being each 100 mm., the uppermost (i) only 50 mm. long. The stem was
laid horizontally at noon (a). After 3^ hours (b) curvature had taken place
in all the zones ; zone No. i had shown the greatest curvature (radius = 16 cm.),
the least curved was zone No. 5. After 5j hours (c) the greatest curvature
was observable in zones Nos. 3 and 4, while zone No. i, which had already bent
beyond the vertical, had begun to straighten itself. After twenty-two hours (d)
zones 1-3 had become erect and the chief curvature (with 7 cm. radius) lay between
the bottom of 4 and the apex of 5. There are two phenomena worthy of
note in this experiment. In the first place, the removal of the region of most
vigorous curvature to the still growing base from the zone of maximum growth
near the apex, where it first appears, and, in the second place, the supra-curvature
of the apical region, occasioned not only by the after-effect of the geotropic stimu-
lus but also by the basal progression of the bending. This supra-curvature is in
some cases much more apparent than in the case of Cephalaria, as may be
seen from a glance at Fig. 135. The supra-curvature is, however, very soon
neutralized, for a new geotropic stimulus begins to operate in the opposite
way, and for other reasons which we have already hinted at (p. 432), but of
which we shall have to speak later on.

The final result is invariably that a definite basal curving takes place

JOST F f

434

TRANSFORMATION OF ENERGY

at the boundary between the completely grown and still growing regions,
and that the entire apical region becomes perfectly perpendicular.

In order that we may clearly appreciate the distribution of growth in
the shoot we will study more in detail SACHS'S numerical results from experi-
mental research on the stem of Cephalaria, as figured at Fig. 134. The letter
U indicates the increase in length on the under side, O that on the upper side,
both in mm. ; and it must be noted that the uppermost zone at the commence-
ment is only half as long as the others ; R represents the radius of the arc of
the curvature in cm.

Zone 5.

Zone 4.

Zone 3.

Zone 2.

Zone i.

U.

O. R.

U.

O. R.

U.

0. R.

U.

O.

R.

U.

0. R.

Stage b

1-2

o-o 47

i-5

o-o 40

43

0-4 30

4.1

1.6

21

3-o

o-o 16

Stage c

i-5

o-o 46

4-5

o-o 1 8

6-0

i-o 15

50

i-3

18

4.4

i-3 17

Stage d

10-0

-i.o 7

95

3-o 7

15-0

15-0 oo

150

13-6

00

5-o

4.0 co

Since we have no data available as to growth in an uncurved control

3.

Fig. 134. Cephalaria procera. a, stem laid
horizontally at noon, showing divisions; zones
-5» -/i 3-, and 2 each 10 cm. long, zone i. 5 cm. long;
A, the same 34 hours later ; c> 2 i- hours later than b ,
d, 1 6 hours later thane. After SACHS (1888). About
•one-tenth nat. size.

Fig. 135. Geotropic curvature in Allium atropurpureum
in various stages (/-/). After SACHS ^1882, p. 839).

experiment it is impossible to say here, as in the case of the root, whether an
elongation takes place over all (measured on an axial line), and we can, therefore,
only affirm that, during curvature, growth on the concave side frequently
remains stationary, or that it shows a distinct retardation. We must not
attempt to generalize on these results, since growth takes place on the concave
side, not merely in the example we are considering, but in other organs also,
and the principal fact is that there is*a differential growth on both sides. As
a second illustration we may take the measurements made by NOLL (1888).
This author established the fact that in Hippuris, a geotropic curvature occurred
when the increase on the under side was (in twelve hours) 5 mm. and on the upper
side 0-25 mm. In the same time the axis increased about 2-6 mm. Comparing
these figures with growth in an erect shoot of Hippuris we find growth in the
latter to amount only to i-o mm. Here, therefore, we have to deal with an
acceleration of growth and not, as in the root, with a retardation. [LUXBURG
(1905) was unable to confirm this growth acceleration in Hippuris ; on the con-
trary, this author has shown that both here and in other plants during the
•curving a retardation of growth occurs.]

GEOTROPISM. I 435

Differential growth has been established not only between the upper
and under sides of an uninjured and complete organ but also in isolated portions ;
thus in stems, at all events, curvatures have been observed in parts cut out
from a shoot, while roots, after wounding, remain for a longer time insensitive to
the influence of gravity. It is by no means remarkable that separated segments
of stems should still show geotropic curvature, but the behaviour of plant organs
when split longitudinally is worthy of note. If a stem be split longitudinally a
curving outwards of each half must occur in consequence of differences in tissue
tension, but if one of these halves be arranged so that its epidermis is upper-
most, and the other half be in the reverse position (the cut surface of the
medulla lying horizontally), geotropism influences each half differently, and
induces in them differential growth between the upper and under tissues ; in
the section which lies epidermis upwards growth in the medulla is accelerated
while the epidermis shortens, in the other half the epidermis increases in
length and the medulla grows less vigorously than in the other half. Tissue
tensions, however, in this case, to a certain extent, obscure purely geotropic
curvature. If similar researches are made with nodes of grasses, i. e. with
the swollen basal regions of the leaf sheaths where tissue tensions of this type
are absent, geotropic curvature may be determined both in the upper and under
longitudinal halves ; it makes no difference which side is uppermost. DE VRIES
(1880) has shown that geotropic curvature occurs in each longitudinal area
even if the shoot be divided in four.

Grass nodes are of interest from another point of view. In the organs
hitherto spoken of the geotropic curvature depends on longitudinal growth ;
where longitudinal growth ceases there curvature is also absent. At all events,
this is the conclusion to which all investigators have come who have examined
the question, with the exception of KOHL (1894), who holds an opposite view.
The nodes of Gramineae are able, however, to develop geotropic curvature in
the full-grown condition, for they are capable of renewing growth each time
they are removed from the position of geotropic rest. In this curvature
the under side undergoes great elongation ; in a very short time it becomes
double or even as much as five times as long as it was, while the upper side
is forcibly compressed so much as to throw it into folds. A few numerical
details (SACHS, 1872, 206) will make this clear.

Cinquantino Maize.

Length of nodes in mm. Upper. Under. Upper. Under. Upper. Under.

Before bending 4-3 4-1 4-0 5-0 5-0 5-0

After bending 2-5 90 3-0 n-o 4-5 12-5

Difference — 1-8 +4-9 — i-o + 6-0 -0-5 +7-5

More recently, from many points of view, it has been shown that not
only grass nodes and related structures, but many other full-grown organs, may
develop curvature when subjected to geotropic stimulus. Branches also which
are secondarily thickened may exhibit geotropic curvature, which cannot
be induced in plants, e. g. palms, which have no power of secondary growth.
It must be assumed that the power of curving rests in the power the cambium
has of producing elements of different lengths on either side. Detailed investi-
gations on these points are, however, not available. (Compare MEISCHKE, 1899 ;
Josx, 1901 ; BARANETZKY, 1901.)

In every case which has been accurately studied the immediate cause
of the curvature is a difference in longitudinal growth of opposite sides. As
is generally the case a stretching of the cell-membranes due to turgor pre-
cedes surface growth, and this is gradually rendered permanent by growth. If
the organ be plasmolysed at the commencement of geotropic curvature it
grows at first in a straight line, later on, however, the curvature is permanent.

F f 2

436 TRANSFORMATION OF ENERGY

Turgor extension is unequal on the two antagonistic sides. The difference
would appear to depend on the fact that the osmotic pressure increases on the
convex and decreases on the concave side, but that is by no means the case ;
on the contrary, the pressure on the concave side appears to remain constant,
while that on the convex side is reduced. Since the rate of growth of the
cell- wall does not depend directly on the amount of osmotic pressure, there is
nothing very astonishing in this. Unequal turgor extension of the two sides
must depend on an alteration in the elasticity of the cell- walls. We have
already seen that there are no reliable data available as to the causes for such
alterations in elasticity any more than there are on the general question of the
mechanics of growth in the cell- wall. On that account it is needless for us to
enter into such controversial points, although geotropic curvatures have
frequently been brought forward in support of different views as to the
mechanics of growth in the cell-wall. Unfortunately, measurements made of
curving organs often do not at once determine whether the concave or the
convex side, or both, are actively concerned in the bending. In many cases,
such as those where the concave side is directly shortened, there can be no doubt
that it behaves passively, and that the curving is the result of vigorous stretching
of the convex side aided by rigidity in the axial region. If, as in grasses,
the concave side be thrown into folds, the passivity of that side makes itself
apparent at once. It does not always behave in this manner, however. Certain
experiments of SACHS (1873 a), where the several tissues were removed during
the bending, tend to show that the axis (medulla in the case of the stem) is
not directly concerned in the process, but this is true only of the uninjured
plants and not of the longitudinally spilt internodes mentioned on p. 435.

Assuming then that curvature in general depends on unequal growth
on opposite sides, that, in positive geotropism, growth is retarded on the
side of the organ facing the soil, while it is accelerated on the upper side,
and that in negative geotropism the distribution of growth is reversed, we have
next to ask how it is that gravity influences growth, and especially how it is able
to influence different organs in different ways. As a matter of fact, the question
has been in a sense already answered by our describing geotropism as a stimulus
reaction ; the significance of this terminology being that gravity is to be regarded
merely as a releasing force and not one which acts in a purely mechanical manner,
and this conception of the phenomena must obviously be looked upon as the
correct one, when we remember that gravity induces diametrically opposite
reactions in positively and negatively geotropic organs. The history of the
science, however, shows (as to the history of geotropic investigations compare
SCHOBER, 1899) that this conception was only arrived at as the result of con-
siderable labour and was by no means self-evident from the very first. As a
matter of fact, even as late as the seventh decade of the last century, an in-
vestigator of the rank of HOFMEISTER (1863) attempted to show that gravity
acted in a purely mechanical way. This author believed that the softness of
the root accounted for its capacity for bending, which in turn was induced
by the weight of the apex. Into his explanation of the negative geotropism
of the stem we need not enter, for it has only an historical significance. The
theory based on the plasticity of the root apex is also of interest only from
an historical point of view, although it is full of lessons for us even nowadays,
as showing how thinking men under the dominance of a preconceived notion
may go blindly in opposition to facts ; it reminds us also that the fact is the
chief thing, the theory only, as it ought to be, the ever-changing expression
of the aggregate of experience. Had HOFMEISTER not been imbued with a
preconceived idea he must have seen that the apex of the root really more
closely resembles a piece of glass than a stick of hot sealing-wax, JOHNSON
(1828) had long previously shown that the weight of the root apex may

GEOTROPISM. I 437

be supported by a force equal in amount and yet that the geotropic curva-
ture was still maintained, and in 1829 PINOT found that the root apex
was able, when bending downwards, to force its way into mercury, and could
overcome a very considerable opposing pressure. HOFMEISTER was, however,
unacquainted with these older observations, and FRANK (1868) was the first
to replace the view of passive sinking of the root apex held by HOFMEISTER
by the correct interpretation, when he said that in geotropism we were dealing
with a special evolution of energy which was released in the interior of the
organ by gravity. Subsequently the effect of gravity as a releasing force was
fully discussed by PFEFFER (1875, 1893 a) and by SACHS. It ought also to be
mentioned that, in 1824, DUTROCHET spoke of gravity as a releasing agent,
although, later, he enunciated another view.

As the downward curving of the root may be accompanied by a recog-
nizable expenditure of energy, so also negative geotropism is the result
of a similar expenditure, because a weight has to be lifted, and because the force
acts on a very much larger lever in the shoot than in the root. PFEFFER (1893 b)
has recently studied the work done in the nodes of grasses subjected to geotropic
curvature, and MEISCHKE (1899) has examined other plants with a similar
end in view. It has been proved that the energy required for carrying out
the work accomplished reaches values such as we might expect to obtain on
the principle (p. 422) that geotropism is a growth phenomenon. It has been
shown more particularly that the energy expended in making a stem of a grass
stand erect is about that required for the purpose, but that, as a general
rule, a large surplus of energy is developed in other geotropic curvatures which
permits of the straightening of the shoot after pronounced over-curving of the
apex. Although we cannot go into details here it will be sufficient for us to know
that the work done during the curving bears no ratio to the energy provided
by gravity. The energy required to produce the movement is supplied by the
growing parts of the plant itself, gravity acts merely as a releasing force.

If gravity acted only through the weight of the moved organs the nature
and amount of the resulting curvature might be determined according to
mechanical principles, but if it acts as a stimulus we must first of all de-
termine experimentally how far the curvature depends on the duration, in-
tensity, and direction of the force. Even in rapidly reacting organs there is
always an interval of about one to one and a half hours, before the horizontally
placed organ shows a noticeable curvature, and this latent period may in other
cases be extended to several hours. It is by no means essential that the plant
should be stimulated continuously until the reaction begins — it is quite sufficient
if the stimulus be applied for a shorter period. As CZAPEK (1898) has shown,
the sporangiophores of Phycomyces, the hypocotyl of Beta, the first sheathing
leaf in seedlings of Avena sativa and Phalaris canariensis, after exposure
to the geotropic stimulus for only fifteen minutes, exhibit later a curving, even
if meanwhile they be placed in a vertical position, or, better still, rotated on a
klinostat. An after-effect following on a geotropic stimulus makes itself apparent
not only if the organ during the curving is gradually placed in its normal rest
position but also if the stimulus be removed long before any visible reaction
occurs. The minimum time during which the geotropic stimulus must be applied
in order that a bending may take place as an after-effect, we term the latent period.
It has in no case been found to be less than fifteen minutes, and amounts to
twenty minutes in the radicles of Zea, Pis urn, Lupinus, and Cucurbita, to fifty
minutes in the epicotyl of Phaseolus, and to several hours in other structures.
[According to FITTING'S (1905) researches CZAPEK' s numbers are not to be relied
on ; the latent period may often amount to only 6-7 minutes, and, according to
MOISESCU (1905), it may be even less than that. There is no doubt that the latter
author's employment of the microscope for the determination of the commence-

438 TRANSFORMATION OF ENERGY

ment of curving suggests further lines of research, but his statements are open
to criticism.] The period of duration of the stimulus affects very markedly
the beginning of the geotropic curving, for while roots of Lupinus, kept in the
horizontal position for thirty-five, forty, fifty, sixty minutes, bend rapidly one
after the other on the klinostat, so that after ninety minutes the reaction is visible
in every one of them, the curving does not begin for two or three hours when the
roots are exposed for only twenty minutes. We may suppose that a stimulus
applied for a still shorter period, e. g. less than the latent period, must still have
some slight effect on the plant, although it does not induce a visible curving.
Indeed, there are observations recorded which confirm this supposition, i. e. ex-
periments on intermittent stimuli. If the root of Linum be placed horizontally
for two minutes and vertically for six minutes alternately, geotropic curvature
takes place after a certain time, although each separate stimulus is far shorter in
duration than the latent period, and hence is unable alone to induce a curving.
If each individual stimulus made no permanent impression on the plant it would
not be possible for a summation of these stimuli to result in a geotropic reaction.
We must assume that every stimulus, however short it be in the period of applica-
tion, produces some internal change, which we may describe as an excitation. This
excitation lasts longer than the stimulus, and hence a summation of excitations
is possible, and when this summation reaches a certain amount then the liminal
intensity is exceeded and curvature begins. Detailed research is still required
before we can say how small the stimulus periods may be when these follow each
other at regular intervals and also how great the intervals may be between the
individual stimuli, for it stands to reason that there must be limits to both.
The matter is of importance, for the whole theory of the klinostat rests on the
results of such experiments. We are as yet quite ignorant, for instance, whether
plants are really geotropically stimulated at all when placed on a klinostat or
whether the individual stimuli neutralize each other. CZAPEK till recently (comp.
1902, 468) held the former view. We may imagine with him the uniform rotation
of the klinostat replaced by four successive jerks, so that the plant for a certain
time is allowed to rest in each of the four chief positions, viz. above, right,
below, left. The plant, according to CZAPEK, must remain in each of these
positions for a briefer time than the length of its latent period ; thus if the latent
period be twenty minutes one complete rotation may be effected in sixty minutes,
so that the plant remains for fifteen minutes in each of the four positions, i. e. less
than the latent period and hence is not stimulated. According to NOLL (1900),
however, we have in this case to deal with intermittent stimuli, for at forty-
five minutes intervals, a .definite face is brought under the influence of gravity
for fifteen minutes, but bending cannot take place because each stimulus is again
neutralized by the corresponding stimulus applied when the plant is in the op-
posite position. In many organs it is quite immaterial, so far as the result is con-
cerned, whether on the klinostat the reaction only or the stimulus as well ceases ;
the nodes of grasses, however, behave quite differently in either case, they must
be able to discriminate between the two possibilities. If laid horizontally they
not only bend but also exhibit renewed growth. If they be placed in a klinostat
they begin to grow (ELFVING, 1884), but their growth is uniform on all sides.
This behaviour of the grass nodes appears to prove (compare PFEFFER, Phys. II,
126, and NOLL, 1902, p. 413) that the movement of the klinostat inhibits the
geotropic curvature but not the geotropic stimulus. If this conclusion be correct,
then undoubtedly an alteration in the rate of growth on the klinostat may be
universally demonstrated where an organ laid horizontally grows axially more
slowly or more rapidly than when placed vertically ; it (e. g. Hippuris) must
also grow more rapidly on the klinostat than under normal conditions. Only
such organs as exhibit a retardation on the concave side equal to the acceleration
on the convex side may go on growing in an unaltered form on the klinostat.

GEOTROPISM. I

439

Experiments on this subject are as yet scanty, so that we need not attempt to
arrive at any definite decision between the two theories. [Meanwhile FITTING'S
(1905) careful investigations have made us more thoroughly acquainted with
these geotropic phenomena. This author has studied in detail the results
of intermittent stimulation, and was able to show that the duration .of a
single stimulus may be shortened at will, and that by summation of these
stimuli a movement is finally induced. FITTING has also studied the relation
between the period of duration of the stimulus and of the interval of non-stimulus
in the case of intermittent stimuli, and has found that geotropic curvature
invariably takes place if the intervals are ten times as long as the periods
during which the stimulus is applied. He has also shown quite clearly that only
the bending and not the perception of the stimulus is impossible on the klinostat.]
In addition to the duration of the geotropic stimulus we must also take
into consideration its intensity. The variations in the amount of the stimulus,
however, as observed in different regions of the earth, are so minute that
it is quite impossible to deal with them experimentally, even if they were
more accessible to the observer than they really are. KNIGHT'S discovery
relieves us from all difficulty in this respect, for we may increase the centri-
fugal force to any extent we please, and we may thus study the dependence
of the latent period on the amount of this force. If we vary the centrifugal
force so that on the one hand it amounts to thirty-eight times the value of g.
(gravity) and on the other hand reduce it down to 0-0005 §• the reaction takes
place (in the root of Vicia faba, CZAPEK, 1895) in the following times : —

•>h. ih. i|h. i|h. 2^h. 3h. 4h. sh. 6h. 8h.

38-35 g- 28-10 7-4-3 3-5-°-9 0>6 0-5-0-4 0-2—02 0-003 o-ooi 0-0005

From these experiments it follows first of all that the plant responds in the
same way but more slowly to a pulling force a thousand times less than g.
As in all movements manifested in response to a stimulus so in the case of
geotropism the stimulus must reach a certain amount, the so-called 'liminal
value', before a reaction follows. The reaction follows all the more rapidly the
more vigorous the releasing force is, and we may, therefore, conclude that the
effect of the stimulus or the excitation in the plant is so much the greater the more
vigorous the centrifugal force is. No investigations have as yet been made on
the effect of still greater intensities of centrifugal force. It must not be assumed
that the excitation increases pari passu with the increase in the centrifugal force,
because this force will in the long run have an injurious effect on the plant, or
at least retards growth (ANDREWS, 1902). It may also be possible to determine
experimentally an apex on the stimulus curve (region of greatest excitation) and
an upper limit of stimulus in addition to the already known liminal intensity.
We may alter not only the duration and intensity but also the incidence of the
geotropic stimulus. If a shoot be so placed that it grows in a straight line upwards,
that is to say, parallel, but in the opposite direction, to that of the geotropic
stimulus, there is no reaction at all, or, to be more accurate, there is no geotropic
curvature. If, however, the shoot be placed at an inclination to the vertical so
that the line of direction of gravity makes an angle with the axis, a curvature takes
place, owing to the fact that on the under side growth is accelerated and on the
upper side retarded. The influence of gravity will have all the greater (but purely
mechanical) effect the more nearly the stem approaches the horizontal. In that
position gravity should have its maximum effect, and if we go on turning it over,
that effect will again be diminished, until finally, in the inverted position, it will
have reached zero. Recent researches do not, however, confirm this view.
CZAPEK (1895), employing various methods of producing the excitation, found
that the maximum effect was produced when the angle 135° downwards was
reached. Roots behave exactly in the reverse way, responding most when placed

440 TRANSFORMATION OF ENERGY

at an angle of 135° with the vertical and pointing upwards, i. e. 45° above the
horizontal. [Several authorities, FITTING (1905) especially, have shown that
the optimum stimulus is given in the horizontal position.] In both types of
organ, however, it may be determined that in addition to the normal position
the inverse position is also a position of rest ; certainly roots bend downwards
in a short time when inverted and shoots upwards.

These reactions are, however, consequent on small curvatures induced
by internal factors, by which a deviation from the position of rest is brought
about. If a plant turned upside down be mechanically prevented from per-
forming any autonomous curvature a geotropic curvature never takes place as an
after-effect on the klinostat. There is always one noticeable difference between
the two positions of rest, the normal position is stable but the inverted position
is labile. Any organ which is inclined somewhat from the inverse position
does not bend back again into that position but attempts to assume the normal
direction. The only point we have to deal with here is the transference of the
stable into the labile position, and vice versa; we have only to alter the end by

Fig. 136. Two shoots of Physostegia^ fixed horizontally in wet sand and in a moist atmosphere, one (the right)
by its base, the other (the left) by the apex. Both show geotropic curvature. (Slightly reduced).

which the plant is fixed (FRANK, 1868 ; NOLL, 1892). If we employ isolated
branches and fix them horizontally by their apices the reaction which takes
place is the same as that seen when the branch is fixed in the normal position,
that is to say, the side facing the ground exhibits growth acceleration, but the
further results are quite distinct, for the base bends upwards and reaches the
stable rest position in the inverted lie (Fig. 136).

Any attempt to discover why a geotropic curvature follows when the plant
is in certain positions, while other positions may be described as positions of rest,
at once suggests the question, what is the initial effect of gravity on the plant ?
Researches which have been carried out during recent years show more and more
clearly that between the application of the stimulus and the movement a whole
series of processes takes place, whose existence is rendered especially prominent
if the application of the stimulus and the performance of the movement affect
distinct and widely separated parts of the plant. Under such circumstances at
least three different operations may be distinguished ; (i) the appreciation or
perception of the stimulus by the receptive or sensitive organ ; (2) the reaction of

GEOTROPISM. I 441

the motile organ ; and (3) obviously the transference of the former to the latter.
The credit of having been the first to clearly distinguish between stimulus and re-
sponse belongs without doubt to CHARLES DARWIN (1881), although the evidence
he put forward in support of the fact has not stood the test of later criticism.

Such localization and separation of perception from response have now
been demonstrated with certainty in the case of heliotropic stimuli (Lecture
XXXVI) ; whether the same is true of geotropisrh also is not quite so well
established. In spite of CZAPEK'S (1895, 1900) ingenious experiments, which we
cannot pause to describe here, it is very doubtful, for many reasons, whether
the seat of geotropic perception lies really in the root apex or only in the root-
cap (NEMEC, 1900, 1901) ; similarly, there is considerable room for suspecting
the truth of the assertion, so often made, that in the case of the seedlings of
certain grasses (Paniceae) it is only the extreme apex of the cotyledon that
is sensitive to the geotropic stimulus. [Even yet this question has not been
decisively answered ; new methods have been invented by PICCARD to deter-
mine this point (1904), but his results cannot be considered as above criticism.
Compare also RICHTER (1902), DARWIN (1902), and MASSART (1902).]

Although a considerable separation between the region of perception and of
movement has not as yet been demonstrated in the case of the stimulus of gravity
there are certain recorded observations of CZAPEK (1898) available which go
to show that there are at least two different series of processes : those concerned
with the perception of the stimulus and those concerned with the reaction.
As in the case of all movements in response to stimulus, geotropism depends
on certain general or formal conditions ; there must be a certain temperature,
a certain nutritive supply, water and oxygen in sufficient quantity, &c., &c.,
before any geotropic response can take place. The conditions which have
to be fulfilled before the stimulus can be appreciated are not the same as those
which govern the occurrence of the reaction, for the stimulus may be perceived
under circumstances where no growth or geotropic response may take place.
Thus at 2° C. a geotropic response may be induced after a sufficiently long
exposure to the stimulus of gravity, but the movement is carried out only when
the plant is exposed to a higher temperature. Again, the stimulus of gravity
may be appreciated in an atmosphere free from oxygen, but that gas is essential
for the carrying out of the movement. In this way, or by methods of a similar
character, we may convince ourselves of the existence of two separate phenomena,
phenomena of perception and phenomena of reaction.

*How comes it to have sensitivity of this kind? What is it that the
plant perceives when gravity affects it ? KNIGHT'S experiments leave no
doubt in our minds that gravity influences the plant only through mass ac-
celeration, which it exercises on all bodies, i. e. by weight. But it is also certain,
as we have seen, that the weight of the overhanging part of the plant above
the zone of curvature has nothing to do with it, since we can neutralize that
without stopping the geotropic movement. What we have to deal with is
an effect of weight operating in the interior of the plant, even in each individual
cell. Since, however, it is not infrequently the case that most of the proto-
plasm exhibits streaming movements it follows that it is only the quiescent
ectoplasm, as NOLL has pointed out (1888, 532), that can be sensitive to the
stimulus of gravity ; it must be able to distinguish between varying pressure
on different sides of the cell. Let us now assume the whole cell contents,
vacuoles, and streaming protoplasm to be the cause of the weight, in the vertical
position of the cell a lateral pressure must be exerted on the ectoplasm, but any
two opposite regions must be subjected to similar pressures. Let us now incline
the cell somewhat out of the perpendicular, then a pressure must be exerted at
once on a certain part of the under side somewhat greater than that on the cor-
responding part of the upper side. But when we remember that a considerable

442 TRANSFORMATION OF ENERGY

pressure on the cell-wall always results from the osmotic activity of the cell-
contents, we can scarcely assume that the limited alteration in pressure resulting
from the inclining of the cells can be perceived by the plant. If, like NOLL
(1902), we make the very modest assumption that the turgor pressure amounts
only to three atmospheres, there rests on the ectoplasm in every position a water
column of 30 m. ; if we assume the diameter of the cell to be o-i mm., the under
side of the organ in the horizontal position must bear an excess of o-i mm. of
water over the upper side ; the plant must be able to appreciate the difference
between a pressure of 30,000 and 30,000-1 mm., and if the inclination of the
cells be less or the turgor be higher it must be able to appreciate even less
marked differences of pressure.

Apart altogether from this conception there are two hypotheses which have
recently been advanced to account for the power plants have of appreciating the
stimulus of gravity. NOLL (1900) imagined that a sensitive apparatus was
formed in the ectoplasm, analogous to the statocysts which occur in crayfish,
adapted to the appreciation of the direction of gravity but beyond the limits of
vision. These must consist of approximately spherical vesicles composed of
sensitive plasma filled with sap, and containing a small but relatively heavy body
in the fluid. This body would correspond to the statolith of the crayfish, and
we may also term it a 'statolith', and it must, according to the position of
the plant organ in space, exert a pressure on some definite part of the sensitive
plasma, and so induce a ' perception ' in the plant. In order to explain this
theory more in detail let us select for study a sporangiophore of Phycomyces,
laid horizontally. The pressure on the outer side of the sensitive plasma would
operate in the statocysts of the under side, and a growth acceleration would
be induced as a response to this ; on the upper side, however, the insides of the
statocysts would be affected by the statoliths and a retardation of growth would
result. If the statolith affects the intermediate limit between the regions which
induce either an acceleration or retardation of growth, i. e. between the outer and
inner hemisphere, then there is no sensation or at least no reaction. In the
horizontal position this would be true of the side walls, but in the vertical posi-
tion the whole of the statocysts come to lie in neutral regions above or below in
the cyst ; in the former case only the statocysts of the sides are unaffected, in
the latter all are unaffected. The application of this hypothesis to a positively
geotropic cell presents no difficulty and so it need not be discussed here, but
certain criticisms may be briefly advanced against this conception of NOLL'S.

We are right in opposing the assumption of special relationships which lie
beyond the limits of vision until the hypothesis in question presents us with an
explanation on broader grounds so comprehensive that we can no longer do with-
out it. As an example we may refer to the atomic theory. But we cannot
compare NOLL'S hypothesis in any way with that theory, because the former at-
tempts merely to explain the phenomena of geotropism and has no further and
wider application. Apart from that, one difficulty presents itself on passing from
a consideration of the uni- to that of the multi-cellular organism. Since this latter
organism exhibits geotropic curvatures not merely in the uninjured condition
but also when cleft longitudinally, we must assume that every individual cell in
it is supplied with statocysts. In each individual cell, at all events in the cells of
a median vertical lamella of the horizontally placed stem, growth acceleration on
the under side and growth retardation on the upper side should be opposed to
each other ; in reality, the cells near the under edge show accelerated growth on
both sides, those near the upper edge show reduced growth on both sides, and the
median zone remains unaffected. Hence we cannot assume the close relationship
between perception and reaction that NOLL does. Response is regulated by the
co-operative action of all cells, correlation plays a part which NOLL'S hypothesis
does not explain. The hypothesis, too, has a certain one-sidedness in the close

GEOTROPISM. I 443

connexion between perception and response. Further, we do not know enough
as to the geotropic response. NOLL appears to assume that the cells of the
concave side are just as active in the reaction as are those of the convex side,
but the retardation of growth in them can be also passive and due merely to the
accelerated growth of the convex side (p. 435). NOLL'S hypothesis brings up
quite special difficulties, however (compare PFEFFER, 1893 a), if the perception
really takes place elsewhere than does the movement, for according to certain
observations of CZAPEK (1898), it is quite certain that the perception of the
under region of a horizontally laid root apex cannot be different from that of
the upper side, and that it is only transmitted to the under side of this motile
zone. On the contrary we must assume equal perception in all the cells of the
apex, and yet the cells of the motile region react quite differently.

Many of the criticisms we have advanced against NOLL'S hypothesis
are applicable also to HABERLANDT'S (1900) and NEMEC'S (1900) views, simul-
taneously and independently promulgated. These authors also employ a
statocyst hypothesis. The sensitive plasma is the ectoplasm of the entire cell,
the statoliths are relatively heavy bodies, such as crystals and starch grains.
Starch grains, which respond quickly to the influence of gravity and press
against a different region of the ectoplasm when the plant is in the upright
than when it is in an inclined or horizontal position, are found in many plants,
regularly in the starch sheath of the stem, and in the root in a central group
of cells of the rootcap. Fig. 137 shows them in the apex
of a cotyledon of a grass. The cells which contain motile
starch grains of this kind are conceived by the authors above
mentioned to be the sense organs for the geotropic stimulus.
This hypothesis has this great advantage that it can be
examined into with the aid of a microscope. It has called
forth a whole series of most interesting observations (HABER-
LANDT, 1903 ; DARWIN, 1903), into the discussion of which
we have unfortunately no time to enter. But difficulties of
various kinds meet us here also (Josr, 1902). We will confine
ourselves to showing that evidence is entirely wanting tending
to prove that only cells with movable starch grains are able
to appreciate the stimulus of gravity. It will be sufficient
to draw attention to the fact that there are plenty of plants F'R- 137- Apex of

i_ • r_ i_"i_'j. J.-L. j. • j.- i_ j_ t_ • i_ j. • the cotyledon ot Pant-

which exhibit the geotropic reaction, but which contain no <mm*&t***m. After
starch, just as, conversely, there are cells with movable starch Ntec (1901).
grains which give no geotropic response, although capable of
growth. [The starch-statolith theory of geotropism has given rise to a flood
of publications, and, although it has certainly found more supporters than
opponents, we cannot as yet accept it as proved. Since, in the course of the
controversy, HABERLANDT has admitted that a perception of gravity may occur
without any change of position of the starch grains, he has placed the theory
in an unassailable position, but at the same time rendered it impossible of proof.
Further details will be found in the literature, of which we quote only the most
recent and most important (HABERLANDT, 1905 ; NEMEC, 1905 ; DARWIN,
1904 ; NOLL, 1905 ; FITTING, 1905).]

Although a study of the newer investigations, which have aimed at analysing
the process of stimulation in geotropism, has led us to no conclusive result, we
must yet admit that our knowledge has been essentially widened on the subject,
for we have had convincing evidence that the whole process is a most compli-
cated one and consists of many inter-related phases. In the first place we
have the purely mechanical phase : gravity acts through weight affecting an
unknown part of the cell — probably the sensitive plasma. It is not at all
improbable that 'pressure on the plasma induces sensation through the weight

444 TRANSFORMATION OF ENERGY

exerted thereby, although accessory changes may first of all lead to this result.
In any case the first influence of the stimulus of gravity must be a purely
physical or purely chemical alteration in the protoplasm, which we may term
perception. Following on this perception conies an excitation in the protoplasm,
which is undoubtedly distinct from the mere perception of the stimulus, since it
may be cumulative, as we shall see in Lecture XXXVI. We know, further,
that it is not coincident with the preliminaries of bending, that it requires
different formal conditions, and may take place in a different part of the plant.
In the latter case the excitation must be transmitted. If we consider the trans-
mission of the excitation as the third phase then we must rank the final reaction
as the fourth. We are quite ignorant whether no perception or no excitation
occurs in the cases where no reaction is manifested, as, for example, when the
organs are in the perpendicular position ; still we must believe that some kind
of response takes place in the resting position also, but it must affect all parts
uniformly and hence bending does not occur. Since stem and root grow
just as quickly on a klinostat as in the erect position it cannot be concluded
that the reaction is absent under normal conditions, especially since we do not
know for certain whether stimulation occurs or not to plants on a klinostat.
Since, however, Phycomyces, Chara and branches of weeping trees grow more
slowly in the inverted position than in the normal, it follows that gravity acts
as a stimulus in these cases.

It must also be noted that attempts have been recently made to solve
the problems of geotropism by chemical and histological methods. NEMEC
(1901) has shown that special rearrangements occur in the protoplasm of cells
which respond to the geo tropic stimulus. Doubtless, we are here dealing
not with a primary effect of gravity but with complex stimulus-phenomena
which bear some as yet unknown relation to the obvious curvatures. These
phenomena are of great interest, however, because it was formerly suggested
that gravity in the first instance induced movements in the protoplasm.

CZAPEK (1898, 1903) has discovered that certain chemical changes take
place in parts which have been geotropically stimulated, and he has succeeded in
proving that tyrosin is oxidized into homogentisinic acid. This oxidation always
takes place in the plant, but homogentisinic acid is formed more abundantly
after the geotropic stimulus has been applied, and apparently this increase
is dependent in some way on geotropism, although what the connexion is is by
no means clear. It cannot have anything to do with the perception of the
stimulus, since it takes place in heliotropic curvatures also, which presuppose
another sort of perception (Lecture XXXVI). If connected with the reaction
it cannot occur first in the root apex and must also be distributed unequally in
the zone of movement. The subject offers a wide field for experimental
research. [Compare CZAPEK, 1905.]

Bibliography to Lecture XXXIV.

ANDREWS. 1902. Jahrb. f. wiss. Bot. 38, i.

BARANETZKY. 1901. Flora, 89, 138.

CZAPEK. 1895. Jahrb. 1 wiss. Bot. 27, 269.

CZAPEK. 1898. Ibid. 32, 175.

CZAPEK. 1900. Ibid. 35, 313.

CZAPEK. 1902. Ber. d. bot. Gesell. 20, 464.

CZAPEK. 1903. Ibid. 21, 243.

[CZAPEK. 1905. Annals of Botany, 19, 75.]

DARWIN, CH. 1881. Bewegungsvermogen d. Pflanze. German ed. by CARUS,

Stuttgart.

DARWIN, FR. 1903. Proc. Roy. Soc. 71, 362.
[DARWIN, FR. 1902. Jour. Linn. Soc. Bot. 34, 266.]
[DARWIN, FR. 1904. Rep. Brit. Assoc. Cambridge.]

GEOTROPISM. II 445

DUTROCHET. 1824. Recherches sur la structure intime etc.

ELFVING. 1884. Ofversigt finska Wetensk. forhandlingar.

[FITTING. 1905. Jahrb. f. wiss. Bot. 41, 221 and 331.]

FRANK. 1868. Beitr. z. Pflanzenphysiologie. Leipzig.

HABERLANDT. 1900. Ber. d. bot. Gesell. 18, 261.

HABERLANDT. 1902. Ibid. 20, 189.

HABERLANDT. 1903. Jahrb. f. wiss. Bot. 38, 447.

[HABERLANDT. 1905. Ibid. 42, 321.]

HOFMEISTER. 1863. Jahrb. f. wiss. Bot. 3, 77.

JOHNSON. 1828. Comp. Linnaea, 1830, ,5, 145, Abstract of literature.

JOST. 1901. Bot. Ztg. 59, i.

JOST. 1902. Biolog. Centrbl. 22, 161.

KARSTEN. 1890. Ber. d. bot. Gesell. 8 (55).

KNIGHT. 1806. Ostwald's Klassiker, Nr. 62. Leipzig, 1895.

KOHL. 1894. Die Mechanik der Reizkrummungen. Marburg.

[LUXBURG. 1905. Jahrb. f. wiss. Bot. 41, 399.]

[MASSART. 1902. Mem. couron. in 8vo, Acad. Bruxelles.]

MEISCHKE. 1899. Jahrb. f. wiss. Bot. 33, 337 (363, note i).

MOISESCU. 1905. Ber. d. bot. Gesell. 23, 364.

NEMEC. 1900. Ber. d. bot. Gesell. 18, 241.

NEMEC. 1901. Jahrb. f. wiss. Bot. 36, 80.

[N^MEC. 1905. Studien iiber Regeneration, p. 334. Berlin.]

NOLL. 1888. Arbeit, bot. Instit. Wurzburg, 3, 496.

NOLL. 1892. Ueber heterogene Induktion. Leipzig.

NOLL. 1 896. Das Sinnesleben d. Pflanzen. Ber. Senkenberg. Gesell. Frankfurt.

NOLL. 1900. Jahrb. f. wiss. Bot. 34, 457.

NOLL. 1902. Ber. d. bot. Gesell. 20, 403.

[NOLL. 1905. Sitzungsb. d. niederrhein. Gesell. Bonn.]

PFEFFER. 1875. Periodische Bewegungen. Leipzig.

PFEFFER. 1 893 a. Die Reizbarkeit der Pflanzen ( Verhandl. Gesell. d. Naturforscher).

PFEFFER. 1893 b. Druck- und Arbeitsleistung (Abh. Kgl. Gesell. Leipzig, 20).

[PICCARD. 1904. Jahrb. f. wiss. Bot. 40, 94.]

PINOT. 1829. Annal. Sc. nat., Ire Ser. 17, 94.

[RiCHTER. 1902. Function d. Wurzelspitze. Diss. Freiburg.]

SACHS. 1872. Arbeit, bot. Instit. Wurzburg, i, 193.

SACHS. 1873 a. Flora, 56, 321.

SACHS. 1873 b. Arbeit, bot. Instit. Wurzburg, i, 385.

SACHS. 1874. Ibid, i, 584.

SACHS. 1879. Ibid. 2, 209.

SACHS. 1888. Ibid. 3. Plates.

SCHOBER. 1899. Anschauungen iib. den Geotropismus seit Knight. Hamburg.

DE VRIES. 1880. Landw. Jahrbucher.

LECTURE XXXV
GEOTROPISM. II

HITHERTO we have confined ourselves exclusively to a study of those
parts of the plant, such as the chief root or chief shoot, whose position of equi-
librium is at right angles to the earth's surface, and which bend back again
by growth movements into that position if they be placed in any other. We
may describe such organs as orthotropic, distinguishing in them two varieties of
geotropism, negative and positive. A casual glance at plants as a whole teaches
us, however, that there are many plant organs whose position of equilibrium
is other than vertical, and these we term plagiotropic, including under that
name oblique directions of growth as well as horizontal. It is possible for
an orthotropic organ to become plagiotropic under the influence of two forces,
i.e. gravity and some other directive force, much in the same way as a body
under the influence of two forces acting in different directions obeys neither
but takes a new direction easily calculated by reference to the law of the

446

TRANSFORMATION OF ENERGY

' parallelogram of forces '. Plagiotropy of this type we will discuss later on ; at
present we will confine ourselves to those organs which are plagiotropic under
the influence of gravity only. An excellent and characteristic example is the
horizontally growing rhizome, such as we meet with in Heleocharis palustris. The
fact that this rhizome grows at a certain depth below the surface demonstrates
that one factor which has often a far-reaching effect on the orientation of organs,
i.e. light, plays no part in this case (Lecture XXXVI). Since also other directive
agents are excluded (Lecture XXXVII) we have only gravity left, and we must
assume that the horizontal position taken up by the rhizome has to do with
geotropism in some form or another. The correctness of this conception
has been demonstrated by ELFVING (1880 a). He planted a subterranean
shoot of Heleocharis in a vessel filled with loose soil ; the vessel had one wall
made of glass so that the direction of the new growth could be studied. When
the rhizome is planted in the normal position the new region maintains the
same line of growth as the old ; if the apex be bent obliquely upwards or down-
wards the new region becomes bent sharply
back into the horizontal position. If, on
the other hand, the axis be twisted round in
the process of planting so that a flank or the
under side now faces upwards no reaction
of any kind follows, the rhizome grows
straight on horizontally without bending or
twisting. From these experiments we may
conclude that the rhizome of Heleocharis
grows, not as in the case of ordinary or-
thotropic organs, parallel with, but at
right angles to, the direction of gravity;
and yet there is no difference between the
sides of the rhizome, which is radial in its
structure. The rhizomes of Scirpus and
Sparganium (ELFVING, 1880 a) and also of
Adoxa and Circaea (GOEBEL, 1880) have
been shown to behave in the same way,
and in all probability the majority of shoots
which develop in a horizontal direction in
the soil have the same characters (e. g.
Paris, Anemone nemorosa, &c.). In many
cases these shoots are the principal shoots
of the plants concerned (e.g. Paris, Adoxa), but lateral branches also exhibit this
special form of geotropism which we may term plagiogeotropism or diageotropism.
The same phenomenon is very obvious in the case of the lateral roots of the
first rank, which always form an angle with the rigidly orthotropic and positively
geotropic main root. It is quite true that in this case the angle is not always
a right angle, it is more often acute, and it is by no means constant in size ;
that it is determined by the direction of gravity was shown by SACHS (1874) by
simply turning the plant round through 180°. He found that in a short time
the new growths made about the same angle with the line of direction of gravity
but a totally different one with the line of the chief root, and that after they
had been again inverted the original direction of growth was resumed. Fig. 138
illustrates SACHS'S experiment, the darkened parts of the lateral roots being
those formed whilst the plant was in the inverted position. Again, the lateral
roots are strongly radial, for they may be twisted at will round about their long
axes without their exhibiting any reaction so long as they remain at the correct
angle with regard to the direction of gravity. Curvature follows at once if there
be any deviation upwards or downwards from the specific * limiting angle '.

Fig. 138. Viciafaba. Chief root with lateral
roots, grown in soil behind a glass plate, first in
the normal, then in the inverted, and finally once
more in the normal position. The increase
while in the inverted position is shown in black.
After SACHS (1874, p. 605'-

GEOTROP1SM. 11

447

cot

CZAPEK (1895) has also proved that no reaction takes place if the lateral root is
turned at right angles upwards or downwards, but if it be only a little removed
from these rest positions a curvature upwards or downwards, according to cir-
cumstances, takes place, which ceases when the limiting angle is again reached.
This latter position only is, however, the stable rest position, the other two posi-
tions must be regarded as labile. Rhizomes correspond to lateral roots so far
as regards the labile rest positions, but differ from them as to the stable rest
position, for that in the case of the root is directed obliquely downwards,
while in the rhizome it is horizontal. One would naturally expect that the
subaerial lateral organs, e. g. many flowers, lateral branches, would find
their stable rest positions when directed obliquely upward ; as a matter of
fact, branches turn back again to their oblique position if they be forced
upwards or downwards. We shall return to this point later, but meanwhile
we may note that many flowers, e. g. Narcissus pseudonarcissus, exhibit another
form of diageotropism (VocHTiNG, 1882). The peduncle is bent over hori-
zontally on the orthotropous scape and if it be taken out of this lie and placed
pointing obliquely upward or vertical it returns to the horizontal once more ;
it is remarkable, however, that
each reaction ceases if the
flowers are directed obliquely or
directly downwards.

While the stable rest posi-
tion of the orthotropic plant
organ is quite constant and is
coincident with the perpen-
dicular, we find that plagio-
tropic rest positions undergo
variations not only when
different organs are considered
but also in one particular organ.
Thus we meet with very marked
differences in a selected ex-
ample, which are due to internal
and external factors. There are
especially two internal factors,
which we cannot always keep
distinct : those which depend on the influence of the state of development,
the ' ripeness ' of the plant, and those which depend on the relationships of
the parts to each other and to the whole (correlations). If we study the lateral
roots of a bean (Phaseolus) which have been grown in uniformly moist soil we
find that from above downwards they form successively the following angles
with the chief root, viz. 130°, 80°, 80°, 90°, 90°, 65°, 75°, 75°, 4°°-

Apart from the individual peculiarities of single roots one notices a decrease
in the size of the angle as the apex of the chief root is approached. Still more
remarkable than these differences between lateral roots is the case where
a single organ in the course of time changes its reaction. Let us study the
development of the horizontal rhizome more closely, taking Adoxa as our
example. The seedling above the level of the cotyledons consists of an ortho-
tropic stem which reaches the light owing to negative geotropism, but later
on this stem bends back and buries itself in the soil. The shoot thus exhibits
a complete inversion of the normal geotropic reaction, since it now behaves like
a plagiotropic secondary root or an orthotropic but positively geotropic organ.
After it has reached a certain depth the rhizome takes a horizontal direction
and produces scale leaves, but it again comes to the surface, acting in a negatively
geotropic manner, when the formation of foliage leaves and flowering shoots

Fig. 1 39. Formation of the rhizome of Adoxa moschafelliiia.
After A. BRAUN (Das Individuum, 1853, Plate II, Fig. 3).
cot, cotyledon ; K, axis of the seedling.

448 TRANSFORMATION OF ENERGY

begins. After that, the apex of the rhizome once more bends downwards into-
the earth, then moves horizontally, and then again upwards. These alternations
of negative and positive geotropic curvatures with the transitional plagiotropic
positions show themselves to be related to the developmental state of the plant,
but they are, as we shall see by and by, partly dependent on external factors.
The rhizome of Paris is originally orthotropic, but when it once becomes plagio-
tropic (horizontal) it remains so if the external relations be unaltered. In the
majority of rhizomes the above-ground flowering shoot does not arise, as in
Adoxa or Paris, as a lateral outgrowth from the chief axis, but the end of the
chief axis itself comes above ground and becomes orthotropic, while a lateral
shoot goes on growing horizontally and continues the rhizome. This phenomenon
is seen e. g. in Heleocharis, Scirpus, Anemone nemorosa, and many other plants.
In all cases the chief axis grows horizontally in the first years until the flowering
period comes on, and, further, the chief axis of the seedling is always originally
orthotropic. The first alteration from orthotropy to plagiotropy is due to
unknown factors, but the annual or more frequently recurring (Heleocharis)
changes from plagiotropy to orthotropy and vice versa appear to be bound
up with alterations in the morphological and physiological characters of the
flowering shoot.

Readjustments of a similar nature, dependent on the degree of maturity
of the organ in question, are known to be frequent in the flower axis ( VOCHTING,
1882). The flower bud of Agapanthus, for instance, is negatively geotropic,
the flower is horizontal, i.e. plagiotropic, the fruit is positively geotropic.
The poppy is another well-known example. Its buds exhibit positive geo-
tropic curvatures, which are compensated later by negative geotropism. WIES-
NER'S (1902) view that the nutation of the poppy bud was due to a special
* weight curving ' is not well founded ; VOCHTING' s thesis, which has been
described above, has not been met by WIESNER ; especially is evidence wanting
that the nutation ceases when the weight of the flower is compensated.

Let us now turn to cases where plagiotropism is obviously dependent on
influences of correlation.

The influence of the chief axis on the lateral branches is markedly shown
when the apex of root or shoot is removed. It has been known for long that
when the apical bud of a spruce is removed the plagiotropic lateral shoots of the
uppermost whorl become erect and that the strongest of them becomes com-
pletely orthotropic and replaces the chief axis. A corresponding experiment
has been successfully carried out by SACHS on the root. It thus becomes a
pertinent question whether the plagiotropism of the lateral branches is a special
character inherent in the branch or whether the rest position in this case is the
resultant of two directing forces, so that, though naturally orthotropic, these
branches are diverted from the perpendicular by a force emanating from the chief
axis. This conception can scarcely be maintained in this form, for BARANETZKY
has recently (1901) shown conclusively that the geotropism of the lateral
branches is not essentially different from that of the chief axis, and that the
actual rest position is conditioned by a special property of the branch. More de-
tailed study will be necessary to appreciate properly what is signified by this
peculiarity which all organs possess and which we may term ' autotropism '

(PFEFFER, 1893).

VOCHTING (1882) was the first to show that when a shoot which had suf-
fered geotropic curvature was removed from the unilateral influence of gravity by
being placed on a klinostat, the plant attempted to compensate that curvature.
The concave side whose growth had been retarded during the curving now
elongated more vigorously than the convex side, and hence the shoot became
straight once more. It has been already shown that not only in geotropic
curvatures of the stem but generally in all cases of induced alteration of form

GEOTROPISM. II 449

of this kind a compensatory reaction afterwards occurs which tends to bring
about the original form. This compensatory reaction makes itself evident
also after a mechanically induced bending of the organ, and it begins, not
only after the cause of the bending has been removed, as in the case of geotro-
pism on the klinostat, but also when the organ is subjected to a weight acting
continuously and unilaterally. Its effect is, certainly, then only to flatten the
curve, not to remove it altogether. Autotropism must also play a part in
the straightening of the geotropic over-curvature, which (p. 434) we preferred
to regard as due to renewed and converse geotropic activity in the overcurved
organ. Autotropism is associated with the increased growth of the side
rendered concave by geotropism (Fig. 135). As BARANETZKY showed, an
organ, such as a shoot of A esculus, may, after suffering geotropic curvature ex-
hibit on the klinostat several pendulum-like movements, because, just as in
the case of geotropism, the autotropism overshoots the mark and is not at
once arrested on the attainment of the straight position but causes a new curva-
ture opposed to the first and with it autotropic growth on the opposite side.

Returning to the discussion of branches of trees, BARANETZKY (1901)
found that during the evolution from the bud these frequently showed negative
geotropism (orthotropism) in a very striking manner, and afterwards took up
the oblique rest position. Apart from other factors, which we need not speak
of here, autotropism is especially responsible for this supplementary curvature
away from the vertical ; it causes the axis of the branch to lie in the direction
already determined by the relation of the bud to the stem. According to
WIESNER (1902) autotropism has nothing to do with this rest position ; he
believes it due to a special peculiarity of the lateral branch, which we have not
as yet referred to, acting in combination with geotropism. This peculiarity is
termed epinasty, and expresses itself in the effort on the part of the lateral organ
to grow more vigorously on the upper than the under side. It must not be
assumed, however, that this is an hereditary characteristic of the morpho-
logically upper side or that dorsiventrality of the branch has been established ;
on the other hand, epinasty must be acquired in the course of the life of the
individual, if we interpret WIESNER correctly, by the influence of the weight of
the branch. It is impossible to pass over WIESNER' s theory without mention,
yet the explanation discussed above appears to us to be more worthy of credence.

A closely related problem, and one of great importance, is, what induces
the bud to take up a definite direction in space ? It may be easily shown, by
cultivation on the klinostat, that buds, like lateral roots, form definite angles
with their parent axes, due to correlations only, and not at all to external factors.
If NOLL'S klinostat theory be correct, then the lateral roots are not withdrawn
from the influence of gravity on the klinostat ; if the chief root be placed in the
axis of rotation of the instrument lessening of the limiting angle must take place
in geo-perception ; if placed at right angles to that axis the angle must be in-
creased. The essential experimental data on this subject are as yet non-existent.
As may be seen especially clearly in the root, the special angle induced in experi-
ments on the klinostat is, as a general rule, larger than the limiting angle formed
with the co-operation of gravity. In virtue of the internal directive force the
lateral roots would lie more horizontally ; their position as observed in nature
is a resultant. As a matter of fact, we can make the lateral roots approximate
to the chief roots and decrease the limiting angle by using a higher speed of rota-
tion (p. 451). The position of lateral shoots is also a resultant of geo- and
auto-tropism, and not infrequently it may be noted that as, in the course of
development, autotropism decreases negative geotropism predominates, so that
axes of inflorescence, for example (e.g. A esculus), come to place themselves quite
vertically. Briefly, we may say that the influence of the chief axis on the lateral
organ expresses itself in the' special direction' and in the angle at which the lateral

JOST G g

450 TRANSFORMATION OF ENERGY

organ is laid down but also in increased autotropism. As already mentioned,
BARANETZKY (1901) assumes other factors still to account for the rest position
of the lateral organs, e.g. the influence of their own weight; into these questions
we cannot enter beyond saying that in our opinion a complete explanation of
the position of branches is not afforded by including the factors to which BARA-
NETZKY draws attention (Noix, 1902 ; [PFEFFER, Phys. II, p. 682]). After
these statements it would seem legitimate to inquire whether on the
whole there is such a thing as plagiotropism without correlation ; the case
of horizontal rhizomes might be alluded to, but it appears to us in the highest
degree probable that plagiotropism in these structures also is due to correla-
tions. We shall presently learn, however, that plagiotropism may be induced
by external factors without correlation.

Relationship to the chief axis is not the only factor affecting geotropism ;
in the shoot itself the capacity for curving is in many cases influenced
by correlations. MIEHE (1902) has shown that absence or incapacity of the
apex in Tradescantia stops or retards the curvature of the basal nodes.

Having now glanced at internal factors we have still to consider external
influences which alter, often very greatly, the geotropic reaction. The first and
most important of these is temperature. SACHS (1874) found that an increase
in temperature decreased the angle between lateral and main roots ; more
remarkable alterations still have been noted by VOCHTING and more recently
by LIDFORS. These authors show that under the influence of a low tempera-
ture (a few degrees above zero) normally orthotropic shoots become plagiotropic.
The shoots of Senecio vulgaris in the wild state behave in this way at the
beginning of winter, as do also those of Sinapis arvensis (VOCHTING, 1898),
Holosteum umbellatum (LIDFORS, 1902), and artificially etiolated shoots of the
potato ( VOCHTING, 1902). The low temperature operates uniformly on all
sides on the shoot, and the plagiotropic rest position cannot, therefore, be
regarded as a resultant of two directive forces ; gravity is the sole directive
agent in this case, but temperature affects the nature of the response on the
part of the plant, the ' disposition ', as one might say. The cases investigated,
first by STAHL (1884), of the influence of light on plagiotropic organs may be
explained in the same way. A certain intensity of light induces plagiotropic
organs to exhibit positive geotropic movements ; thus, illuminated rhizomes of
Adoxa and Circaea bury themselves more or less vertically in the soil, not that
they turn away from light because they have become negatively heliotropic
(Lecture XXXVI), but because they have become positively geotropic, as
may be proved by a simple experiment. This alteration in disposition is of
the greatest service to the plant, since it prevents the rhizome from growing
out of the soil if it lives in suitable surroundings. Other rhizomes certainly
react quite differently, and yet quite in accordance with the necessities of the
case, for they exhibit negative geotropism when illuminated, and at the same
time alter into leafy shoots. Lateral roots behave in the same way as the
rhizomes of Adoxa, for they, on illumination, make far smaller angles with the
chief root than they do when grown in the dark (Fig. 140).

The medium also in which the root grows is of great importance in deter-
mining the reaction of the lateral roots ; SACHS (1874) found that the rest position
of such roots was not the same when cultures were made in soil as when made in
water or damp air. The greatest difference appears between the behaviour of the
side-root in the soil and in the air ; it stands out prominently on the main root
(ELFVING, 1880 b). If the root be placed in the soil with its apex directed
upwards, a vigorous curving sets in, which almost always results in the reinsti-
tution of the normal direction of growth ; if the same experiment be carried
out in moist air the downward curvature is only slight and the apices of the roots
grow more or less horizontally. Obviously culture in damp air either weakens
the geotropic reaction or strengthens autotropism ; at all events, by supplying

GEOTROPISM. II

a stronger centrifugal force (50 g.), by increasing the geotropic excitation, we
can induce the same reaction in the chief root in moist air, and re-establish
the same orthotropic behaviour as we see in roots grown in the soil and subject
to gravity only.

We have now to speak of a fourth external factor which may affect the
way in which the organ reacts, the amount of centrifugal force. We know
from SACHS'S experi-
ments (1874) that the
angles formed by
secondary roots may be
altered by centrifugal
force and the roots made
to exhibit more nearly
orthotropic reactions,
and, according to
CZAPEK (1895), the same
is true of the rhizome.
The last-mentioned fac-
tor has certainly no effect
in nature ; it is owing to
the combined influence of
light, temperature, and
the nature of the medium
that the rhizome main-
tains a certain constant
depth of position in the
soil; we may indeed
affirm that its existence
would not be possible
unless its geotropic re-
action could be altered
and its ' disposition ' be
changed.

To speak of a
' change of disposition '
presupposes of course a
' geotropic disposition '
in the plant. We are
unable, however, to dis-
cover at what point in
the chain of processes
this makes its appear-
ance. Whether it makes
itself evident in con-
nexion with the actual
perception, in the struc-
ture of the perceiving
apparatus (NOLL, 1896),
or later on, between the
act of perception and the
reaction (PFEFFER, 1893;
CZAPEK, 1898) is still a
disputed point. NOLL'S view, viz. that the ' disposition ' is governed by the
structure of the perceiving apparatus has the great advantage of simplicity and
ease of conception, but the assumption of the close relations existing between

G g 2

Fig. 140. Roots of the seedling of Phaseolus multiflorns^ grown behind
glass. /, in diffuse light ; //, in darkness.

452 TRANSFORMATION OF ENERGY

perception and reaction is, as we have already said (p. 441), quite arbi-
trary.

The organs we have hitherto dealt with are radially constructed both as
regards their anatomical structure and branching, but from the physiological
point of view they only partially assume that position in space which we
primarily regard as the normal position of radial organs, viz. the erect ; a large
number behave plagiotropically, taking up an oblique or even horizontal posi-
tion. We should expect a priori that all organs with dorsiventral structure
would be plagiotropic, and, as a matter of fact, this assumption is correct in by
far the majority of cases, for only a few dorsiventral parts, e. g. the shoot
of Vicia faba, are orthotropic. Dorsiventrality, as previously mentioned,
is conditioned very often by external factors, more rarely (as in lateral members,
by relations to the chief axis. Among external factors light takes the first
place and gravity may combine with it in producing the result. Since many
causes co-operate in inducing dorsiventrality it is of course probable that
movements of orientation of dorsiventral organs are for the most part condi-
tioned by several factors, the analysis of which is often difficult. Dorsiventral
shoots of subterranean rhizomes would form very suitable subjects for the
study of movements of orientation, but as such shoots are not at present known
we are compelled to have recourse to aerial organs, in which, owing simply
to withdrawal of light, functional disturbances take place of so radical a nature
that it is impossible to carry out experiments on them. Still some examples
are known in which it is possible to exclude the light, when gravity alone makes
itself evident. These cases will be sufficient to give us answers to the funda-
mental questions on the subject.

Let us study first of all the experiments made by FRANK (1870) on hori-
zontal branches of the yew or spruce. The dorsiventrality of these bodies
expresses itself externally in the difference in size of the needles on the upper
as opposed to the under side, and also in their mode of orientation. If such
branches be bent vertically upwards whilst emerging from the bud, or at least
during the period when the shoot is still growing, we find that, both in light
and in darkness, a curving takes place in them, by means of which they become
bent downwards into the normal horizontal position, and a corresponding upward
curvature when they are bent towards the ground. So far the shoots of Coni-
ferae conform entirely with the rhizomes of Heleocharis, and possibly they also
have labile rest positions directed perpendicularly upwards or downwards.
A difference from the radial plagiotropic organs makes itself obvious if we twist
round the branch on its long axis so that the under side becomes upper side, the
whole branch remaining meanwhile horizontal. It makes efforts at once to
invert itself once more so as to make the originally under side face downwards,
a result which may be achieved in one of two ways, either by a bending, which
will be complete when the branch has described a curve of 180° and turned its
apex towards the chief stem, or, more quickly, by retaining the same direction
of growth, i.e. radially outwards from the stem, and by inducing a torsion of 180°.
The former method, by curving through 180° in one plane, has been observed
by SACHS in isolated lateral branches of Atropa belladonna ; the latter, torsion
through 180°, is illustrated by FRANK'S experiments on branches of Coniferae
still attached to the tree. If a dorsiventral branch be fixed after being twisted
through only 90°, so that one flank faces upwards, the other downwards, it is
obvious that curvature in one plane cannot effect a return to the normal posi-
tion in relation to gravity, and torsions must take place, torsions which have
been observed by FRANK (1870) in branches of the second rank after the
vertical fixing of branches of the first rank ; the twigs of the second rank then
take up a position pointing obliquely upwards, those on the other side pointing
obliquely downwards.

Flowers also exhibit this phenomenon, especially dorsiventral or zygo-

GEOTROPISM. II

453

morphic forms, as do also leaf -blades, which in most cases are markedly dorsi-
ventral. As an example of a dorsiventral flower which owes its position to
the influence of gravity only, and not, as in many other cases, to light, we
may take Aconitum napellus, which has been studied by NOLL (1885-7). If
the inflorescence of this plant be so bent that the terminal portion of the axis
with its buds points vertically downwards, and if it be fixed in this position
in some appropriate way, after a short time the peduncle becomes curved as
shown in Fig. 141. The curvature ceases when the upper part of the peduncle
comes to form once more an angle of 30-50° with the direction of gravity, that
is, when the hooded sepal has again attained the upper position. If the lie
of the flower be regulated by the force of gravity only, this lie (Fig. 141, //) must
be that of rest. The relation of the flower to the axis of inflorescence has also
to be considered, for it is only when the opening of the flower points outwards
that insects can visit it and the flower can perform its normal functions.
Thus we see that, after this median bending inwards of the peduncle, there
follows a complex movement resulting in a torsion of the peduncle and an
outward turning of the flower itself. We must leave it undetermined whether,
as is to be expected from NOLL'S account, a purely geotropic movement is always
combined with another autonomous one induced by internal causes (NOLL'S exo-
tropism), or whether (SCHWENDENER and KRABBE, 1892) a torsion may take
place without any median inbending. This
much at least is certain that in these orienta-
ting movements in flowers of this kind a
correlative influence of the axis plays a part ;
this is especially noticeable in the flowers of
the Orchidaceae. The flowers of this order
are orientated inversely in the bud, i.e. the
labellum is posterior, but owing to a torsion
in the ovary during the evolution of the bud
they come to assume the normal position.
It is possible to prevent this torsion, how-
ever, and compel the flower to open in the
inverted position if the plant be rotated on
a klinostat or if the inflorescence be fixed in
the inverted position. The torsion must,
therefore, be induced by gravity. If, how-
ever the axis be cut off above a flower which has not yet suffered torsion,
the flower assumes its normal position by a simple curvature without any
torsion, and inclines itself outwards over the top of the cut end of the axis ;
that is, it performs only the first of the movements which have already been
described as occurring in Aconitum.

Foliage leaves, as we might expect, behave in principle in the same way
as do dorsiventral flowers. If the axis be fastened in the inverse position they
could regain their geotropic angle and correct orientation of upper and under
sides by a simple geotropic axial curvature of the petiole or base of the lamina ;
they would, however, succeed in carrying out their orientating movements by
turning the tips of their blades inwards, though they seldom find space enough
available. As a matter of fact, we find them performing the same movements
which we have found characteristic of flowers (NoLL, 1887), i.e. a median inward
curvature is often followed, as in Aconitum, by an exo tropic movement, while
in other cases, especially leaves with short petioles, the same result is achieved by
torsion only. It is impossible to decide at present whether this torsion consists,
as NOLL believes, always of two combined movements, or whether it is a single
movement (geo-torsion, SCHWENDENER and KRABBE, 1892).

Other torsions also occur, induced by gravity, as in many plagiotropic
shoots, e.g. Philadelphus. The leaves in the bud of this plant are decussate,

nape,

'ellu

141. Inverted flower of Acontlum
s in two stages. After NOLL. (1885-7).

454 TRANSFORMATION OF ENERGY

but in the full-grown shoot the leaves are no longer so, but all are arranged
in two rows along the sides of the branch. This alteration in position is due to
torsion of the internodes, and since also it takes place in darkness (SCHWENDENER
and KRABBE, 1892) it must be dependent on gravity, although in the majority
of cases such torsions are due to light (Lecture XXXVI).

Since the mechanics of all these torsions have not as yet been fully explained
we will not consider them further. Leaves, however, suggest another aspect
from which we may study geotropism. Hitherto we have referred all geotropic
movements to growth, and have emphasized the fact that they take place only
if the organ in question is still in a state of growth or is capable of starting
growth anew. In the leaves of many plants (e.g. Leguminosae and Oxalidaceae)
movements occur which may take place without any growth and merely by
inequality of osmotic pressure on opposite sides of the petiole.

Leaves and petioles do not possess the power throughout their entire
extent of altering their state of turgor and so elongating or shortening opposite
sides ; on the contrary this peculiarity is limited to one special organ, which may
be identified externally. Since these organs occur at the base of the leaf -stalk,
or (as in pinnate or bipinnate, &c., leaves) at the bases of the pinnae as well,
forming relatively short connecting regions between non-motile parts, we may
term them articulations. For the most part they stand out from parts in the
vicinity by reason of their greater thickness, and hence are termed ' cushions '
or pulvini. Their anatomical structure is very characteristic and closely
related to their function. An examination of the pulvinus of Phaseolus multi-
florus shows that all the vascular bundles run together in a single axial stand
imbedded in parenchyma. The walls of the parenchyma cells are very elastic
and extensible, and when in a state of turgor marked differential tissue tensions
are induced with the scarcely extensible vascular bundle. If the parenchyma
be isolated it elongates very considerably, while the vascular system contracts
scarcely at all. % As may be easily seen, an increase in turgescence on one
side or a decrease on the other, or the occurrence of both phenomena simul-
taneously, must result in an elongation of one side and contraction of the
other, so that the articulation bends. The vascular bundle is of course also
bent but undergoes no change in length. Bending of the joint naturally results
in a passive movement of the parts of the leaf distal to the articulation.

Movements in the pulvini may be induced by the most varied external
stimuli, as also when the leaf is turned upside down by inversion of the stem
(SACHS, 1865). In order to bring it back once more to the normal position, for
the most part not only are simple curvatures necessary but torsions also
occur, which have been even less investigated here than in other cases. In
the curving of the joint there is no permanent elongation of the convex side,
as PFEFFER (1875) demonstrated by microscopic measurements, and after the
inversion of the stem the leaf soon resumes its previous position. From the
fact that no growth can be determined, it may be concluded that the curving
here is due to an increase of osmotic pressure on the convex side and a corre-
sponding diminution on the concave, a conclusion confirmed by the plasmolytic
method. Thus HILBURG (1881) found that in the articulation of Phaseolus
plasmolysis began to appear on the morphological upper side after a geotropic
stimulus, only when a 4 per cent, solution of potassium nitrate was used,
although before the stimulus a 3 per cent., solution was sufficient ; conversely,
the osmotic value of the cell contents of the under side, as measured in terms
of potassium nitrate, fell from 3 J per cent, to 3 per cent. When the stimulus
was permitted to affect the organ for a longer period growth, it is true, finally
ensued on the convex side, and when the stem was placed once more in its
normal position the leaf was incapable of attaining its original orientation.
Although geotropic curvature induced by turgor is limited to the articulations
of certain leaves, it is still theoretically of great interest, since it is probable

GEOTROPISM. II 455

that the perception of the stimulus of weight is a phenomenon of much wider
distribution than the occurrence of geotropic reactions would lead us to
suppose. It can scarcely be doubted that perception of the stimulus in very
many cases leads to no visible result because the capacity for response is
wanting in the organs concerned. The leaf articulations are organs of this
type which have retained their power of reaction long after growth has
ceased in them.

We may now close our discussion of dorsiventral organs and of the external
agencies which often to a very marked degree affect their reaction, and conclude
this lecture by a reference to twining plants which are distinguished by very
special behaviour towards the geotropic stimulus.

In the plants we have been studying hitherto the organs which exhibited
movements of orientation had sufficient strength to carry these out in opposition
very often to great resistance, evolving considerable energy in the process.
We have seen that not infrequently plants are capable of performing far more
work than is required for producing movement in the passive regions. A
certain rigidity is a fundamental condition of activities of this nature ; when
that is absent, the organ, such as a prostrate stem, can never completely raise
itself in spite of its possessing a negatively geotropic capacity. There are
many plants whose shoots would lie on the ground were they not capable of
making use of the rigidity of other plants for their own elevation. Plants
of this kind we term climbers. The simplest type of climber is represented
by such plants as Galium aparine, which, after reaching a certain height
by its own unaided efforts, sinks to the ground unless it manages to come in
contact with some other plant to which it adheres by means of the prickles
which it is provided with. Other climbers exhibit more complicated apparatus
designed for the same purpose, such, for example, as the hooks of Uncaria
or Strychnos (TREUB, 1882-3 ; SCHENCK, 1892 ; EWART, 1898). These organs,
however, attach themselves to the support accidentally only, so to speak, and
exhibit no active movements designed for the purpose of bringing about attach-
ment, such as are shown by the two great series of tendril-bearers and twiners.
From the biological point of view these two series have much in common,
since both save themselves the trouble of forming any special skeletal tissue
in their axes, and make use of some other rigid structures as a means of support-
ing the weight of their leaves. In nature these supports are always living
or dead plants, and hence twiners and tendril-bearers are dependent, quite as
much as epiphytes, on other vegetation. Indeed, one might compare them in
a sense with parasites, since, if the supporting plants be alive, they shade the
light from them and thus inflict injury upon them though, it is true, only
indirectly. The movements, however, which the twiners exhibit in their efforts
to grasp their supports are so distinct in ^physiological sense from those exhibited
by tendril-bearers, that a combined treatment of the two series is not advisable.
At present we will consider twiners only, since these plants carry out their move-
ments by means of a special form of geotropism (Noix, 1892-1902) and are
thereby brought into close relation to the subject we have just been discussing.

Twiners grow round the stem of the supporting plant in a spiral manner,
and since those spirals lie close to and exert pressure on the support, and since,
further, the twining stems are frequently rough, the attachment is thus rendered
very secure and slipping away from the support is practically precluded. On
closer examination of a twiner, e. g. Calystegia, which begins to develop in spring,
we note that its shoots are at first strongly orthotropic and hold themselves
erect in virtue of a certain rigidity which they possess. After reaching a certain
height, however, the apex of the shoot bends over, and that too in consequence
of an active movement, not merely in consequence of the weight of the apex,
and takes up an almost horizontal or plagiotropic position. At the same moment
a special type of movement begins to make itself apparent, which may be termed

456

TRANSFORMATION OF ENERGY

a revolving motion, since the horizontal apex rotates round the fixed vertical
basal region like the hand of a clock. This movement continues so long as the
shoot remains capable of growth and as a rule is always in one definite direction.

In the majority of cases the revolution, looked
at from above, is opposite in direction to the
clock-hand, i.e. to the left', the direction of
motion in the hop and honeysuckle is, how-
ever, on the contrary, to the right, while
alternating left and right revolutions have
been observed in Bowiea volubilis and Loasa
lateritia. Plants which revolve to the left
(Fig. 142) twine round their supports to the
left, and those which revolve to the right
twine to the right (Fig. 143). Since there is
obviously a close connexion between the mode
of twining and the revolving motion it will be
necessary to study this new type of movement
somewhat more closely.

The rotatory movement may be most
easily explained by means of a simple model.
Fasten a thick-walled caoutchouc tube by
its base to a vertical peg, and place in its
apex a piece of lead tubing sufficiently heavy
to bend the tube over into the horizontal
position, and rotate with the hand the apex
of the tube in the reverse direction to that of
the clock-hand (Fig. 144). If a small indi-
cator, say a needle, be fastened to the end
of the lead piping, pointing downward at
the commencement of the movement, we
find that this needle points to the left after
a quarter of a revolution (looked at from
without), after half a revolution it points
upwards, and so on ; but one also sees that
the torsion of the horizontal part on its long
axis, as seen from without, is in the direction
of the clock-hand. One may easily convince
oneself of the fact that the torsion is in the
reverse direction to the rotation by drawing
a line on the long axis of the tube which is seen
to revolve round the horizontal part of the
tube. The advancing surface of the tube thus
changes moment by moment as the revolution
proceeds. If we desire the movement to
be carried out in such a way that the same
surface always lies in front, we must hold the
apex of the tube firmly during the revolution,
but if one revolution be completed under
such conditions a torsion ensues in the lower
end of the tube, a torsion which is immedi-
ately undone as soon as the end is released,
in consequence of which there develops at

once the same torsion in the apical region as we have previously noted during
the complete revolution. If now we mark on the horizontal and the bent region
of a shoot of Calystegia one definite surface, by painting a line on it with indian-
ink, we may note that, just as in the model, this line twists round in the

Fig. 142. Fig. 143.

Fig. 142. Left-handed twining shoot of
Pharbitis. Fig. 143. Right-handed twining
shoot of Myrsip)tyllum asparagoides. From
the Bonn Textbook.

/

\

/

Fig. 144. Model to illustrate twining.

GEOTROPISM. II 457

direction of the hands of the clock. When the rotating end of the shoot has
passed through 360° in a direction contrary to the clock-hand, the end of the
inked line has twisted round through 360° in a direction conformable to the
motion of the clock-hand ; in other words, a complete revolution in one direc-
tion is accompanied by a complete twisting in the other, and in the end the
shoot is just as it was in the beginning and no torsion of any kind is observable
in the vertical part of the shoot.

This rotatory movement has for long been considered autonomous, resulting
from the operation of internal factors, and, as a matter of fact, we shall presently
become acquainted with similar movements which are autonomous. The
revolutions of twining plants are, however, conditioned by gravity only ; in other
words, they are geotropic, but here we have to deal not with increased growth
on the upper or under surfaces of the organ but on its flanks ; in plants which
twine to the left it is the right flank (seen from above) that grows more
vigorously than the left, and vice versa. The whole shoot, however, does not
exhibit such 'lateral geotropic' reaction; on the contrary, it is limited to the
intermediate region between the erect and the horizontal portions of the shoot,
for the erect region is simply negatively geotropic while the horizontal region
is diageotropic. As soon, therefore, as the right flank of the bent portion is
induced to grow more rapidly by the stimulus of gravity the horizontal region
begins to rotate, and, in order to avoid torsion in the basal region, it must, as we
have seen, twist on its axis and turn another surface towards the right flank.
Thus new surfaces directed to the right are successively subjected to the lateral
geotropic stimulus.

In proof of the geotropic nature of these rotating movements we may advance
the following evidence.

1. These movements, as SCHWENDENER (1881) first pointed out, and as
BARANETZKY (1883) afterwards confirmed, cease to be exhibited on the klinostat,
the axis straightens and performs only certain irregular oscillatory movements,
obviously due to internal causes (autonomous nutations, Lecture XLI). If the
revolving movements are to be correlated with these nutations and also termed
autonomous (WORTMANN, 1886), and if it be said that gravity merely influences
their direction, then all geotropic movements must be considered autonomous,
since such nutations are universally distributed. As a matter of fact, C. DARWIN
(1881) attempted to show that all movements in response to stimulus were
modified nutations, a contention which has not been verified.

2. If we place the tip of the revolving stem against a stick, it stops the
movement and a tension arises in the shoot, because the right flank proceeds to
elongate more rapidly than the left. If it were the fact that after a time, for
inherent reasons, another , e. g. the under, surface proceeded to grow more vigor-
ously, the apex would be lifted up, the flank lying against the stick must
grow more rapidly and the tension would be counteracted. But nothing of
the kind is to be seen ; indeed NOLL (1885) draws attention to the fact that in
his experiments the tensions were maintained for days.

3. If we twist a revolving shoot of Calystegia through 180° and fix it so that
what was the upper surface is now the lower, and so that the previously convex
side now faces to the left and the concave side to the right, the curve becomes
flatter, and finally a new curving commences in the reverse direction. The rapid
reaction of a surface which is suddenly brought into the proper position to
receive the stimulus, and the absence of any after-effect in the previously stimu-
lated surface, must at least awake in us the suspicion that we have to deal here
with other conditions than those of ordinary orthogeotropism.

The cause and the mechanism of the revolving movement ought now to
be generally obvious. We need not go into the complications which arise when
irregularities of movement occur in the apex of the shoot, but merely note

458 TRANSFORMATION OF ENERGY

that the apical region does not always lie horizontally. We have yet to explain
the significance of this rotatory movement in the twining.

Twining commences if we place a more or less upright support beside the
greatly overhanging apex of a revolving shoot. The support is enclosed at
first by slack, very flat spirals, which gradually become steeper. This elevation
of the apex takes place in obedience to negative geotropism, and if the support
be subsequently removed — other conditions being suitable — the spirals are
straightened out and the shoot appears twisted ; if the support be not with-
drawn the successive spirals become tighter, compressing the support. The char-
acteristic of the climbing plant lies in the gradual transition from the diageotropic
horizontally placed apex to the negatively geotropic base through the laterally
geotropic overhanging region. The twining arises from a combination of the re-
volving movement and negative geotropism, the support rendering the otherwise
inevitable straightening of the shoot impossible. The twining movements take
place, however, without any support, just as they do when it is present, in proof
of which we may cite the free twinings which many shoots exhibit when they
have grown beyond their supports or when isolated and placed in water (SACHS,
1882) ; far too little is known, however, about the origin of such movements
to justify us in drawing any conclusions from them. If they also arise as a result
of revolution and negative geotropism combined, the capacity for growth must
disappear very much earlier in them than in normal shoots. Ordinarily
speaking no such free twining is observable at the free ends of the shoots of
climbers ; on the contrary, after one complete revolution the apex remains
essentially unchanged, exhibiting in specially simple cases a curving almost in
one plane. WORTMANN (1886) and, earlier, DE VRIES (1873) tied a fine silk
thread round the apex of the shoot, placed it over a pulley, and supported the
weight of the overhanging shoot by a small compensating weight. Under such
conditions according to WORTMANN, the simple revolving movement, such as one
sees in the free apex, no longer occurs, but the shoot begins to exhibit twining,
the spirals being at first flat but afterwards becoming gradually steeper. The
support acts often like the weight in this case, i. e. it prevents the shoot from sink-
ing down and permits negative geotropism to have its legitimate effect. Whether
the silk thread merely supports the weight of the apex, as WORTMANN affirms,
without altering the movements in any way cannot be decided ; it would appear
to us at least as if the thread must hinder the torsion of the apex on
its axis.

A detailed analysis of the twining phenomena is still unfortunately unavail-
able, and by no means all investigators hold the view that twining may be
accounted for by revolving movement and negative geotropism alone. SCHWEN-
DENER (1881), for example, postulates as essential a so-called ' grasping move-
ment ' in addition to these factors. The apex of the twining shoot must come
in contact with the support from time to time, and in consequence of the tensions
set up thereby the incurvings of the shoot becomes transformed into permanent
twinings. It is impossible to deal with this question here, for details of which
reference must be made to the literature. (In addition to the authors already
cited see also AMBRONN, NOLL, and KOLKWITZ.) Similarly, we must omit any
discussion of the torsions which are of such frequent occurrence in twining
stems and which have been so variously interpreted.

Instead of discussing difficult problems like these we prefer in conclusion to
direct attention to certain important phenomena which strongly support our
conception of the co-operation of geotropism in twining movements. If we turn
a twining shoot of Calystegia upside down some of the youngest spirals first of all
begin to open out, then to bend backwards, and proceed to curve upwards in the
original direction, viz. to the left. The undoing of the spirals already formed
plainly shows that the lateral geotropism is not yet obliterated in them, but when

GEOTROPISM. II 459

inverted it is the inner flanks of the spirals which exhibit increased growth,
hence bringing about a straightening of them.

If geotropism be so active a factor in the twining phenomenon we are led to
conclude that twiners can regularly embrace only such supports as are more or less
erect. Indeed the longer the overhanging part of the shoot is, the more will the
inclined supports be seized, as one may observe especially in plants like the hop.

The clasping of the support at right angles is compensated for by a pecu-
liarity of the climber, which we have hitherto not drawn attention to. As in
the case of many etiolated shoots, twiners have inordinately long internodes with
at first very small leaves, and there are many advantages in these leaves not
reaching their full size until after the stem which bears them has obtained a firm
hold on the support, first, because the weight of the stem would be appreciably
increased, and, secondly, because the interruptions in the rotation which would
result from the striking of large leaves against the support are thereby avoided.
The advantages of possessing long internodes require no special explanation,
so that we may at this point close our very fragmentary observations on geo-
tropism in general and twining in particular.

Bibliography to Lecture XXXV.

AMBRONN. 1884. Berichte math-phys. Kl., Gesell. d. Wiss. Leipzig.

BARANETZKY. 1901. Flora, 89, 138.

BARANETZKY. 1883. Mem. de 1'Acad. de St. Petersbourg, Ser. 2, 31.

CZAPEK. 1895. Sitzungsber. Wiener Akad. 104, 1197.

CZAPEK. 1898. Jahrb. f. wiss. Bot. 32, 175.

DARWIN. 1881. Das Bewegungsvermogen d. Pflanze. German edition by CARUS,

Stuttgart.

ELFVING. 1880 a. Arb. d. bot. Instit. Wiirzburg, 2, 489.
ELFVING. i88ob. Acta Soc. Fenn. 12.
EWART. 1898. Annales Jard. bot. Buitenzorg, 15, 187.

FRANK. 1870. Die natiirl. wagerechte Richtung von Pflanzenteilen. Leipzig.
GOEBEL. 1880. Bot. Ztg. 38, 790.
HILBURG. 1 88 1. Unters. bot. Instit. Tubingen, i, 23.
KOLKWITZ. 1895. Ber. d. bot. Gesell. 13, 495.
LIDFORS. 1902. Jahrb. f. wiss. Bot. 38, 343.
MIEHE. 1902. Jahrb. f. wiss. Bot. 37, 527.
NOLL. 1885. Bot. Ztg. 43, 664.

NOLL. 1885-7. Arb. bot. Inst. Wiirzburg/3, 189 and 315.
NOLL. 1892. Heterogene Induktion. Leipzig.

NOLL. 1896. Das Sinnesleben d. Pflanzen. (Ber. Senkenberg. Gesell. Frankfurt.)
NOLL. 1901. Sitzungsber. niederrhein. Gesell. Bonn.
NOLL. 1902. Ber. d. bot. Gesell. 20, 403.
PFEFFER. 1875. Die periodischen Bewegungen. Leipzig.
PFEFFER. 1893. Die Reizbarkeit d. Pflanzen. Leipzig.
SACHS. 1865. Experimentalphysiologie. Leipzig.
SACHS. 1874. Arb. bot. Inst. Wurzburg, i, 584.
SACHS. 1879. Ibid. 2, 226.
SACHS. 1882. Ibid. 2, 719.

SCHENCK, H. 1892. Beitr. z. Biologic u. Anatomic der Lianen. Jena.
SCHWENDENER. 1 88 1. Sitzungsber. Berlin. Akad. 1077.
SCHWENDENER. 1 886. Ibid. 663.

SCHWENDENER and KRABBE. 1892. Abhandl. Berlin. Akad.
STAHL. 1884. Ber. d. bot. Gesell. 2, 383.
TREUB. 1882-3. Annales Jard. bot. Buitenzorg, 3.
VOCHTING. 1882. Die Bewegungen d. Bliiten u. Fruchte. Bonn.
VOCHTING. 1898. Ber. d. bot. Gesell. 16, 37.
VOCHTING. 1902. Bot. Ztg. 60, 87.
DE VRIES. 1873. Arb. bot. Inst. Wurzburg, i, 317.
WIESNER. 1902. Sitzungsber. Wiener Akad. in, 733.

WORTMANN. l886. Bot. Ztg. 44, 273 ; 6OI J 617.

460 TRANSFORMATION OF ENERGY

LECTURE XXXVI
HELIOTROPISM

THE distribution of light also induces the plant to take up a certain orien-
tation in space ; one may speak of phototropism just as we have of geotropism,
or — since in nature the sun is the only source of light which need betaken account
of — we may term it more particularly heliotropism. There are many analogies
between geotropism and heliotropism, of which the following are the chief : —

1. Just as geo tropic curvature arises only from the unilateral influence of
gravity, so heliotropic curvatures are induced only by unilateral incidence of
light. The same apparatus which withdraws the plant from the unilateral
action of gravity, the klinostat, enables us also to prevent heliotropic curva-
tures by rotating the plant in its normal position round a vertical axis. The
same effect may naturally be attained by revolving the light round the plant
or if we permit the light to stream on the plant on all sides equally (e. g. diffuse
daylight).

2. The different organs of the plant react in different ways to unilateral
illumination. Orthotropic organs arrange themselves parallel with the path of
the incident ray, and grow either in a positively heliotropic manner towards
the source of light or negatively away from it. Plagiotropic organs, on the other
hand, arrange themselves so as to make a definite angle with the incident rays.
Organs which are orthotropic as regards gravity are almost always orthotropic
as regards light, and the plagiotropic behaviour of other organs shows itself
also in heliotropism just as in geotropism. For the most part each organ
possesses a power of appreciation of the stimulus of light and of gravity, and
if the reactions to these two forces do not annul each other, a positively geo-
tropic organ must be negatively heliotropic and vice versa ; that this is the
case for the most part experience teaches us.

From the analogies which exist between them the conclusion has been
drawn that geotropism and heliotropism are closely related phenomena, and
hence they have frequently been considered in conjunction. The reason for
treating them separately here is that, as will be shown later, there are great
differences between them.

As we have noted above, in nature light and gravity act for the most part
simultaneously on the plant, and we must assume without deeper knowledge
that the one interferes with and not infrequently neutralizes the other. If,
however, both affect the plant in the same way, then one could not learn
anything as to the one component which is of interest to us at the present
moment, viz. heliotropism, unless it were possible to exclude geotropism alto-
gether. This, however, can be done, for we may cause the plant to rotate in
the vertical plane on a klinostat and arrange it so that it is illuminated
on one side only by placing the axis of the klinostat parallel with a window.
Experience has shown that so complicated an experiment is by no means es-
sential, since the effect of light often makes itself so prominent that geotropism
would appear to be absent altogether. In the present lecture we will confine
ourselves to the establishment of this fact— one of the highest importance—
and consider the combined action of light and gravity later (Lecture XXXVII).

The fundamental phenomenon of heliotropism may be seen in any plant
which has been cultivated in a room at a distance from the window. Let
us consider a seedling of Sinapis alba growing in a culture solution (Fig. 145).
It will be seen that in a very short time the stem bends towards the window until
its axis is approximately parallel with the line of incidence of the light ; it is
positively heliotropic and orthotropic. The root behaves in the converse
way, it is orthotropic but negatively heliotropic. The leaves on the other

HELIOTROPISM 461

hand take up their positions partly passively, owing to the bending of the stem,
partly actively, so that their upper surfaces are exposed to the light, i. e. they
are plagiohelio tropic. Positive heliotropism is a common phenomenon among
the shoots of the higher plants, especially in the seedling condition ; it occurs
in orthotropic leaves also, such as one meets with in the seedlings of many
Monocotyledons. It is by no means limited to green plant organs, for it is to
be met with in many Fungi. Thus the stalks of the fruits of Peziza fukeliana
and of Coprinus, the perithecia of many Pyrenomycetes and the unicellular
sporangiophores of Phycomyces, Mucor, and Pilobolus all bend towards the
light. A certain small number of roots also, e.g. those of Allium sativum, are
positively heliotropic.

We are not nearly so well acquainted with the process of positive helio-
tropic curvature as we are with geotropic movements. The little we do know
tends to show that heliotropic does not differ fundamentally from geotropic
curvature. The comparison comes out first of all in the fact that — apart from
leaf articulations — the curvature is due to unequal growth of opposite sides,
and that apparently growth is retarded on the concave side and accelerated
on the convex; at the same time the median zone,
frequently at least, grows on at a uniform rate during
the curving. Generally speaking the first beginning of
the curving is manifested at the zone of most vigorous
growth in heliotropism also, which then proceeds basi-
petally, and becomes stationary in the last segment
capable of growth, while the apex straightens again.
We shall meet with exceptions to this behaviour later.

In days gone by positive heliotropic curvature
was explained in a very simple and to a certain extent
purely mechanical way (DE CANDOLLE, 1832). The
explanation was based on the fact that growth in
length in many organs was retarded by illumination
and accelerated by shading. Hence, if an organ is more

brilliantly illuminated on one side than on the other p. i Seedlin ofSt-ua^s
the former must remain shorter and become in conse- aWR<IThe *m^ mar? the
quence concave, and so the organ must exhibit positive Item^f positiyeiy^fhe root
heliotropic curvature. This hypothesis was, however, negatively heliotropic. After
overthrown by closer study of negatively heliotropic SACH:
organs. Negative heliotropism occurs in many terres-
trial roots, but most of all in aerial roots, in many tendrils, the hypocotyl
of the mistletoe, and, among unicellular structures, in the roothairs of
ferns and liverworts, &c. These organs all grow more rapidly in darkness
than in light, and on being illuminated on one side ought to exhibit the
same curving as is exhibited by positively heliotropic organs. Since they
do not do so, and since the illuminated side grows more rapidly and becomes
convex, we may fairly conclude that heliotropic curvature does not arise
directly from a difference in the amount of illumination on opposite sides of
the organ. As was the case in geotropic curvature, so here also we have to do
with a uniform reaction of the entire plant to an external stimulus.

In order to prove this view of the case completely it has to be shown that
one and the same organ, may, according to the conditions, react positively
or negatively. Evidence of the most convincing character to this effect has
been brought forward by BERTHOLD (1882) in certain marine Algae, which were
positively heliotropic in weak light and negatively heliotropic in brighter light.
STAHL (1880) has observed the same phenomenon in Vaucheria, and OLTMANNS
(1897) has advanced exact experimental proof of a similar nature in the case
of Phycomyces, by allowing the fungus to grow at varying distances from an
electric arc lamp. Half an hour after the commencement of the experiment

462 TRANSFORMATION OF ENERGY

he found the sporangiophores exhibiting positive phototropism at a distance
of 80 cm. from the source of light (the light intensity being the equivalent
of 8,000 Hefner lamps) ; at a distance of 20-30 cm., on the other hand, with
a light intensity equal to about 100,000 Hefner lamps, they showed negative
curvature. As a result of these experiments it is obvious that there is a certain
intermediate light intensity which has no phototropic effect at all. Indeed
OLTMANNS' experiments have proved that sporangiophores remain erect if
they be farther than 30 cm. from and nearer than 80 cm. to the source of light.
An hour after the beginning of the experiment the negative curvatures had
become still more apparent, but at a distance of 60-70 cm. no phototropic
reaction was induced.

Each organism may, therefore, be found in one of three different conditions
determined by light intensity, viz. (i) a condition of positive heliotropism ;
(2) a condition of indifference ; (3) a condition of negative heliotropism.

Before considering OLTMANNS' experiments further in detail it must be
first of all emphasized that the helio tropic effect depends essentially on a factor
we have not as yet noticed, viz. the intensity of the light. WIESNER, some time
before (1878), studied the influence of the intensity of light on helio tropic
curvature by illuminating, unilaterally, etiolated seedlings by means of a gas flame
of constant but very limited intensity. He placed the plants at different
measured distances from the source of light and calculated the intensity of the
light which fell on the exposed face of the plant. He noted further the begin-
ning of the curvature and the inclination from the vertical which each had
attained after four hours. He found that in the case of Vicia sativa, with a light
intensity equal approximately to a half spermaceti candle power, curvature was
most quickly induced (after seventy minutes), and reached its maximum (to the
horizontal position). The increase or decrease of the light afforded a measure
both of the time before reaction commenced and of the angle of inclination,
conforming with a decrease of the heliotropic stimulus. Quite close approxima-
tion to the flame induced no bending nor even growth, a result which at first does
not appear to correspond with the results obtained by OLTMANNS with Phyco-
myces. Closer examination shows that the intensity of the light in WIESNER' s
experiments was very low (about 200 candles), while OLTMANNS, both in respect
of Phy corny ces and of the etiolated seedlings of cress and barley, first saw curva-
tures taking place when the intensity of the light was the equivalent of from
10,000 to 15,000, likewise 400,000-500,000 and 500,000-600,000 Hefner lamps.
In WIESNER' s experiments injurious effects must have been induced by the gas
flame, and this exhibits itself in the fact that WIESNER found growth ceased
in the indifferent position, which was not the case in OLTMANNS' experiments.
WIESNER attempted to determine the lower limits of light intensity also, but
in place of his results we will quote the more recent experiments of his pupil
FIGDOR (1893). This latter investigator found that light intensity approxi-
mately equal to 0-002 of normal candle power was no longer sufficient to
induce any heliotropic curvature in the seedlings of Vicia saliva. At the same
time other plants behaved in a different manner; Lepidium sativum, for instance,
still exhibited heliotropism with light of 0-0003 candle power, and in Raphamts
sativus light of 0-016 candle power had no longer power to induce heliotropic
curvature.

Summarizing these results we may say that there is a certain minimum
intensity of light at which heliotropic curvature commences but that this
liminal stimulus varies in different plants ; and that there is also a certain
light intensity at which the curvature takes place in the shortest time and
exhibits the greatest angle; and, thirdly, a light intensity at which no heliotropic
curvature follows, an intensity which OLTMANNS regards as the best for the
organism or organ in question, i. e. the optimum illumination for its general
welfare, while the second intensity referred to above is to be regarded as the

HELIOTROPISM

463

optimum for positive helio tropic curvature. Finally, by further increase in
light intensity, we reach a new liminal value, viz. that for negative curvature.
Both negative and positive curvature have for their object the placing of the
plant in the position of optimum light intensity. It may be possible to establish,
at least for some plant organs, that all light intensities which produce negative
curvature do not act equally, but that as the intensity increases the negative
curvature also at first increases and later decreases. We would thus have
a curve with three zeros, regarding the abscissae as indicating light intensities
and the ordinates as representing the amount of reaction (relatively their
reciprocals). Such a curve is illustrated at Fig. 146.

As OLTMANNS has pointed out (1892) the whole curve is not given by every
organism ; at the most we may expect that shade-loving plants (amongst which
we may include Phycomyces) will exhibit both positive and negative halves
of the curve. Photophilous plants, on the other hand, demand so much light
that we must be satisfied if we can prove generally that they are no longer
positively heliotropic when the light reaches a certain intensity, for we are
unable to recognize negative heliotropic curvature in them. On the other
hand, roots perhaps show only the negative section of the curve, for they
are negatively heliotropic only when the light is very bright, and to all lower
intensities they are quite indifferent.

intensity of

Fig. 146. Diagrammatic curve, to show the relation of heliotropic reaction to light intensity.

The problem of heliotropic reaction is complicated by the fact that the
cardinal points of the curve (the zeros and maxima) are not constant in the case
of any selected plant. They vary both according to external and internal condi-
tions. Amongst the former light itself is by far the most important. OLT-
MANNS (1892) found that the longer Phycomyces was illuminated the more its
adaptability to light increased, for sporangiophores which were negatively
heliotropic at the commencement of the experiment became, after a few hours,
indifferent or even positively heliotropic. One culture, which had been strongly
illuminated for ten hours, showed next day a strong tendency to bend over
in the reverse direction. Further, the previous illumination has a great influence
on the initiation of the positive curvature, for it is known that the commence-
ment of the reaction takes place at a much lower intensity in etiolated plants
than in those which have been grown in light. Among internal factors which
influence light adaptability the age of the organ has to be considered. Young
sporangiophores of Phycomyces are more adaptable than old ones ; indeed, the
latter respond more readily in negative manner. The heliotropic adaptability
of certain flowers has long been a familiar fact ; such flowers are positively
heliotropic when young but react negatively after being fertilized, e.g.
Linaria cymbalaria (HOFMEISTER, 1867) and many others (WiESNER, 1880 ;
HANSGIRG, 1890). In these cases the power of adjustment is obviously of
service to the plant, for flowers exposed to light will be more readily seen

464

TRANSFORMATION OF ENERGY

and visited by insects for purposes ot cross-pollination, whilst it is preferable
that the fruits should be sheltered from sunlight. Without proving it in detail
it may be generally accepted that heliotropic movements have in each case
some definite purpose to fulfil.

So far we have considered only unilateral illumination. In nature such a
condition of affairs is best seen in the case of plants which grow against a wall or
under trees or bushes, and in these heliotropic curvatures are specially observable.
At the same time freely exposed plants may be expected to exhibit heliotropic
curvatures also, since, as the sun circles round them, the north side of the plant
will receive markedly less light than the south side (at least in our latitudes).
We might expect that such plants would curve towards the brightest light,
i. e. towards the south, or that they would take up different positions during
the course of the day, following the course of the sun. WIESNER (1901) has,
as a matter of fact, shown experimentally that heliotropic curvatures may
appear with sufficient rapidity to enable the plants to follow the sun's course
in the heavens. When he allowed seedlings of Vicia
sativa to revolve once in twenty-four hours, the plants
being unilaterally illuminated through a slit in an
apparatus designed for the purpose, he found that, not-
withstanding the rotation, the seedlings bent continu-
ously towards the slit. In nature, however, we find
that the plant neither follows the sun's course nor does
it bend towards the south. The reason for this is that
even direct sunlight is too bright to bring about helio-
tropic curvature ; only diffuse, not direct, sunlight has
the power of inducing heliotropic movements, as
WIESNER has again and again proved.

It was already noted that in WIESNER' s ex-
periments (1878) the apices of the seedlings arranged
themselves exactly parallel with the direction of the
incident ray only when the intensity of the light reached
a certain degree, and so, as we may assume, attained
the position of heliotropic equilibrium ; in other cases
they formed larger or smaller angles with the horizontal,
and we cannot say whether this deviation from the
position of equilibrium is due to autotropism or whether
it is really the resultant of the combined action of geo-
tropic and heliotropic stimuli. Both views are possible,
though the former seems to us to be the more probable ; this at least is certain,
that autotropism acts in opposition to heliotropism, and can be neutralized
only when the stimulus reaches a definite amount. There is a third possibility,
the structure in question may not be orthotropic but plagiotropic. This is
certainly not the case with the principal axis, but many lateral branches,
although radial in structure, are plagiotropic. Since, however, there are no
investigations recorded on this subject which enable us to settle the question,
we may turn at once to dorsiventral organs whose plagiotropism undoubtedly
bears a definite relation to light. The leaves of the higher plants will serve
as an illustration of this relationship.

Foliage leaves have, in the great majority of cases, a quite distinct dorsiven-
tral structure, since the upper side normally receives a greater amount of light than
the lower. The light position of the leaf -blade demonstrates itself very clearly when
we examine a simple case, where diffuse light plays upon one side only or at least
affects one side more than the other. If the strongest diffuse light is directly
overhead, as is the case in a wood (WIESNER, 1899), the leaf -surfaces are exactly
horizontal. If, on the other hand, it be directed horizontally, the leaves must
behave very differently according to their positions on the axis, if they are to

Fig. 147. Heliotropic curva-
ture exhibited by the leaves of
Polygonumfagopyruin. The
arrows indicate the direction
of incidence of the light rays.
After FRANK (Lehrbuch der
Botanik).

HELIOTROPISM 465

get the maximum amount of light. In general, however, we may recognize four
principal positions which the leaf may occupy with regard to a laterally
placed source of light ; it may be illuminated from the front, from behind, or
from either flank. The movements exhibited by the leaf have been described
by FRANK (1870) in the following terms : — ' Leaves facing the light curve
until their upper sides are convex, they then bend downwards so as to expose
their surfaces to the light. The same relative position is obtained by leaves
on the shaded side by the upper surface becoming concave and bending
upwards. Lateral leaves obviously cannot attain the optimum light position
by bending in one place, and hence they exhibit torsions which place the laminae
in a vertical position, one margin pointing upwards, the other pointing down-
wards, the surface being orientated at right angles to the light/ These
phenomena are illustrated at Fig. 147. If, finally, the source of light be beneath
the leaf, a torsion of 180° takes place, so that the normal orientation of upper
and under surfaces with reference to light is again attained. This latter
case is illustrated by the leaves of * weeping trees', and it may be induced
experimentally in other plants by artificial illumination from the under side,
e. g. by aid of a mirror.

The region of heliotropic movement entirely depends on the structure and
distribution of growth in the leaf. Naturally, the basal parts of the leaf are
those most concerned, whether the base be in the form of a petiole or merely
part of the lamina. Leaves with long petioles are especially worthy of obser-
vation, since in such cases the angle which appears between the blade and
the petiole shows that the two component parts of the leaf do not react in
the same manner to the external stimulus, the petiole curving more into the
erect, the lamina more into the horizontal position. We shall find later on,
however, that the movement of the petiole is not independent but is initiated
by the lamina. If the leaf be branched, heliotropic movements are carried
out as a general rule by the individual leaflets, at all events after they reach
a certain age.

Heliotropic curvatures are readily recognized as growth phenomena, but
the mechanics of the torsions have not as yet been fully explained. It has
long been believed that these torsions were occasioned only by the action
of a series of external factors, such as light, gravity, weight of the organ, which
individually led to curvatures, but in combination induced torsions, but later
investigations have shown that torsions might appear when light only was the
functional external factor. Thus VOCHTING (1888) has clearly demonstrated
torsions of purely photic origin in the leaves of the Malvaceae, and SCHWEN-
DENER and KRABBE (1892) have proved that such torsions do not occur in
the great majority of leaves when experimentally examined with the aid of
the klinostat, although they certainly made their appearance in the peduncles
of certain flowers. We cannot enter here into a discussion of the possible reasons
which may be advanced to account for the frequent non-occurrence of torsions
when plants are subjected to unilateral illumination on the klinostat; we
need only note that, generally speaking, dorsiventral organs, when placed on
the klinostat, carry out special movements, such as the remarkable epinastic
curvatures of leaves (compare p. 449), and that it has not as yet been shown
whether these organs are stimulated geotropically on the klinostat or not
(compare p. 438). Although, among external factors, light alone is sufficient to
induce torsions, still there is one internal factor which might co-operate
(exotropy, NOLL, 1885-7), but as to this we are not in a position to give a
final decision. If the torsions cannot generally be regarded as due to the
combination of two curvatures (SCHWENDENER and KRABBE, 1892) we are
completely in the dark as to. the mechanics of their production. One fact only
seems to be established, viz. that, in general, growth also plays a part in the
production of these torsions.

JOST H h

4^6 TRANSFORMATION OF ENERGY

Certainly that is not so in all cases. A not inconsiderable number of leaves,
as we have already seen, are able to carry out geotropic curvatures, by altera-
tion of turgor only, in special articulations, and without growth, and these
articulations are capable also of exhibiting helio tropic responses. In Robinia
pseudacacia, for instance, the main petiole and its articulation remain almost
stationary, while the leaflets exhibit continuous movements during the day.
In light of weak intensity the leaflets take up the same positions that we
have seen them assume in movements occasioned by growth ; if we imagine,
for the sake of simplicity, that the chief petiole stands horizontally and that the
strongest diffuse light falls on it from above, we shall find that all the leaflets
arrange themselves horizontally also ; if, however, the light strikes the leaf in
front in the same direction as that in which the petiole lies, the leaflets twist
round at their articulations through an angle of 90°, so that they may adjust
their surfaces vertically at right angles to the incident ray. If a bright light,
e. g. direct sun's rays, be allowed to fall on the leaf from above, we find that
an entirely new phenomenon makes its appearance, for the horizontally placed
leaflets elevate themselves, and each forms with its opposite neighbour an
angle of 90° or less, instead of 180°, so that the upper surfaces approxi-
mate. No matter what be their position the leaflets execute movements, so
that they make with the incident ray a very small angle or even turn their
edges to it. We have thus two extreme positions to consider, a surface position
adapted to feeble light and a profile position for strong light. These move-
ments must have some purpose, whether over-bright light is injurious or
whether the object is only to avoid excessive transpiration. The intermediate
positions are especially purposeful, since they obviously render possible the
absorption of an amount of light adapted to its intensity.

Leaves which have no articulations are, however, unable to alter their space
relationships continuously, hence they assume, during their growing period, a
fixed light position, which is determined not by direct sunlight but by the
strongest diffuse light ; to this light the leaf -blades place themselves at right
angles. Such a position insures that, on dull days at least or in shady situations,
a maximum amount of light shall be absorbed, while direct sunlight never
appears to injure these leaves, simply because it is constantly changing its
direction during the course of the day. In certain plants, however, the so-called
'Compass plants' (SxAHL, 1881), the fixed light position is determined by
direct sunlight. These plants, among which the indigenous Lactuca scariola
may be included, show the ordinary leaf orientation in shady situations, but
in exposed places their leaves perform certain bendings and twistings so as to
turn the laminae vertically and approximately north and south. The leaves
thus stand in profile at midday, while in the morning and evening the surfaces
are exposed to the incident rays. So far as the leaves are concerned which
arise on the north or south aspects of the stem, a torsion at the base is sufficient
to bring them into the profile position ; those pointing east and west, on the
other hand, have to carry out complicated movements in order to attain the
vertical N-S position, for they do not rest content with merely bending upwards
so as to press their upper surfaces against the stem, but exhibit curving on
their midribs as well, northwards or southwards.

The leaves of the Compass plants in the mature condition are not dorsi-
ventrally constructed, their east and west surfaces have the same structure. A
bilateral construction and an accompanying vertical position of the leaf surfaces
occur very frequently, e. g. in Iris, and in many New Holland species of A cacia
and Myrtaceae. In the case of Iris, the vertical position of the leaf, although
the orientation is not associated with definite points of the compass, is never-
theless an adaptation by which the brightest rays, at least in the height of
summer and in southern lands, do not fall on the lamina so completely as
would be the case were the blade horizontal ; on the other hand, individual

HELIOTROP1SM 467

leaves of Acacia and of the Myrtaceae, which are arranged so as to be
vertical not to the soil but to the branch which bears them, are exposed at
right angles to the full midday sun. It is doubtful, therefore, whether the
orientation of the leaves is to be regarded in this case as a protection from
excessive insolation. It is possible that these leaves are in general not affected
at all by light. This is certainly true of many radially constructed leaves, such
as those of the pine, Sedum acre, &c., whose fixed light position is such that the
upper sides have as often the highest photic ration as the lower sides.

It is impossible for us to enter into the wider problems connected with
the actual photic ration of leaves of different types, or with the relative
photic ration of the upper as contrasted with the under side ; we must
content ourselves with quoting only one fact from the elaborate researches
carried out by WIESNER (1899). This author showed that in addition to leaf
types which avoid excessive light by their orientation (articulated leaves,
Compass plants) there were also others that achieve the same results by
their form. It may be experimentally proved that more light always falls
on a flat leaf than on one which is concave or convex, and hence it may be assumed
that the very common occurrence of leaves with uneven surfaces indicates
an adaptation for the protection against excessive insolation.

It will be necessary for us to strictly limit ourselves in the selection of
further illustrations of dorsiventral helio tropic organs. Omitting the consideration
of flowers altogether (NOLL, 1885-7 5 SCHWENDENER, 1892) let us glance only at
certain organs, which are not from the beginning dorsiventral but which become
so as the result of the action of external factors. The seedling of the cucumber
is orthotropic and positively heliotropic; but when it has reached a certain
size a sharp curvature appears above the cotyledons, which is determined by the
source of light, but which does not bring the epicotyl into a rest position in the
direction of the light, but places it horizontally. The shoot continues to grow
in this direction, becoming quite dorsiventral, for roots arise on its under
surface (CZAPEK, 1898 b). We have a counterpart to the cucumber in Hedera
helix (SACHS, 1879, p. 272). In this case the shoot is negatively heliotropic
and grows in an approximately horizontal direction. Thus seedlings of Hedera
and Cucurbita, planted side by side and exposed to unilateral illumination,
bend their shoots in opposite directions, and we may watch the development
of a perfectly plagiotropic shoot from an orthotropic, negatively or positively
heliotropic plumule. Aerial roots also arise on the under side of the stem of
Hedera, but this dorsiventrality is by no means inherent and fixed, for, as is well
known, it is possible, by altering the direction of the light, to make any flank
dorsal or ventral at will, and probably the same is true of Cucurbita. When
rotated on a klinostat, both plants, if illuminated equally all round, are ortho-
tropic. Markedly dorsiventral shoots are generally not so, if they be taken out
of their rest position they do not attempt under all conditions to regain it
by curving or torsion, but endeavour to adapt themselves to the new position
by altering their structure.

Marchantia behaves quite differently. The gemma, from which we
may cultivate the plant, is bilateral, and the side which is more strongly
illuminated becomes the upper side of the thallus ; the dorsiventrality, once
established, cannot be altered by altering the illumination. If the direction
of the light be changed, the plant behaves essentially like a foliage leaf, for FRANK
has shown that not simple curvatures of the thallus only but torsions as well
are set up which bring it back to the original light position (compare SACHS,
1879 ; CZAPEK, 1898 a). We may also refer here to the behaviour of certain
lateral branches, whose initials are radial, but which, under the influence
of unilateral light, become dorsiventral, just as they do when influenced by
gravity (p. 453). The leaf insertions may remain unaltered, and only the
petioles or the leaf bases induce a twisting of the lamina towards the flanks

H h 2

468 TRANSFORMATION OF ENERGY

(Acer, Abies), or photogenic torsions may be set up in the stem (Cornus
mas, &c.), which serve the same purpose, i. e. of bringing the point of inser-
tion of the leaf on to the flank.

Our summary of heliotropic phenomena has been brief and that for two
reasons. It would be quite superfluous to treat of orthotropic organs in great
detail, because in their case the phenomena are generally speaking perfectly
obvious, but that is not the case with the plagiotropic organs. The phenomena
exhibited by such organs are very complicated, and investigations in individual
cases leave much to be desired. What we have said, however, may serve as
an introduction to the more important general problems we have yet to study.
It was pointed out at the commencement of this lecture that the old
explanation of heliotropic curvature was applicable only to orthotropic, positively
heliotropic organs and that on that account it must be rejected. We must
now investigate more closely what exactly takes place in heliotropic movements,
so as to obtain if possible some insight into the mode of operation of the stimulus.
Our knowledge of stimulus action has been greatly extended since DARWIN
(1881) showed that, in certain cases, the heliotropic movement could make itself
apparent at a considerable distance from the place which was unilaterally
illuminated. Since then ROTHERT (1894) has repeated and extended DARWIN'S
experiments, and nowadays this field of research is perhaps the most accurately
studied in the whole range of plant physiology. With
the view of obtaining some conception of the charac-
teristic relations which exist let us observe the behaviour
of seedlings of Setaria or some other member of the
Paniceae group of the Gramineae. As in all members
of that order, a primary sheathing leaf is formed above
/ tf Jff the scutellum, and this leaf we will, for the sake of

Fig. 148. Seedlings of Se- simplicity, term the cotyledon. It is of a tapering form,
etiS.ted'7eBdiing 'of Median an(^ ^ a short time reaches its definite length of 3-6 mm.,
age. 77 the same curved while the underlying, somewhat narrower region, the
afte^bem^yonge^ex^d^o hypocotyl, grows much longer and may become
unilateral illumination. After e_6 cm> jn length. In seedlings of a certain age the

ROTHERT, 1894. (About nat. .-. Y^ , S-, ljt & , ,-, -, . ^ , MI

size.j cotyledon is full-grown although the hypocotyl still

goes on growing vigorously. While, at first, growth

in the hypocotyl is general through its entire length, later on the base
ceases to exhibit any signs of increase, the most vigorous growth being con-
fined to a region just below the apex. If such a seedling be illuminated on
one side, a sharp heliotropic curving takes place at the apex of the hypo-
cotyl, which creeps gradually backwards as far as growth will permit (Fig.
148). This curvature makes itself apparent only if the cotyledon be illumi-
nated from one side whether the hypocotyl be exposed to light or not. If the
cotyledon be shaded and the light be permitted to fall on one side of the hypo-
cotyl no heliotropic curving takes place. Hence we may conclude that it is
only the cotyledon that is sensitive to the light stimulus, and that it is only
the hypocotyl which can carry out the movement. The excitation which the
light effects in the cotyledon must be transmitted to the hypocotyl, and
the curvature takes place only from such a transmitted excitation. We have
thus in this case a definite organ for the perception of the stimulus of light,
viz. the cotyledon, and, as ROTHERT has shown, it is more especially the apex
of that organ that is the sensitive part ; on the other hand the motile organ,
the hypocotyl, is some distance away from the sensitive organ, and in it the
power of perception is entirely absent. From the behaviour of these organs
we may draw the further conclusion that perception and heliotropic excita-
tion are two distinct phenomena — a point to which we have already drawn
attention in speaking of geotropism (p. 444) — which depend on different pro-
perties of the protoplasm and which are independent of each other in so far

HELIOTROPISM

469

that although an excitation always follows a perception (direct excitation) it does
not follow that every excitation must be the direct consequence of a perception
occurring in the region concerned, since excitations may also be transmitted
(indirect excitation). We may therefore conclude from this experiment that
these two types of excitation are fundamentally distinct processes, for it is
only after indirect or transmitted and not after direct excitation that a reaction
occurs in the case of the seedlings of the Paniceae. Since, however, we observe
that a curvature follows in the young cotyledon also after a direct excitation,
we may take it as certain that the phenomena of excitation in the cotyledon
are in all cases identical with those in the hypocotyl, and that the non-appearance
of curvature in the cotyledon in its later developmental stages is solely due
to the cessation of growth.

The phenomena just described as occurring in young plants of Setaria are
comparable to those which are seen in the majority of Gramineae (i. e. Poaeoideae).
In them no hypocotyl is developed, and the cotyledon attains very considerable
dimensions. It is sensitive to unilateral illumination throughout its entire
length, but the excitation, and also the reaction, is most vigorous when the apex
is subjected to unilateral illumination. This conclusion may be arrived at from
a study of several facts. The course of ordinary helio tropic curvature supports
this view. If we consider this pheno-
menon in Avena (Fig. 149), we note
that that curvature begins just below
the apex (b), but after 3j hours (c), the
whole organ, has become affected ; later
on the apical region (after several oscilla-
tions forwards and backwards, probably
of an autotropic nature) again becomes
straight and the curving is localized at
the base, the radius of curvature being
at the same time reduced (d). If we
next attempt to determine the distri-
bution of growth in the seedling, we
find that the maximum is somewhere
about 5 to 10 mm. from the apex, and
that from this point upwards it be-
comes reduced very rapidly, but that, basally, the decrease is quite gradual.
The heliotropic curving does not, therefore, begin in this case at the zone of
greatest growth, but in a region where growth is very feeble, and hence it follows
that the excitation must be greater at the apex than lower down, since, if the
excitation were equally great throughout, the curvature must obviously start
in the region of most vigorous growth. We may arrive at the same conclusion
with regard to the distribution of heliotropic sensitivity by entirely different
means. Heliotropic curvature appears in Avena if we illuminate the whole
cotyledon on one side or only its base or only its apex, but the results are not
identical in the three cases. The most remarkable result is obtained when
we arrange, by means of a sheath of black paper of the form represented at
Fig. 150, /, that the light falls on the upper end of the seedling from the right
only, and on its base from the left. After i J hours the parts exposed to the light
have bent in the form of a double bow, and the whole structure takes the form of
an S. After five hours (Fig. 150, II, III) the excitation transferred from the apex
downward has annulled the tendency of the base to bend to the left or even
transformed it into a reverse tendency ; in other words the transmitted ex-
citation prevails over that locally developed. The apical zone, specially affected
by the light, is very limited in extent, being at most 3 mm. long. From these
experiments it follows that the excitability of the base of the seedling (as measured

a

Fig. 149. Heliotropic curvature in Avena. After
ROTHERT (from DETMER'S Practical Botany). \\ nat.
size, fl, at the commencement of the experiment ;
6, after i J hours ; c, after 3^ hours ; rf, after 9^ hours.

470 TRANSFORMATION OF ENERGY

by its reaction) is not less than that of the apex, but its power of perception
must be less well developed. When we speak of a more limited sensitivity, we
mean the antecedent perception and not the excitation, still less the movement,
but, generally speaking, the greater the sensitivity the greater the excitation,
although movement, as we have seen, may fail to make itself evident in spite
of a very great excitation.

From among the other examples which are known as supporting the
hypothesis of a distinction between perception and reaction in heliotropic
phenomena, or at least as confirming the idea of unequal excitation and the
transmission of excitation to other regions, we will select the case of the leaves
of the Malvaceae, whose behaviour has been carefully observed and described
by VOCHTING (1888). We have to deal with, in this case, a surface which,
generally speaking, exhibits no active movements, and we have also an articula-
tion immediately adjacent which may bring about curvatures by alterations in
turgor, and finally a petiole showing growth movements. Necessities of space
compel us to limit ourselves to the consideration of the movements of the

articulation. Such movements may be in-
duced in it directly, or they may result from an
excitation transmitted to it from the lamina.
If unilateral illumination affects the
pulvinus only, it behaves almost like a
. positively heliotropic stem, and when it

\ y, bends it gives thereby a different inclination

to the leaf-blade. The leaf surface, how-
ever, so influences the pulvinus that it
places the blade at right angles to the
* MM. incident rays. When VOCHTING succeeded

Fig 150. ^ Arrangement for partial darken- by appropriate experimental means in bring-
ing of a seedling of Avena. The arrows - J Y\ * i • 1,1 TIT-, •
indicate the direction of the light. II and Ulg the pulviUUS and the blade UltO OppOSl-
/// show the result after 5 hours. The finn VIA fnnnH fhat f VIA rmlvinnc in /VhAHiAnr**
horizontal line indicates the limits of the tlOI1» ne 1OUna tnat tne PUlvmuS> Ul ODCQience
regions subjected to the influence of the ravs tO the impulse transmitted to it frOHl the

^S^SSni^SSebSr/S^ !amina» responded much more readily than

seedling has curved to the right in its upper it did to 3. direct Stimulus, and the direct
part, and to the left at its base. After , . , 1,1 1-11

ROTHERT (1894, pp. 17 and 59). stimulus was completely neutralized by a con-

trary stimulus transmitted to it from the

blade. The perceptive power of the blade in other cases also is doubtless
responsible for the movements of the petiole.

Further, proof has been adduced by KOHL (1894) in support of the view-
that the negative heliotropism of roots arises from a power of perception
located in the root apex. As a result of our studies in the localization of helio-
tropic perception in the apices of grass cotyledons and of roots, we might
be inclined to conclude that geotropic perception is also localized, but such
a conclusion drawn from analogy must be rejected, first because geotropism
and heliotropism are very different phenomena looked at from a physiological
standpoint, and secondly because, from the biological point of view, heliotropic
perception, in the apex of the cotyledon, in the apex of the root, and in the
surface of the leaf-blade, appears to be entirely purposeful, while the object of
localizing geotropic perception in the root apex is not apparent on the face of it.

Neither is it true that a definite organ is constructed in all cases to perceive
the light stimulus, nor is the power of perception less intense at the region of
the movement than elsewhere, for there are plant organs which exhibit as great
powers of perception in the motile zone as in other regions, and yet in their
case also the heliotropic excitation can be transmitted (ROTHERT, 1894).

Having now studied sufncientlyin detail thebehaviour of the grass seedling,
and having, in the course of our study, seen that a heliotropic curvature must

HELIOTROPISM 47I

consist of at least four distinct processes, i. e. perception, excitation, transmission
of the excitation, and curving, let us examine these processes separately, as
far as the present state of our knowledge will permit. Let us look at perception
first. In the case of gravity, we could at least affirm with certainty that the
first purely physical effect on the plant must be the influence of weight on the
sensitive protoplasm, and we arrived at that conclusion because the effect of
gravity could be replaced by centrifugal force. We cannot, however, so far
replace sunlight by any other agent, and so we are compelled to seek some other
solution of the problem. It may certainly be supposed that light operates in
this case just as gravity does ; in this case, as in that, we have to consider in the
orthotropic organ a change in the direction of the active agent concerned,
here as there we have to deal with phenomena due to unequal growth. SACHS
laid special stress on the fact (compare MULLER-THURGAU, 1876, and SACHS,
1882) that the heliotropic rest position of an organ lies in the direction of the
incident rays, just as the geotropic rest position lies in the direction in which
gravity acts. A stimulus is administered whenever the long axis of the organ
forms an angle with the line of operation of the stimulus. We do not know
whether, from this correspondence between geotropism and heliotropism, SACHS
himself drew the conclusion it suggests that gravity and light were com-
parable from a purely physical point of view ; at all events it might be possible
to conclude so and to advance the hypothesis that gravity like light depends
on undulatory motions of ether ; one might do so if indeed the likeness between
geotropism and heliotropism, established by SACHS, were proved to be one of
complete conformity of the two stimulation phenomena ; but this is by no means
the case.

It is possible, indeed it is highly probable, that the reaction or curvature
is the same in both cases, at present, at least, we know of no difference. The
heliotropic excitation may indeed be the same as the geotropic, but there is
a great difference in the perception in the two cases, as is shown quite clearly
by certain observations made by CORRENS (1892). When CORRENS studied
the effect of oxygen on tropistic movements, he was able to establish the fact
that the seedlings of Helianthus in the presence of the slightest traces of oxygen,
were still capable of exhibiting geotropic curvatures, while a far higher per-
centage was needed (i per cent, of the normal amount) for the performance
of heliotropic movements. Geotropic curvature ceased when growth came to
an end, hence it is possible that in this case perception .was quite independent
of the presence of oxygen, while heliotropic perception appeared only when
relatively large amounts of oxygen were present.

Although it follows of necessity from this fact that geotropic and helio-
tropic perception are essentially different, still SACHS'S view might yet be
correct, according to which it is the direction of the incident ray that is perceived.
In order to estimate the truth of this theory more accurately we must compare
it with the earlier conceptions of heliotropism. According to these the essential
condition for heliotropic curving is an unequal intensity of light on either
side of an orthotropic organ. The rest position would thus be in a line
parallel with the path of the light, because when so placed all sides of the
organ would be equally brightly illuminated ; when unilaterally illuminated,
the fact that the light rays stream through the plant in an oblique direction,,
would not lead to perception, but that result would be attained by the unequal
illumination of the two sides.

If we compare this conception with SACHS'S views, we are driven to the con*
elusion that it is impossible to accept the explanation previously offered by DE
CANDOLLE, that the result is due to the direct action of light of different intensity
on the rate of growth. We have already (p. 461) refuted this theory, and the
behaviour of the Gramineae, studied above, is sufficient in itself to disprove it»

472 TRANSFORMATION OF ENERGY

We may rather inquire as to the origin of the stimulus which brings about the
perception, and here we meet with two alternatives, ' Does the plant perceive
the direction of the light rays or the difference between the illumination on either
side of its body ? ' As a matter of fact the side of the plant turned towards
the source of light must be more brightly illuminated than the side turned away
from it, and if the long axis of the plant be parallel with the light ray ah1 sides
will be equally illuminated. For simplicity let us limit ourselves to ortho-
tropic organs only. SACHS'S hypothesis has never been proved, and the
facts which are recorded in MULLER-THURGAU'S memoir (1876), and on which,
according to SACHS, these views are founded, appear to us to support equally
well the other view. The heliotropic curvature exhibited by unicellular
and very translucent plants or plant organs (such as Fungi or roothairs) appears
at first sight most in harmony with the facts on which SACHS'S theory is based,
for in these cases it may be affirmed that there is practically no difference in
the light intensity on the concave and convex sides, seeing that the amount
of light lost by absorption in the cell is not worth considering. The difference
has not, however, been measured, nor do we know how great it must be in
order to lead to perception in the plant.

More recently SACHS'S views have been vigorously attacked by OLTMANNS
(1892). This author placed the plants on which his experiments were made
in a box into which light entered from one side only, the light being direct

sunlight. The light was made to pass first of
all through a hollow glass prism filled with
gelatine tinted with indian ink. When the

apparatus was arranged as is shown at Fig. 151,
the sunlight struck the

prism at right angles,

-. and hence the light rays passed (in the direc-
tion of the arrows) into the space below,
parallel to each other, while the intensity of the
Fig. 151. OLTMANNS' apparatus. GI, light obviously diminished from left to right,
glass vessel containing the plants to be Behind the prism straight -growing filaments

experimented on ; TK. prism. 1 he direction /. T7 -, .f ijj-j. t j

of the light and its intensity are indicated by of Vauchena Were placed, and it Was found

the arrows. after several hours that those which were sub-

jected to light of medium intensity had re-
mained quite straight, whilst those to right and left had curved apically towards
them. The curvatures took place in a plane parallel to the outer wall of the
box, at right angles to the direction of the rays, positive or negative according
to the intensity of the light. Unfortunately the distribution of the light
intensity and the course of the rays were as simple as that described only
in a few of OLTMANNS' experiments, and even in this one it is still a matter
of doubt whether the rays do not also travel in the same directions as those
which the curvatures followed. Although, doubtless, a decrease in the intensity
of light may theoretically be obtained independently of the direction of the
rays, in practice, in the most carefully planned experiment, owing to the
reflection of the light from the walls of the vessel, the dust particles, and
from the plant itself, deviations must occur.

DARWIN (1881) also carried out a research, which aimed at proving that
heliotropic curvature does not take place in the direction of the rays. In order to
carry out this experiment most appropriately a seedling of Setaria is illuminated
on two sides, which we shall term ' right ' and ' left ', by parallel beams of light
of equal intensity, under which conditions naturally the seedling remains
uncurved. If the whole of the posterior half of the photosensitive cotyledon
be darkened by means of a tinfoil cap while light is permitted to penetrate
the anterior half as before, one would expect, if SACHS'S view were correct,
that no heliotropic curvature would take place. As a result of DARWIN'S

HELIOTROPISM 473

experiment, however, there can be no doubt that heliotropic curvature does
take place and that too, forward, i. e. in a plane at right angles to the incident
ray. According to the hypothesis which regards the inequality of illumination
on opposite flanks as the origin of the stimulus in heliotropism, this result is
quite intelligible, but we have yet to inquire whether this result really contra-
dicts SACHS'S hypothesis. Apparently it does not, since it is possible to imagine
that, owing to refraction and reflection in the interior of the cotyledon, the rays
may be deflected in a direction at right angles to the course followed outside.

It is perhaps impossible to arrive at a decision as to which of the two
hypotheses is the correct one, because it is scarcely possible to obtain a definite
direction of light rays without inducing differences in intensity at the same time,
and because differences in intensity cannot be attained without light diffusing
from a brighter to a less bright region. It is a matter for individual judgement
which hypothesis is to be accepted ; on the ground of analogy (Lecture XXXVII)
we lean to the hypothesis based on difference of intensity of light. If that be
so, we must assume that the plant has the power of comparing the degree
of illumination on different regions. Or tho tropic organs perform heliotropic
curvatures when opposite flanks are unequally illuminated, which curvatures
have the effect of cancelling this difference. Plagio tropic organs, on the other
hand, are attuned to unequal illumination. Looking at or tho tropic organs
only, it is obvious that the liminal stimulus may be exceeded when a certain
difference is reached, but this can only be determined by experiment. Such
experiments have been carried out by MASSART (1888). He exposed Phy-
comyces in appropriate ways for two hours to unequal illumination on opposite
sides, and found that a positive heliotropic reaction took place when the
relationship between the two light intensities was at least in the ratio of
100 to 118. This ratio was found to be constant for light of varying intensity.
Thus MASSART was enabled to prove in the case of heliotropism the validity
of WEBER'S law as to the relation subsisting between the amount of the
stimulus and sensitivity, a law which we shall have to refer to later on in
reference to other stimulus phenomena, and thus he was able to confirm an
earlier suggestion made by PFEFFER (1884). Further investigations are urgently
needed, however, since the results we have already arrived at (p. 463) as to
this power of adjustment show most clearly that the law can apply only to
light of certain intensity. With light of higher intensity no reaction occurs
{in the indifferent condition), and obviously also when the degree of illumina-
tion on opposite sides reaches higher relative proportions than 100 to 118.
In opposition to M ASSART'S results, we must expect also the difference in
illumination leading to a stimulus movement to vary with the absolute light
intensity, but the laws governing this change have yet to be discovered. The
answers to very many questions, which cannot even be indicated here, will
depend on such experimental researches.

Experiments are certainly not wanting in modern literature which have
for their object the determination of the liminal intensity in heliotropism (com-
pare p. 462 ; FIGDOR, 1893). They all deal, however, with unilateral light alone
and only one special case is considered, viz. how weak the light may be which
still induces heliotropic curvature, if the opposite side be shaded as much as
possible.

The latent period of the heliotropic stimulus has already been determined.
'According to CZAPEK (1898 a) it amounts to 7 minutes in the cotyledons of Avena
and in Phy corny ces ; 10 minutes in the hypocotyls of Sinapis alba and Beta vul-
garis, 20 minutes in the hypocotyl of Helianthus and 50 minutes in the epicotyl of
Phaseolus. If one of these organs be unilaterally illuminated for the specified
lime, heliotropic curvature ensues afterwards in the dark, that is to say, we
meet with an after-effect in this case as in geotropism. We are quite ignorant,

474 TRANSFORMATION OF ENERGY

however, as to whether and how far the latent period is dependent on the
intensity of the light.

While in the case of gravity in nature we have to consider only one variable,
i. e. the direction in which the force acts, in the case of light as a stimulus we
have several variables, viz. the direction, the intensity, and yet another, which
we have not as yet considered, viz. the quality of the light. It has long been
recognized that rays of different wave length do not act in the same way. The
more highly refrangible rays which are more especially concerned in the forma-
tive activity of light (p. 310) have been found to be also those especially con-
cerned in heliotropism. WIESNER (1878) found that the rays at the limits-
of the violet and ultra-violet regions were the most active, and that the activity
decreased from that point so that, in yellow light, practically no heliotropic
curvature took place at all. [Compare DANDENO, 1903.] The movements
begin again, however, in red light and increase towards the ultra-red, although
this is not true of every plant examined. Negatively heliotropic organs,
according to WIESNER, behave like positively heliotropic ones.

Examining more closely the action of light, we have to recognize in the
first instance in all cases a purely physical or chemical action which may lead
to a heliotropic stimulus if the light falls unequally on different sides ; wherein
the primary effect lies, however, we are quite ignorant. Possibly light may
first of all induce certain chemical changes; but an action like that which
takes place in silver salts is inconceivable, because the red rays, which perform
no function in photography, were found to be active in WIESNER' s experi-
ments. It is certainly possible that heliotropic curvature in red rays is a
phenomenon sui generis, which may be compared with thermotropic processes,
which we shall consider later on (Lecture XXXVII), and if that be so,
heliotropism might be, in the restricted sense, included amongst those light
effects which, for the sake of brevity, may be termed photographic. On the
other hand it is also conceivable that definite chemical reactions are rendered
possible owing to the action of those kinds of rays which WIESNER found to act
heliotropically.

Meanwhile we need not consider further the so-called chemical effect
of light. If it can be proved that the plant reacts, as we may say, not
to light but to an effect produced by light, then heliotropism may be regarded
as a special instance of chemotropism (Lecture XXXVII) and would, for that
reason, be still further removed from the category of phenomena that geotropism
belongs to, where, as we have seen, it is not gravity itself as such, but the actual
weight associated with gravity that is perceived. Still the difference between
geotropism and heliotropism is sufficiently great to cause us to treat these twa
phenomena separately ; the likeness lies in the nature of the reaction, in the
character of the curvature ; the differences lie in the nature of the perception.

In addition to the chemical effect of light we have also to consider a mechani-
cal influence. According to MAXWELL, a pressure amounting to about 0-5 mg.
per sqm. is induced in any medium through which a light wave is propagated,
in the direction of the path of propagation. The existence of this pressure
has been more recently confirmed experimentally by LEBEDEW. Apart alto-
gether from its limited amount there are other reasons for doubting whether
heliotropic perception is at all dependent on this pressure. [HABERLANDT
(1905) has recently advanced a unique hypothesis as to light perception in plants.
On the upper epidermis of many plants papillae are to be found which, according
to HABERLANDT, act as converging lenses. The papilla concentrates a spot
of light on the protoplasm lining the inner wall of the epidermal cell. The
protoplasm of the inner wall is sensitive to this light and is able so to orientate
itself in response, that, according to the position of the leaf in regard to the
source of light, the light is concentrated at different places on the inner wall

HELIOTROPISM 475

of the cell. As yet the essential basis, however, for this very ingenious theory
is still wanting. It is remarkable that these ' ocular ' papillae are not present
on or tho tropic organs nor on the highly sensitive seedlings of the Gramineae.
Further there is no evidence forthcoming in support of the view that the upper
side of the leaf only is capable of appreciating the light stimulus. The experi-
mental evidence in favour of the function of the ocular cells will scarcely stand
critical investigation.]

We have now to inquire into the other processes which follow on perception
of the stimulus. We have already referred to the excitation, the transmission
of the stimulus, and finally the reaction. Meanwhile there is still a further
phenomenon which must be noted ; we saw that the plant must be able to compare
the different intensities of light on different sides, so that we are now met with the
question as to whether the plant compares the perception itself or the excitation.
It is possible to hold the view that it does neither, but that the case is of a
more complicated nature. Probably the light operates on each individual cell
as a stimulus and induces in each a reaction, at present unknown to us. If
these individual reactions be uniformly alike, then no further effect is produced,
but if they be dissimilar, this difference induces an helio tropic excitation. As
to the nature of excitation we know nothing, and as to its transmission also
very little is known. In ROTHERT'S (1894) experiments with grass seedlings,
it was possible to determine only that the excitation travelled towards the
base, following the path of the parenchyma. We are compelled to assume that
the intercellular protoplasmic threads are the immediate agents in the
transference of the excitation from cell to cell, but such a fibrillar structure as
that described by N£MEC (1901) cannot play any special part in the transmission
(HABERLANDT, 1902). Finally, as to the last link in the chain, viz. the reaction,
the immediate cause we are at least acquainted with, and that is either
differential growth or differential turgor-stretching.

We see, therefore, that our knowledge of the very important problems
which heliotropism presents to us is as yet very meagre ; many of these questions
lend themselves to experimental treatment and possible solution, and these
may in turn throw unexpected light on the remainder.

Bibliography to Lecture XXXVI.

BERTHOLD. 1882. Jahrb. f. wiss. Bot. 13, 569.

DE CANDOLLE. 1832. Physiologic vegetale, 3, ioS6.

CORRENS. 1892. Flora, 75, 87.

CZAPEK. 1898 a. Jahrb. f. wiss. Bot/32, 175.

[CZAPEK. 1898 b. Flora, 86, 425.]

[DANDENO. 1903. Science, 18, 604.]

DARWIN. 1881. Bewegungsvermogen. ^German ed. by CARUS. Stuttgart.

FIGDOR. 1893. Sitzungsber. Wiener Akad. Math.-nat. Kl. 102, I. Abt. 45.

FRANK. 1870. Die natiirl. wagerechte Richtung von Pflanzenteilen. Leipzig.

HABERLANDT. 1901. Biolog. Centrbl. 21, 369.

[HABERLANDT. 1905. Die Lichtsinnesorgane d. Laubblatter. Leipzig. See also

Ber. d. bot. Gesell. 22, 105.]
HANSGIRG. 1890. Ber. d. bot. Gesell. 8, 353.
HOFMEISTER. 1867. Die Lehre von der Pflanzenzelle, p. 293.
KOHL. 1 894. Die Mechanik d. Reizkriimmungen. Marburg.
LEBEDEW. 1901. Annalen d. Physik, IV, 6, 433.
MASSART. 1888. Bull. Acad. Bruxelles, III, 16, 590.
MULLER-THURGAU. 1876. Flora, 59, 65.

NEMEC. 1901. Reizleitung und reizleitende Strukturen. Jena.
NOLL. 1885-7. Arb. bot. Instit. Wiirzburg, 3, 189 and 315.
OLTMANNS. 1892. Flora, 75, 183.

476 TRANSFORMATION OF ENERGY

OLTMANNS. 1897. Flora, 83, i.

PFEFFER. 1884. Unters. a. d. bot. Institut Tubingen, i, 407.

ROTHERT. 1894. Cohn's Beitr. z. Biol. 7, i.

SACHS. 1879. Arb. bot. Inst. Wiirzburg, 2, 226.

SACHS. 1882. Vorlesungen iiber Pflanzenphysiologie. Leipzig.

SCHWENDENER and KRABBE. 1892. Abh. Berliner Akademie.

STAHL. 1880. Bot. Ztg. 38, 412.

STAHL. 1881. Ueber sog. Kompasspflanzen. Jena.

VOCHTING. 1888. Bot. Ztg. 46, 501.

WIESNER. 1878-80. Die heliotr. Ersch. i. Pflanzenreich (Denkschriften d. K.K.

Akad. Wien, 39 and 43).
WIESNER. 1899. Biol. Centrbl. 19, i.
WIESNER. 1901. Ibid. 21, 801.

LECTURE XXXVII

COMBINED ACTION OF GEOTROPISM AND HELIOTROPISM.
THERMOTROPISM AND OTHER TROPISMS

HAVING now discussed separately helio tropic and geo tropic movements, we
have still to glance at the movements which result from the simultaneous or
rapidly consecutive stimuli of light and gravity. We will confine ourselves to
a discussion of the data which CZAPEK'S (1895) recent researches have given us
access to.

CZAPEK experimented with plants which, like the seedlings of A vena and
Lepidium, exhibited a heliotropic curvature under optimum conditions in time
and degree, like that of geotropism. He convinced himself that the plants
mentioned when placed on the klinostat began to exhibit a curvature, when
they were subjected to unilateral illumination, simultaneously with other seed-
lings placed horizontally and kept in the dark ; further that the heliotropic
curvature progressed in the same manner as the geotropic, and that the
maximum amount of curvature (90°) was reached in both cases at the same
time. The influence of consecutive stimuli was next investigated. The seed-
lings were kept horizontally in the dark until the first trace of geotropic
curvature became apparent, when (about an hour after the beginning of the
experiment) they were placed vertically and illuminated from one side, so
that the ensuing heliotropic curvature might operate antagonistically to the
geotropic curvature. Under these conditions a reduction in the geotropic
curvature took place exactly at the same time that control plants which had
not previously been subjected to geotropic stimulus began to show heliotropic
curvature. This experiment, according to CZAPEK, teaches us that a primary
geotropic induction has no effect on a subsequent heliotropic stimulation.

The case was entirely different if the seedling was first of all heliotropically
stimulated and then laid horizontally in the dark in such a way that the side
which was more brightly illuminated faced downwards. There was a very marked
delay in the initiation of the geotropic reaction as compared with control
plants which had not been heliotropically stimulated, and this delay was all
the greater the longer the heliotropic stimulus was allowed to influence the plant;
after a stimulus of ten minutes' duration it amounted to a quarter of an hour,
after sixty minutes' stimulation the delay amounted to two hours. It would
appear, therefore, that the geotropic reaction is affected by heliotropism,
although as far as we are concerned the matter must not be taken as proved.
CZAPEK'S supposition that the two stimulus reactions, in the case of a solitary
induction, take place in exactly the same time has yet to be more definitely

COMBINED ACTION OF GEOTROPISM AND HELIOTROPISM 477

established. In another memoir CZAPEK (1898) has shown that, in the case of
A vena and Lepidium, the latent geotropic period is twice as long as the latent
heliotropic period, the former being 15' the latter 7'. It may therefore be
affirmed that, under given conditions, these plants are less sensitive to geotropic
than to heliotropic stimuli. There is another important point which comes out
on a critical examination of these results, viz., * the moment of the commence-
ment of the negative geotropic reaction in seedlings which had been curved
backwards heliotropically was naturally considered as that at which the angle
with the horizontal began to decrease.' Geotropic curvature, in other words, was
observed when it had overcome the heliotropic after-effect. Obviously it would
have been more scientifically correct to compare the experimental plants with
controls which had been heliotropically stimulated for the same length of time
and then rotated in the dark on a klinostat. The moment at which the
curvature of the seedlings which had been geotropically stimulated took place
after the initiation of the same movement in the plants on the klinostat would
then have been the moment of the initiation of the geotropic movement. It is
very probable that no delay in the initiation of the geotropic curvature would
then have been noticed.

We believe that only one conclusion can be drawn from these experiments,
a conclusion already suggested by the difference in the latent periods, viz.
that in Avena and Lepidium, of two stimuli of equal duration, one geotropic
and the other heliotropic, the latter has the greater effect. Hence a geotropic
reaction in full swing is soon overcome by a heliotropic, whilst geotropism
exerts only a gradual influence on heliotropic activity. CZAPEK' s assertion
that subsequent heliotropism influences geotropism appears to us to be entirely
without foundation. It must also be noted that CZAPEK himself did not observe
it to occur universally. In plants like Helianthus, which responds more
rapidly to geotropic than to heliotropic stimuli, no retardation of the geotropic
reaction in consequence of a previous unilateral illumination was noticeable.
In Helianthus geotropic curvature began at the same time as in Avena, only it
was much more vigorous and hence earlier observable.

The behaviour of plants subjected to the antagonistic but simultaneous
influence of light and gravity is of great interest. CZAPEK has carried out
many experiments on this subject also, in continuation of those previously
instituted by MOHL (1851), MULLER-THURGAU (1876), VOCHTING (1888 b), NOLL
(1892), and others. As a detailed discussion of all these researches would carry
us too far, we will limit ourselves to a summary only.

1. If unilateral light falls on normally orientated plants, many of them
place themselves directly in the line of the light rays (Phycomyces, Pilobolus,
Vicia sativa), others (Lepidium, Avena) place themselves at a small angle
with the path of the incident ray, others again (Phaseolus, Helianthus) bend
but slightly from the vertical. The beginning of the heliotropic curving
follows in all cases at the same time as it does in plants placed on the klino-
stat, but the heliotropic rest position is reached in general at a much later
period.

2. If horizontally directed light falls on a plant already lying horizontally,
the final result is almost the same as in i. Phycomyces and Pilobolus continue
their horizontal growth, Avena seedlings and those of many other plants
form an angle of less than 20° with the horizontal, Helianthus and Ricinus
find their rest positions at an angle of under 45°, and Cucurbita at an angle
under 50°.

3. If light be projected from below in a vertical direction on the plant
lying horizontally, Avena and Phycomyces bend at right angles downwards ;
others remain horizontal, others again (Helianthus) curve upwards at an
angle of under 45°.

478 TRANSFORMATION OF ENERGY

4. If, finally, light from beneath be projected on an inverted plant, e. g.,
Helianthus, it takes up a rest position at an angle of 45° downwards, while other
plants keep on growing vertically downwards.

It is obvious, therefore, that in certain plants, such as Avena, heliotropism
is always the dominant factor, whilst in other cases it is always geotropism that
is predominant and such plants are less sensitive to heliotropism. CZAPEK has
shown that, after Avena has taken up its rest position after stimulation by
horizontal illumination, first of all an obvious geotropic upward curvature follows.
Further he has found the same initial geotropic curvature occurring in hori-
zontally laid plants which have been illuminated vertically from below, and
this indeed occurs at the normal time and delays the helio tropic downward
curvature. In this case one cannot assume that the geotropic curvature
is finally overcome by heliotropism, and we must agree with CZAPEK in
believing that the geotropic perception cannot be obliterated. The antago-
nism expresses itself in the reaction, although we are still entirely unable to
show where it makes itself apparent in particular. Again it is quite possible
that in other cases also the perception may be destroyed.

The observations which have been recorded on the combined action
of geotropism and heliotropism do not as yet lead to any satisfactory general
conclusions ; they require revision and correction in many respects, because
possibly the individual variants in CZAPEK' s experiments have not been sufficiently
excluded. The chief difficulty in all such experiments lies in obtaining equally
great excitations by different stimuli. The goal aimed at is perhaps quite
unrealizable, if different and non-comparable excitations correspond to different
stimuli.

If the problem as to the combined influence of geo- and helio-tropism
in orthotropic organs presents great difficulties these are greatly increased
when we turn to plagiotropic, and more especially dorsiventral organs ; so much
so, indeed, that we will not attempt to enter on a discussion of the phenomena
presented by them.

We have by no means exhausted the movements leading to orientation
of plant organs, for there are still quite a number of stimuli which lead to move-
ments of this character. That most closely related to light is heat, which
also induces a special kind of movement. Since WIESNER (1878) established
the occurrence of heliotropic curvatures due to the action of red and ultra-red
light, so we may with equal justice speak of thermotropic curvature, for rays
which pass through a solution of iodine in bisulphide of carbon are known
as dark heat rays. Positive curvatures induced by such rays have been observed
by WIESNER in Vicia sativa and in the cress. WORTMANN (1883), in a lengthy
series of experiments, endeavoured to prove thermotropic curvatures in the
sporangiophores of Phycomyces and in seedlings of Lepidium, Linum, Zea, &c.
He employed as a source of heat a large vertically placed metal plate, which
had been warmed and which gave off the heat rays from a darkened surface
facing the plant; but in repeating WORTMANN' s experiments STEYER (1901)
showed that WORTMANN'S apparatus was not a suitable one. This latter investi-
gator by careful experimental means showed that in the case of Phycomyces
no sign of thermotropism was exhibited, and that a seedling of Lepidium
was positively thermotropic at high temperatures and negatively so at low.
STEYER' s statements as to seedlings are also, however, wanting in detail,
without which the subject cannot be considered as presented in a clearly
intelligible form. At the same time we know for certain that shoots are
positively thermotropic, since VOCHTING (1890) has proved that the peduncle
of Anemone stellata follows the course of the sun only on account of its positive
thermotropism. That heliotropism has nothing to do with this reaction is
shown by the fact that these movements go on in darkened chambers where
the peduncle bends towards that part of the wall of the vessel which is most

THERMOTROPISM 479

heated by the sun at the moment. Anemone nemorosa and Tulipa silvestris
behave in the same manner.

More exact investigations have been carried out by WORTMANN (1885)
on roots. When roots are grown in water or moist air and heated on one side
no curvatures take place, but when they are grown in sawdust thermotropic
•curvature was very evident. The response was the result of heat conduction not of
radiation. WORTMANN placed the sawdust in zinc boxes 6 cm. broad, one longi-
tudinal wall of which was heated to about 40° C. by being brought into close
proximity to a gas jet, while the other was kept at a temperature of about
•9° C. by means of running water. The temperature of the sawdust on either
side thus showed a difference of about 30°, roughly 5° C. for every cm. of
diameter. Every root grown in this medium was thus necessarily warmer on
one side than on the other ; further, the roots collectively were also, according
to their position, subjected to higher or lower temperatures. The thermotropic
curvatures resulting were markedly different, those exposed to higher tempera-
tures showing negative curvature, those exposed to lower temperatures positive.
This result reminds us of OLTMANNS' experiments on Phycomyces where helio-
tropic curvature was positive or negative according to light intensity. Just as
there a state of indifference was brought about by light of medium intensity, so
also we may anticipate a similar condition to arise in the root when it is exposed
to medium temperatures. As a matter of fact, WORTMANN found that Ervum
lens exhibited negative curvature only between 27-5° C. and 50° C., and positive
only between 26° C. and 12° C. At 27° C., the critical temperature, the reaction
was sometimes positive, sometimes negative, and sometimes there was no
reaction whatever. In the case of Pisum the critical temperature was about
32°, in Zea mais rather higher, i. e., about 38° C. In other cases (e. g. Phaseolus)
negative curvatures only could be induced.

Additional investigations on roots have been carried out by J. AF KLERCKER
-(1891), who measured the angle made by the curved root with the vertical
after the completion of the reaction. The following results on an average of 28
experiments were obtained from Pisum : —

Temperature 26°-29° 29°-320 32°-35° 35°-38° 38°-4i°

Inclination 8-9° 12.9° 27-2° 38-4° 43-9°

In these experiments we are dealing only with negative curvatures, and we see at
once that the stimulus increases markedly with the temperature ; in like manner
we observe, in the case of positive curvatures, an increase of the stimulus
as we recede from the critical temperature. This is very well shown by
Sinapis alba, where, at 24°-29° C., the angle is only 2-4°, but amounts to 19°
when the temperature is i9°-24° C. On the analogy of the heliotropic curve
(p. 463) it is very probable that when the temperature falls still further the
stimulus will again decrease ; e. g., KLERCKER found that in Sinapis, when the
temperature was I4°-I9° C., the angle was only 10-5°. It is very desirable that
a complete curve should be obtained for one and the same organ, from the highest
to the lowest temperatures, for it is obviously only in this way that the funda-
mental facts with reference to thermotropism can be fully established.

If, in spite of the imperfect nature of our knowledge, we inquire
into the cause of the stimulus in thermotropism, we meet with the same
differences of opinion as in the case of heliotropism. VAN TIEGHEM (1884),
who was the first to draw special attention to thermotropism, put forward
a theory, corresponding in all respects to that advanced by DE CANDOLLE
for heliotropism. He argued from the known facts as to the influence
of heat on longitudinal growth (p. 300). When an organ is subjected to a
temperature lower than the optimum, the warmer side elongates more vigorously
than the other, and the curvature is consequently negative in character ;
if, however, the temperature be above the optimum, the cooler side will grow

480 TRANSFORMATION OF ENERGY

more vigorously and a positive curvature towards the source of heat will ensue.
As we may readily understand, the response in the case of the root does not
at all correspond to that of the stem, and hence WORTMANN (1885) was perfectly
right in repudiating VAN TIEGHEM'S hypothesis. In some cases, certainly, this
theory may explain the facts. Thus VOCHTING (1888 a) carried out a careful
investigation into the mode of unfolding of the buds of Magnolia, where
the curvatures are negative owing to the illuminated side growing more rapidly
than the shaded. That this is due to heat alone and not to light, has been
conclusively proved by VOCHTING. In all probability, however, this is not a
case of thermotropic movement, for in other organs, e.g. fruits, similar movements
may be induced, if they be made to grow more rapidly on one side than the other.
Heat does not act in this case as a specific stimulus, but as a * formal condition '.
Generally, however, this is not the case, for WORTMANN (1885), in some experi-
ments, found that those parts of the roots grew most vigorously which were
subjected to a temperature, which, operating on all sides of the organ, did
not permit of any further growth (supra-maximum temperature).

After having refuted VAN TIEGHEM'S hypothesis, WORTMANN assumed that
in thermotropism we are dealing with the direction in which the heat rays pene-
trate the plant. He has advanced no proof of this, however, and it could only
occur in cases where radiant heat was the cause of the thermotropism. As far as
regards roots grown in sawdust, we cannot speak of heat radiating in a definite
direction since diffusion of heat by conduction is the first consequent ; there is
indeed a direction of temperature decrease, bu, not of heat rays. So far as we know,
however (compare VOCHTING, 1888 a), thermotropism due to radiant heat cannot
be distinguished from thermotropism due to conduction. We are entitled to
assume that the cause of the stimulus is the same in both cases, and that it
lies in the dissimilar temperature on opposite sides, a difference which the plant
recognizes and to which it reacts. Obviously, growth may express itself on the
individual sides quite differently from what it does when all sides are subjected
to a uniform and equally high temperature. In heliotropic curvature also we
saw that the illuminated side under certain conditions grew more rapidly than
it did when all sides were equally illuminated. We are ignorant how great the
difference in the temperature affecting the two sides must be before a stimulus
is effected, nor has the effect of the height of the absolute temperature on the
liminal intensity of the stimulus been determined, nor how the stimulus is in-
creased by rise of temperature. In this direction there is ample room for
experimental inquiry.

As to the purely physical or chemical aspect of heat as a stimulus, looked
at from the point of view of perception, we know nothing. We have compared
thermotropism with heliotropism, but at the same time we do not mean to imply
that the perception which precedes curvature is the same in both cases. That
goes without saying, for one of the data which WORTMANN has established
points indeed quite the opposite way ; roots which have their apices removed
exhibit thermotropic curvature, and hence the root apex cannot be the organ
of perception, or at least cannot be so exclusively, although we must admit it to
be so in the case of heliotropism.

Associated with heat is electricity, which is propagated in waves by radiation
or conduction. Although it has recently been shown that radiant light, heat, and
electricity are closely related forms of energy, which differ from each other only
in wave length, we must not conclude that on that account they operate on the
plant in the same way. The variation in the amplitude of the waves, which
in the case of light are infinitely small in relation to the plant, but which in
the case of electricity markedly exceed the diameter of a seedling or a Phyco-
myces, may induce markedly different results in organisms. It is impossible
to draw any conclusions in this connexion from the solitary experimental
investigations conducted by HEGLER (1891), in which he established negative

ELECTROTROPISM. CHEMOTROPISM 481

electrotropism in Phy corny ces ; hence it is preferable to await the accumulation
of further data.

More comprehensive investigations are forthcoming as to the influence of
electric currents than as to that of electric radiations. ELFVING was the first
(1882) to observe curvatures in the root when galvanic currents were sent
through the water in which the roots were bathed. These curvatures were some-
times positive, the root apex curving towards the positive pole, sometimes they
were negative. According to ELFVING' s statements the direction of curvature
depends in the first instance on the nature of the plant, according to BRUNCHORST
(1884) it depends on the strength of the current. A strong current induces
positive, a weak current negative curvature, while medium currents produce no
effect at all. The medium current is not the same, however, for every plant. At
first sight the results obtained in this relation appear to exhibit a certain like-
ness to the variation in the heliotropic response induced by different intensities of
light. The likeness is, however, superficial since, as BRUNCHORST has shown, the
positive galvanotropic curvatures are generally not phenomena of stimulation,
but originate entirely from the fact that growth on the positive side of the root
is injured by the current. Probably certain chemical changes are induced by the
current which operate first of all inhibitively to growth and finally fatally ; at all
events a root which exhibits positive curvature of this character, always dies,
not only on that side but altogether, at the latest after 24 hours. The negative
curvatures on the other hand appear to be genuine phenomena of stimulation,
where the root apex acts as the organ of perception. We have even less ground
for assuming that the plant has the power of appreciating electric waves them-
selves than we had for believing in its power of perceiving light and heat waves ;
in all probability the action of the current is to develop certain bodies by electro-
lysis, which lead to appreciation of the stimulus (compare Lecture XLIII).

The unequal distribution of certain soluble substances may also bring about
directive movements. These have been termed chemotropic movements and
are especially well seen in Fungi and in pollen-tubes. MIYOSHI (1894 a) has
confirmed this in the case of certain Mucorinae and in Penicillium, Aspergillus,
and Saprolegnia, the occurrence of movements in which was recognized long
before and which can be interpreted only as cases of chemotr opism . Thus KIHL-
MANN (1883) found that the cells of Isaria became bent out of their previous
path of growth when placed in the neighbourhood of germinating spores of M e-
lanospora parasitica and ultimately grew over them, and DE BARY (1884, 393)
has suggested that the entry of parasitic Fungi into their host-plants is due
to stimuli of a chemical nature. From MIYOSHI'S investigations there can be
no doubt that chemotropic movements are widely distributed amongst Fungi and
generally speaking serve the purpose of guiding the fungus to a suitable nutritive
substratum, although chemotropic attraction is also effected by substances
which are not good nutrients. MIYOSHI advanced further proof of chemotropism
by injecting leaves, such as those of Tradescantia, with certain experimental
solutions, and sowing spores of a fungus on the moistened epidermis. The in-
jection diffused out through the stomata, and, if it was positively chemotropic, he
found that the hyphae curved into the stomata, while they grew beyond them
when the leaf was injected with water only. MIYOSHI obtained similar results
by sowing the spores on finely perforated plates of mica smeared over with
a chemotropic layer of gelatine. Finally he employed fine capillary tubes filled
with the solution, the ends of which he inserted into the drop of fluid of an
ordinary slide culture, thus permitting of a diffusion of the stimulating agent.
Whenever a straight fungus hypha encountered dissimilar concentrations of the
stimulant on different sides, its growing apex bent round until the new growth
had placed itself parallel with the diffusion flow of greater concentration, towards
which it grew. WORTMANN'S experiments on the thermo tropic curvatures of
roots (p. 479) present us with quite similar phenomena, for we may well com-

JOST I 1

482 TRANSFORMATION OF ENERGY

pare the temperature income with the diffusion inflow in this case. No one,
however, would desire to defend the view that the direction of the diffusion
inflow determines the orientation of the long axis of the cell ; it is much more
probable that the plant reacts to unequal distribution of the stimulant, and
endeavours to place itself in such a position that all sides are in contact with
equal concentrations of the stimulant. This, however, cannot be directly
proved, because it is not possible to bring about stimulation on different
sides by different concentrations, without producing at the same time a
diffusion current.

MIYOSHI investigated a large number of substances in different states of
concentration, and was able to prove that some were good attractive media,
others only moderately so, whilst others still, if stimulants at all, were repul-
sive. Repulsions, such as those exhibited by the cells of Fungi, have been
observed after treatment both with free organic and inorganic acids ;
they were also induced by alkalis, alcohol, certain salts, such as potassium
nitrate, magnesium sulphate, potassium and sodium tartrate, all in solutions of
weak concentration. That the successful media did not act equally well on all
Fungi examined is easily intelligible. The Hyphomycetes behave very like each
other, but Saprolegnia, living as it does under very different conditions, reacts very
differently. Exact investigations on the behaviour of such Fungi as are extreme
specialists so far as their nutrition is concerned, are calculated to teach us much
with regard to the special phenomena of chemotropism. Generally speaking,
ammonium compounds, phosphates, peptone, asparagin, and sugar are good
attractive agents ; ammonium phosphate among inorganic compounds is espe-
cially so. The different types of sugar, prominently grape sugar and cane sugar,
are excellent attractives for Hyphomycetes, but Saprolegnia responds but feebly
to them. Glycerine and gum arabic act neither attractively nor repulsively,
which indicates that the chemical action does not depend only on the
nutritive value of the substance. This is especially clearly seen in the case of
potassium nitrate, which acts repulsively, although it is a nutrient in many cases.

Apart from the specific action of the individual substance dependent on its
chemical constitution, the concentration in which the solution is presented is of
importance. Mucor stolonifer, for instance, reacts to a o-i per cent, sugar solution
in a positive manner but more markedly to a 2 per cent, solution, and the re-
action becomes more vigorous as the concentration is increased. At 15-30 per
cent, the response is less apparent, and at 50 per cent, a repulsive reaction ensues.
An exact determination of the critical concentration between positive and nega-
tive response is required, and experiment has yet to show what is the lowest con-
centration or liminal intensity which has a stimulating effect. The statements
as to repellent concentrations are very meagre, because very often before
repulsion takes place the plants have suffered injury. MIYOSHI (1894 a) records
the following results, however: — a 10 per cent, solution of ammonium phos-
phate was repellent in the case of Saprolegnia, while a 5 per cent, was still
attractive ; a 3 per cent, solution of ammonium j chloride (doubf ully lower
concentrations) was repellent to the same plant; a 50 per cent, solution
of cane sugar in Hyphomycetes and a 20 per cent, in Saprolegnia ; a 50 per
cent, grape sugar in Hyphomycetes and a 10 per cent, in Saprolegnia ;
a 20 per cent, beef extract in Saprolegnia and some Hyphomycetes but not in
Mucorinae. In the last case the action was doubtless due to the phosphates
present in the extract. The liminal values of the attractive solutions as
inducing stimulation were naturally lowest in the case of the best media. The
liminal value of meat extract is 0-005 Per cent- for Saprolegnia, of grape sugar
o-oi per cent, for Mucor mucedo, and of ammonium nitrate 0-05 per cent.
for the same fungus. The determination of the liminal value for differ-
ences in concentration on opposite sides is obviously of greater importance
than the determination of the absolute liminal value, which cannot be

CHEMOTROPISM 483

determined exactly. How great must this difference be so that perception may
follow, and how does this value vary with the absolute concentration ? In
order to obtain a definite but at the same time constant difference in concentra-
tion on opposite sides, MIYOSHI sowed spores of a fungus on a collodium mem-
brane perforated in the middle, and laid it between two strips of filter paper
crossing each other at right angles. If streams of different concentration were
allowed to pass through the filter paper on both sides, the difference in con-
centration affecting the germ tubes was kept approximately constant. When
Saprolegnia was made to grow between a o-i per cent, and 0-3 per cent, solution
of sugar no curvature of the hyphae took place ; the same result was obtained
on using a o-i per cent, and 0-5 per cent., but when the solution on one side was
o-i per cent, and on the other I per cent., positive chemotropic curvature took
place. The same relative percentages must be maintained at higher concentra-
tions if perception is to follow ; thus a 0-5 per cent, solution of sugar must be
opposed to a 5 per cent. MIYOSHI thought he was entitled to conclude from
these experiments that in general the solution must be ten times as strong on one
side as on the other if curvature was to take place (WEBER'S Law; compare pp.
473 and 543). Investigations must, however, first of all be made as to whether
this relation holds good near the critical concentrations, and whether it is
effective at high concentrations, where negative curvature appears. It is very
probable that it is not so ; moreover the repellent results obtained at higher
concentrations are in all probability due, at least in part, to osmotic and not to
chemical activity (MASSART, 1889 ; compare Lecture XLIII, p. 542).

In pollen-tubes as well as in Fungi, well marked capacity for responding to
chemotropic stimuli has been established (MOLISCH, 1889, 1893 ; MIYOSHI,
1894 b ; LIDFORS, 1899).

If we place a portion of a stigma, a style, or an ovule of Scilla patula on
sterilized gelatine and sprinkle pollen of the same plant over the gelatine, keeping
the whole preparation moist and in the dark, we find that the pollen-tubes in-
variably grow towards the tissue and finally pierce it. The fact that Fungi
behave in the same way makes it very probable that what attracts the pollen-
tube is nothing out of the common but merely some kind of sugar or other body
commonly found in the plant. Since the stigma contains glucose, and since the
ovule has been shown to contain a polysaccharide, we naturally think at
once of cane or grape sugar as the exciting agent. As a matter of fact MIYOSHI
(1894 b) has shown that pollen-tubes react vigorously to cane sugar and other
soluble carbohydrates, such as levulose, dextrose, dextrine, and lactose, while
the other substances which are active in the case of Fungi are in this case in-
different or repellent. MIYOSHI was also able to determine the liminal differ-
ence in concentration for pollen-tubes by the same method as he adopted in his
experiments on Fungi. In the case of Agapanthus, chemotropic curvature
always took place when the concentration of the stimulant was at least five times
greater on one side than it was on the other, a fact which was established for
percentages of 0-5, i and 2. Starting from this basis MIYOSHI was able to
deduce the degree of concentration of the cane sugar solution which escaped
from the ovules of Hesperis matronalis, assuming that the sensitivity of the
pollen-tubes of this plant was exactly the same as of those of Agapanthus.
When pollen- tubes and ovules were both placed on gelatine containing a known
amount of cane sugar, and whose surface was also moistened with a sugar
solution of the same concentration, approximation of the pollen-tubes to the
ovules took place only when the secretion from the ovule was at least five times
as concentrated as that in the gelatine medium. Growth towards the ovule took
place only if the substratum contained 0-25 per cent, to 2 per cent, of sugar but
not at higher concentrations, hence the concentration of the cane sugar solution
in the ovule must have been at least 10 per cent.

It is well worthy of note that MIYOSHI has determined in a large number of

I i 2

484 TRANSFORMATION OF ENERGY

plants that an excretion of cane sugar takes place from the ovule and that the
pollen-tube reacts to this substance. There can be no doubt therefore that the
ovules of one plant must attract the pollen of entirely different plants, a fact
which has indeed been definitely proved in many cases by experiment. Since,
however, in nature entry of foreign pollen is prevented, other conditions must
co-operate, especially the conditions necessary for the germination of the pollen,
where chemical stimuli, often of a highly specific nature must play a special
part (pp. 317 and 372). Further, we must not imagine that all pollen-tubes react
only to sugars as stimulants. In this relation LIDFORS (1899) has recorded
observations which have extended and completed those of MIYOSHI. He was
able to show that in Narcissus tazetta the attractive substance was not a carbo-
hydrate at all, and, after various attempts, he succeeded in proving that it was
a proteid that induced the chemotropic reaction. The decomposition-products of
the proteid, however, were quite inactive. [As to the chemotropism of roots
compare SAMMET (1905), LILIENFELD (1905), NEWCOMBE and RHODES (1904).]

Hitherto we have considered only liquid or soluble bodies in relation to
chemotropic activity ; but it is obvious that gases may also have this effect, for
they also diffuse and might affect different sides of plants if in different degrees of
concentration. Chemotropic curvatures due to gases have, as a matter of fact,

been observed by MOLISCH (1884) in
roots and later in pollen-tubes (1893),
phenomena to which he has given the
name of aerotropism. MOLISCH' s method
of experimenting was as follows : — he
separated two chambers from each other
by a vertical plate, and placed different
gases in each. The plate was pierced by
a narrow slit, in front of which, at the
smallest possible distance off he placed
the radicle of a seedling ; the opposite
sides of this radicle were thus in different
atmospheres. When the root was placed
at the boundary between ordinary air
Fig. ,52. Hydrotropism in the root After SACHS and air poor in oxygen, a curvature took

(from DETMER'S Smaller Practical Physiology). place towards the atmosphere which Was

richer in oxygen, and this capacity must

obviously prevent the root from penetrating too deeply into the lower layers of
the soil ; in other words aerotropism is a factor in determining the depth to
which roots penetrate the soil. Negative aerotropism to oxygen has also to be
taken into account. This phenomenon makes itself evident when the root has
to choose between an atmosphere of ordinary air and one composed of pure
oxygen ; the root bends towards that which is poorer in oxygen. By altering
the oxygen concentration on both sides a condition is at last reached when
neither positive nor negative curvature takes place, a condition of indifference
in short so far as this gas is concerned. MOLISCH found that the root re-
sponded only negatively to all the other gases he investigated, viz. carbon-dioxide,
chlorine, hydrochloric acid, coal gas, ammonia, and chloroform. When the con-
centration of these gases was increased, a positive curvature certainly often
appeared, but that was merely due to injury to the concave side of the root, and
it was no more a genuine stimulus reaction than the positive curvature already de-
scribed as occurring in galvanotropic reactions. [MOLISCH'S results have recently
been called in question by BENNETT (1904) ; compare also SAMMET (1905).]

Since, according to MOLISCH, the aerotropic movements took place after
removal of the root apex, we must conclude that the act of perception of the
stimulus takes place in the growing zone, and this constitutes a difference
between aerotropic and hydrotropic curvatures, which have also been observed

HYDROTROPISM 485

on roots although in other respects the two series of phenomena closely resemble
each other. As in the case of aerotropism we had to take account of perception
of unequal distribution of a gas, so in hydro tropism we have to deal with unequal
distribution of water vapour. SACHS (1872) demonstrated hydrotropism in the
root in a very simple way (Fig. 152). He covered a shallow zinc cylinder on
one side with a large-meshed netting, filled the cylinder with wet sawdust, and
suspended the whole apparatus so that the perforated under side of the cylinder
formed an angle of 30°-45° with the horizontal. Peas were then planted in
the sawdust, and the roots on germination soon exhibited positive geotropism,
growing out of the sawdust through the meshes into the air. ' If the air outside
be completely saturated or nearly so, the roots grow straight out into the air ;
if the air be not saturated but only moderately damp, the roots bend sideways
and curve over until they again reach the under side of the sawdust. Not
infrequently they grow backwards closely adpressed to the oblique surface and
sometimes the root apex re-enters the moist sawdust through the meshes, at
once curving out again geotropically and repeating the performance several
times thus lacing itself through the meshes/

MOLISCH (1883) has shown that the stimulus in positive hydrotropic curva-
ture is perceived by the root apex. He surrounded the root tightly with moist
blotting paper so that only about i mm. of the apex was exposed. When a
psychrometric difference was established, positive hydrotropic curvature ensued,
just as when the growing region was subjected to dissimilar amounts of water
vapour on either side. PFEFFER (1894) has more recently extended this research
by showing that the stimulus is perceived only by the root apex. If the apex be
uniformly wet on all sides hydrotropic curvature never occurs in the growing zone.
Further experiments have yet to be undertaken to determine whether special
emphasis is to be laid on this difference between aerotropism and hydrotropism,
viz. the great senitiveness of the apex in the latter case and the absence of that
sensitivity in the former.

We will only note further the fact that hydrotropism is by no means confined
to the root. Positive hydrotropism is exhibited also by the rhizoids of the
Marchantiaceae and negative hydrotropism by certain, but by no means all,

Elumules, e.g. Linum (MOLISCH, 1883), potato (VOCHTING, 1902), (compare
INGER, 1903, Ber. d. bot. Gesell. 21, 175) ; negative hydrotropism is on the
other hand of very general occurrence in Fungi (Mucor, Phycomyces, Coprinus).
The sporangiophores of Phycomyces are remarkably sensitive to water (WORT-
MANN, 1881) and lead to quite special phenomena in this case. If Phycomyces
be grown on bread, it may be noticed that the sporangiophores, though geotropic,
do not, in darkness, stand perfectly erect ; they form rather a tuft of diverging
filaments. Each sporangiophore, owing to transpiration, renders the air in its
immediate vicinity damp, and hence the neighbouring sporangiophores curve
away from it, and since the same applies to all of them, the result is this
outward divergence of the sporangiophores, the one from the other. The
marked attraction of Phycomyces by certain insoluble bodies such as iron, which
ELFVING (1890) attributed to physiological stimulation at a distance, is due to
hydrotropism (£RRERA, 1892 ; STEYER, 1901). Iron is hygroscopic and hence
the air in the neighbourhood of that metal is somewhat drier than before ;
Phycomyces bends therefore negatively hydrotropically towards the drier region.
Obviously it reacts to very minute psychrometric differences, but exact experi-
mental data on the subject are not as yet forthcoming.

The behaviour of Phycomyces when placed on a klinostat is also quite
peculiar ; the sporangiophores stand at right angles to the surface of the rotat-
ing medium, just as if there were a radially acting force emanating from it. In
reality the position which Phycomyces takes up in relation to the substratum
can be explained by hydrotropism only, since it alters when the air is saturated.
At the same time it must be noted that an ordinary moist chamber is not ab~

486 TRANSFORMATION OF ENERGY

solutely saturated, but that the saturation is always increasing sufficiently
to induce directive movements on the part of the fungus. In the case of other
plants, however, grown on a klinostat, e. g. Lepidium sativum, orientation with
special reference to the substratum may easily be observed. If Lepidium be
cultivated on a cube of turf the plumules arrange themselves at right angles to
the four faces of the cube which are parallel to the axis of the klinostat, while
they assume somewhat oblique relations to the two other sides, turning their
convex sides to the axis. These positions cannot be explained by hydrotropism
since the plumules of Lepidium are not hydrotropic. Probably we have here
to deal with heliotropic movements which in the conditions of illumination
under which the experiment is generally conducted are by no means excluded
(DiETZ, 1880). In the dark such orientation in relation to the substratum com-
pletely disappears.

We may conclude this lecture by drawing attention to two other tropisms
about which little is known. DARWIN (1881) was the first to describe traumato-
tropism in roots. If the growing point (SPALDING, 1894) of a root be injured on
one side by some chemical or by heat, a curvature takes place in the growing
zone, which has the effect of removing the end of the root from the injurious
substance. Data with regard to this tropism are almost entirely wanting [com-
pare BURNS, 1904] ; we do not know whether the stimulus is to be sought for
in some chemical change in the root or whether even the curvature itself has any
purpose in nature.

We are better acquainted with the phenomena of rheotropism first described
by JONSSON (1883) as occurring in roots, more especially in those of seedlings,
but not limited to them. When such roots are grown in running water, they
exhibit a curvature in the opposite direction to the course of the flow (positive
rheotropism). NEWCOMBE (1902 a) found that in the case of the roots of Cruci-
f erae, which respond best to this stimulus, the minimum rate of water flow which
was capable of acting as a stimulus was 2 cm. per minute ; the best results were
obtained when the speed had reached 100-500 cm., and at about a i,ooocm.
negative curvature ensued, although these would appear to be due to purely
mechanical causes and not to be stimulation phenomena at all. JUEL (1900)
has made similar experiments with like numerical results for Zea mais and Vicia
sativa. Rheotropism, however, is not a peculiarity exhibited by all roots,
and the individual variations in different cases are, according to the state-
ments of all investigators, very considerable (compare BERG, 1899).

More recently (1900) JUEL has shown that decapitated roots still react
rheotropically, and NEWCOMBE (1902!)), considers it probable that the stimulus
makes itself felt not only in the growing zone but also at the apex and in older
parts up to a distance of 15 mm. from the apex. That rheotropism has nothing
in common with hydrotropism, as one might at first sight imagine it had, has
been shown experimentally by JUEL, who has proved that the immediate percep-
tion is due to the pressure of the flowing water. Hence rheotropic curvatures
should be correlated with the movements in roots due to unilateral contact
(Lecture XXXVIII).

We have now gained some insight into tropistic curvatures due to a large
number of different stimuli, but we must not attempt to consider the combined
action of several stimuli, as we attempted to do in the case of geotropism
and heliotropism.

Bibliography to Lecture XXXVII.

DE BARY. 1884. Vgl. Morphologic u. Biologie d. Pilze. Leipzig.
[BENNETT. 1904. Bot. Gaz. 37, 241.]
BERG. 1899. Lunds Univ. Arsskrift, 35.
BRUNCHORST. 1884. Ber. d. bot. Gesell. 2, 204.
£BuRNS. 1904. Beihf. bot. Centrbl. 18, I, 159.]

HYDROTROPISM 487

CZAPEK. 1895. Sitzungsber. Wiener Akad. 104, 337.

CZAPEK. 1898. Jahrb. f. wiss. Bot. 32, 175.

DARWIN. 1881. Bewegungsvermogen d. Pflanzen. German ed. by CARUS.

Stuttgart.

DIETZ. 1888. Unters. bot. Inst. Tubingen, 2, 478.
ELFVING. 1882. Bot. Ztg. 40, 257.

ELFVING. 1890. Commentationes var. Univ. Helsingfors.
ERRERA. 1892. Annals of Botany, 6, 373.
HEGLER. 1891. Verb. d. Gesell. d. Naturforscher, 108.
JONSSON. 1883. Ber. d. bot. Gesell. i, 512.
JUEL. 1900. Jahrb. f. wiss. Bot. 34, 507.
KIHLMANN. 1883. Act. Soc. Fennicae, 13.

KLERCKER. 1891. Oefvers. Vetensk. Akad. Forhandl. Stockholm, 10, 778.
LIDFORS. 1899. Ber. d. bot. Gesell. 17, 236.
[LILIENFELD. 1905. Ber. d. bot. Gesell. 23, 91.]

M ASSART. 1889. Arch, de Biologic (van Beneden and Bambeke), 9, 515.
MIYOSHI. 1894 a. Bot. Ztg. 52, i.
MIYOSHI. i894b. Flora, 78, 76.
MOHL. 1851. Die vegetab. Zelle, p. 140. Braunschweig. (R. WAGNER'S Hand-

worterbuch d. Physiologic.)

MOLISCH. 1883. Sitzungsber. Wiener Akad., Math.-nat. Kl., I. Abt. 88, 897.
MOLISCH. 1884. Ibid. 90, in.
MOLISCH. 1889. Sitzungsanzeiger Akad. Wien.

MOLISCH. 1893. Sitzungsber. Akad. Wien, Math.-nat.-KL, I. Abt. 102, 423.
MULLER-THURGAU. 1876. Flora, 59, 65.
NEWCOMBE. 1902 a. Bot. Gaz. 33, 177.
NEWCOMBE. i9O2b. Annals of Botany, 16, 429.
[NEWCOMBE and RHODES. 1904. Bot. Gaz. 37, 23. ]
NOLL. 1892. Ueber heterogene Induction. Leipzig.
PFEFFER, quoted by ROTHERT. 1894. Flora, 79, 212.
SACHS. 1872. Arb. bot. Inst. Wiirzburg, i, 209.
[SAMMET. 1905. Jahrb. f. wiss. Bot. 41, 6 n.]
SPALDING. 1894. Annals of Botany, 8, 423.

STEYER. 1901. Reizkrummungen bei Phy corny ces. Diss. Leipzig.
VAN TIEGHEM. 1884. Traite de botanique. Paris.
VOCHTING. 1888 a. Ber. d. bot. Gesell. 6, 167.
VOCHTING. 1888 b. Bot. Ztg. 46, 501.
VOCHTING. 1890. Jahrb. f. wiss. Bot. 21, 285.
VOCHTING. 1902. Bot. Ztg. 60, 87.
WIESNER. 1878-80. Die heliotrop. Erscheinungen. (Denkschr. W. Akad.)

WORTMANN. 1 88 1. Bot. Ztg. 39, 368.
WORTMANN. 1883. Ibid. 41, 457.

WORTMANN. 1885. Ibid. 43, 193.

LECTURE XXXVIII
HAPTOTROPISM

AT the end of the preceding lecture we referred to curvatures which occurred
in the root as a consequence of contact stimulus. Such haptotropic or thigmo-
tropic movements are most conspicuously illustrated by tendril-bearing plants
(DARWIN, 1876 a ; PFEFFER, 1885 ; FITTING, 1903 a) since these plants are pro-
vided with special organs or ' tendrils*, whose function it is to attach themselves
to supports by haptotropic curvatures. Just as in the case of the twining
plants, the tendril-bearers are unable to stand erect by their own unaided efforts,
and hence they make use of any rigid bodies available as supports, whether these
be dead or alive. The attachment to the support is effected by a spiral winding
of a tendril round it. Since the tendrils are generally either leafless lateral

TRANSFORMATION OF ENERGY

branches or leaves without laminae, it may be said that support is effected by
means of lateral organs while the chief shoot grows straight on. In this respect
tendril-bearers differ markedly from most twining plants ; but there is another
and more important distinction, viz. that the twining stem can hold on from
below upwards only to a more or less erect support and twine in a definite
direction, i.e. to right or left, while tendrils can attach themselves also to
horizontal supports and can twist round them to the right or to the left, upwards
or downwards. This points to quite different physiological properties of these
two closely related biological groups of plants — a point which will come out with
perfect clearness in the following treatment of the subject.

Starting with typical tendrils, such as we find in the Leguminosae, Cucur-
bitaceae, or Passifloraceae, we find them to be long, slender, flexible structures,
which, as in Passiflora, arise singly in the axils of the leaves, or which, as in
the Cucurbitaceae, arise singly or in greater numbers on a tendril-bearer,
alongside a leaf on the chief axis (Fig. 153. Compare GOEBEL, Organographie,

F'K- fS3- Tendril-bearer of Sicyos angulatus. 6, feebly stimulated ; c} strongly stimulated ; </, a tendril which
has attached itself to a support. At W a reversion of the direction of twining has taken place. From DETMER'S
Smaller Practical Physiology.

p. 610) ; in the Leguminosae the tendrils generally occur at the ends of leaves.
A transverse section of a tendril is, as a rule, circular, but often it varies from
that shape and becomes flattened. Commonly the anatomical structure is
markedly dorsiventral (O. MULLER, 1887) ; it is possible to distinguish an upper
and an under side and a right and a left flank. Even if, anatomically, there be
no difference between the sides it is possible to recognize dorsiventrality by other
evidence, for very frequently the tendrils, as they develop from the bud, are
rolled up in a spiral manner and the convex side is then always the under side.
They begin to grow rapidly and in a few days reach their definite length by
straightening. During this period they perform peculiar movements, i. e. of cir-
cumnutation, which recall those of the twining plants, but which are of a purely
autonomous character and which, for that reason, cannot be discussed in this
connexion (Lecture XLI). It may be noted only that the tendril, in consequence
of the fact that one side is growing more rapidly than the other, is slightly bent

HAPTOTROPISM 489

and that it describes approximately a conical curve in space, because the zone of
more vigorous growth affects successively new surfaces. At first the axis of the
cone is directed steeply upwards, but it gradually sinks lower and lower below the
horizontal, in which position finally the circumnutations cease. The growth
which now takes place is very conspicuous, for the whole tendril, under suitable
conditions, may elongate from 50 per cent, to 90 per cent, in one day, and indi-
vidual zones may show extension amounting to 100 per cent. Growth is inter-
calary, for no apical elongation takes place. It is most vigorous in the basal half
and continues in that region for a longer time than in the apical half. After about
3 to 5 days the tendril appears to be full-grown, but more accurate measure-
ments show that it is still growing though feebly, i.e. about 0-5-2 per cent, in
24 hours. After this feeble growth has continued for a few days vigorous
growth recommences, although it never reaches its original activity. The
growth moreover is always unilateral, the upper side growing more rapidly, and
hence inducing the formation of a coil or spiral, the concave side being the under
side morphologically. This spiral coiling begins as a rule in the middle or in
the basal part of the tendril and spreads towards the apex.

The specific sensitivity of the tendril, the capacity for twining round a sup-
port, makes itself apparent as soon as it has reached from J to J of its ultimate
length, and it may be still observed when spontaneous inrolling takes place.
The sensitivity ceases when the tendril is full grown. In order to understand
how the enclosure of the support by the tendril is achieved, we must study
first of all the incurving movement which takes place after a brief contact with
some solid body. If, for instance, the tendril of Passiflora or of Cyclanthera
pedata be rubbed on the under side with a splinter of wood or a pencil, after a few
minutes, or even a few seconds in some cases (Cyclanthera, 5-7 seconds ; Passi-
flora and Sicyos 25-30 seconds), a vigorous incurving takes place, the under
side to which the friction was applied becoming concave, and this incurving
progresses so rapidly that it is often possible to follow the movement with the
naked eye. After 30 seconds, or a longer period in less sensitive tendrils, the
curvature ceases and soon thereafter the tendril becomes straight once more, but
the undoing of the curvature always takes a much longer time than its formation.

So far as the result is concerned it is by no means immaterial to which part
of the tendril the friction is applied , we find that the under side is not equally
sensitive throughout its entire length. The same stimulus produces a more rapid
response if applied to the upper third of the tendril than if applied in the middle,
and no visible response is given by the base of the tendril where the growth is
most vigorous. On the flanks the sensitivity also decreases from the base to the
apex, and it is generally less there than on the under side. Further, on the flanks
the stimulated part is always towards the concave side, but the curvature is
less marked and slower than when an equally strong stimulus is applied to the
under side. Finally, if the upper side be stimulated, as a rule no curvature follows.
This is not the case, however, with all tendrils ; those of Cobaea scandens, Eccre-
mocarpus scaber, Cissus discolor, &c., curve quite as vigorously when the dorsal as
when the ventral side is stimulated. We shall find later on that these tendrils
also are physiologically dorsiventral. Such tendrils as those of the plants just
named may be considered as uniformly sensitive on all sides, as contrasted with
the others previously referred to, which were sensitive on one side only (DARWIN,
i876a). FITTING (1903 a) had shown, however, that the dorsal side of unilaterally
sensitive tendrils can receive a stimulus although it does not respond by curving.
The facts on which he bases this conclusion are as follows : If a tendril be touched
with a stick on a reacting side only a short distance away an incurving
takes place only at the stimulated place, and this curving is propagated about
2-5 mm. on either side of the region of stimulation. If two places be stimulated
about i J-2 cm. apart, two curvings occur, the region between remaining straight.
If the whole of one side of the tendril be stimulated from base to apex, the tendril

490 TRANSFORMATION OF ENERGY

rolls itself up into a spiral, from the apex downwards. If a tendril which is
sensitive on all sides be stimulated first on one side and, at the same time or
shortly afterwards on the opposite side, no curvature results, provided the
stimulus on one side be equal to that on the other. If the same experiment be
tried with a tendril sensitive on one side only and if both upper and under sides
be stimulated, one would expect that the response would be the same as if the
under side only had been stimulated. That is, however, by no means the case,
for the tendril remains quite straight. If a small part of the upper side and
at the same time the whole of the under side be stimulated, curvature takes
place only at the places on the under side which lie opposite to the unstimu-
lated regions of the upper side. The sensitiveness to contact is thus as well
developed on the upper as on the under side, and the difference between the
two sides lies in the fact that while stimulation of the under side induces
curvature, stimulation of the upper side induces no visible result or simply inhibits
curvature on the under side, according to circumstances. Following FITTING,
we must therefore recognize tendrils which react equally on all sides and tendrils
which do not do so. As to the behaviour of the latter type we shall have a clearer
conception after we have analysed the stimulus movement into its different
phases. First of all let us examine the nature of the perception of the stimulus.

On this subject DARWIN has already made certain statements. He
assumed that the tendrils reacted to a definite pressure and therefore laid light
weights, such as wires, threads, &c., on the parts of the tendril capable of
movement, taking the utmost precautions against inflicting a shock. He found
that the tendril of Passifiora gave curvature-response to the pressure of
a piece of platinum wire 1-23 mg. in weight, and to a piece of cotton
thread 2-02 mg., while the tendrils of other plants required greater weights,
up to 4-9 mg., before any response could be recognized. According to
PFEFFER (1885), to whom we owe an elaborate investigation into the
phenomena of perception of contact stimulus by tendrils, these experiments of
DARWIN'S do not meet the case at all, since as a matter of fact much greater
pressures may be exerted on the plant without any visible response resulting.
A continuous or statical pressure generally never induces curvature, and even
in DARWIN'S experiments, in spite of every care being taken, vibrations could
not be avoided when the weights were placed on the tendrils or afterwards.
The shocks, slight as they were, operated as a stimulus. If shocks be not elimin-
ated, far lighter bodies than those employed in DARWIN'S experiments will induce
curvature, e.g. a particle of cotton thread 0-00025 mg-> which was placed in posi-
tion simply by a draught of air. On the basis of PFEFFER' s researches the
perceptive capacities of tendrils may be estimated in the following way.

We may first of all inquire whether liquids as well as solids may act as
stimuli to the tendrils. This is obviously not the case, since if we inflict only
very slight blows on the tendrils with a solid body curvature appears at once,
while an equal shock from a liquid never induces any reaction. PFEFFER allowed
water, watery solutions of various substances, oil, and finally mercury to strike
the sensitive region of the tendril at greater or less velocities and obtained no
reaction, although the mercury had an injurious effect on the tendril. These
facts are of great importance, for they show that tendrils cannot be stimulated
by raindrops ; a capacity for reacting to such stimuli would be obviously quite
meaningless. If, however, there be any small solid particles in the liquid, such as
crystals in the oil, mud in the water or accidental impurities in the mercury,
stimulation is at once set up. It would appear therefore that tendrils are able
to discriminate between different conditions of aggregation of bodies and to
react to the solid but not to the liquid. Yet this is by no means the case, for
tendrils are unable to distinguish gelatine containing 10-14 per cent, of water
from a liquid, although that substance is solid at ordinary temperatures. This
fact suggested a whole series of interesting experiments, for PFEFFER was enabled,

HAPTOTROPISM 491

first of all, by employing a 14 per cent, gelatine solution which was kept moist,
to fix bodies to the tendrils under investigation without inducing stimulation ;
further the gelatine was smeared over a glass rod and this rod was employed in
the study of the influence of different types of contact on tendrils. By its means
the effect of constant pressure, both uniform and gradually increasing, was
tried ; solitary or numerous successive blows, light or heavy, were inflicted on
the tendril, or the tendril was rubbed with the rod ; in no case, however, did
any stimulation occur, nor did any curvature follow after constant pressure
or blows, single or successive, or after friction ; each and all were quite ineffective.

In the course of his experiments PFEFFER proved that blows, administered
by a solid body (apart from gelatine) were stimulants provided they were of
sufficient intensity. Thus thin smooth glass threads, sticks of wax, filter paper,
animal membrane in the dry and the wet condition induced a positive reaction,
and the significance of the velocity of the impacts could be easily demonstrated
by means of particles of clay suspended in water. On the other hand it was seen
that solid bodies induced no reaction if the pressure they exerted were statical,
that glass threads or needles, if they were pressed against the tendrils cautiously
and without any friction and without any sudden increase in the pressure,
produced no effect. Nor was there any result when a short piece — about 4 mm.
long — of the tendril was subjected to a constant pressure of a solid body, on
which there were several different points of contact (e.g. a rusty nail, emery
paper). In all these cases, however, a reaction took place at once when the
bodies in question were gently rubbed on the tendril no matter how limited the
surface of contact was. Of the greatest importance is the fact that small blows
of this kind, though unable to induce a reaction when administered singly,
induced curvature by summation of stimuli. The tendril remains sensitive for
a remarkably long time to constantly recurrent stimuli, and while the reaction is
in full progress new stimuli may be perceived until gradually the tendril
becomes accustomed to them.

As a result of his inquiries PFEFFER summarized the perceptive powers
of tendrils in the following words : ' In order that a stimulus may be effective
definite points of limited extent in the sensitive region of the tendril must be
affected by a push or a pull of sufficient intensity, simultaneously or in adequately
rapid succession. On the other hand, the tendril does not react as soon as the
blow affects all points of a larger surface with approximately equal intensity
in such a way that compression of closely adjacent regions is sufficiently nearly
uniform* (gelatine). Thus it is that tendrils are not excited either by mechanical
shaking in general or by rain. ' Under all conditions a local compression,
decreasing with sufficient rapidity, is a condition of stimulation, which is
not induced by statical pressure only, even if such a pressure affects widely
separated parts with considerable intensity.'

In order to have a single word to express the mechanical conditions of
tendril stimulation we will use PFEFFER' s term, contact, although that word
might be more appropriately applied to statical pressure ; we may say, in
a word, that tendrils possess contact sensitivity. We shall find that other
bodies as well as tendrils possess the capacity of responding to contact-stimulus.
Whether or not a similar mechanical influence leads to perception in the interior
of the cells in the case of geotropism, it is impossible at present to say ; there,
apparently, statical pressure does act as a stimulus.

We must assume that the perception of the stimulus takes place in the
epidermis, and PFEFFER has shown in this relation that the deformations of the
protoplasm necessary for perception might be simplified in many tendrils by
certain histological arrangements such as the so-called 'sensitive pits '. Still
it must be remembered that there are many sensitive tendrils which do not
possess such histological differentiations, so that these pits cannot obviously
be a necessary condition of stimulation. It is needless therefore for us to

492 TRANSFORMATION OF ENERGY

discuss them at length, and we may content ourselves with referring to HABER-
LANDT'S (1901) exhaustive memoir on the subject. HABERLANDT, in addition to
anatomical data, records certain physiological arguments as to the nature of
contact sensitivity. He points out that a vigorous radial pressure, a com-
pression of the protoplasm, does not lead to a stimulation, and that the
stimulus-movement is induced rather by tangential tensions in the protoplasm.
Whether these observations help to explain the nature of the phenomenon we
are not prepared to say.

It is very remarkable that tendrils which possess so great a capacity for
distinguishing different mechanical influences should also react to stimuli very
different in character from those of contact. We are indebted to CORRENS (1896 a)
for proof of the fact that tendrils exhibit curvatures, when subjected to sudden
changes of temperature (cooling or heating), quite similar to those induced by
contact. As far as we know (FITTING, 1903 a, 614) the mechanics of growth are
the same as those seen in contact curvatures. Further CORRENS has shown that
chemical stimuli also induce curvature in tendrils, as for instance when they are
treated with such diverse substances as iodine solution, acetic acid, chloro-
form, or ammonia. Finally PFEFFER has succeeded in obtaining responses by
employing weak induction currents as stimuli.

We shall return to these phenomena at the close of this lecture, but we must
not omit to draw attention to them here by way of showing that the sensitivity
of tendrils is not so restricted as one might think from the publications on the
subject. On the other hand curvatures due to heat or chemical stimuli may be
the more readily disregarded since these are entirely unnatural and do not affect
tendrils in the wild state.

Let us now turn to the curving itself that generally follows on the application
of a stimulus. The extraordinary rapidity of the process has not unnaturally
led to the assumption (DARWIN, 1876 a; MACDOUGAL, 1896) that the movement
was due to a diminution of turgidity on the concave side, afterwards rendered
permanent by growth. From FITTING'S (1903 a) researches it would appear,
however, that turgor has no special part to play in the process, which is rather
to be attributed to special growth phenomena. FITTING showed, by making
microscopic measurements of the movements of ink-lines made at appro-
priate distances apart on the upper and under sides, that immediately after the
stimulation the marks on the convex side separated much more rapidly than
before. The elongation may be so great in the course of a few minutes as to
amount to an increase of 50, 100, or even 160 per cent, in an hour, values not
attained by the non-stimulated tendril in 24 hours, and this, too, independently
of the age of the tendril. At the same time the indices on the under or concave
side approximate somewhat, resulting in an absolute contraction of about I per
cent, per hour. From these measurements it is obvious that not only do all the
tissues of the stimulated zone lying between the axis (middle line) and the convex
outer surface suffer extension, but that most of those towards the concave side
also participate in the growth increment, in other words, the neutral line, which
is neither elongated nor contracted, lies on the concave side of the medulla ;
the middle line itself exhibits a marked increase in growth. That can only be
definitely established by direct measurements on the flanks which suffer
elongation to the same degree as the middle line.

We cannot, however, draw any conclusions from these calculations as to
whether the growth acceleration, which reaches a maximum at a point on the
other side opposite to the point of contact, is the first and the only factor
leading to curvature, or whether an inhibition of growth on the concave side
takes place at the same moment or j ust previously. The observed approximation
of the indices on the concave side may be active, or they may be caused passively
by compression induced by the curvature. It is more probable that the growth
acceleration is not active in all the parts which it affects but that the deeper

HAPTOTROPISM

493

Convex side

layers behave passively in relation to the peripheral layers where vigorous
growth takes place. Unfortunately space will not permit of our discussing the
question at greater length (FITTING, 1903 a, p. 615).

Some time after the completion of the curving a cessation of growth takes
place and thereafter begins, as we have already seen, a straightening of the tendril.
Measurements show that the hitherto convex side remains quite unaltered while
the movement of the indices on the concave side indicate that growth is taking
place, less vigorously it is true than on the convex side during the period of
curvature, but still so as to show that a marked acceleration in growth is occur-
ring as contrasted with that exhibited by the unstimulated tendril. This
acceleration can be traced beyond the middle line. A graphic representation
of these phenomena is given at Fig. 154.

The recorded facts show in the clearest manner that the whole movement
following on stimulation is exceedingly complicated, since the curvature is not
effected by a simple contraction of the stimulated side but by an acceleration of
growth which is most marked at the spot on the convex side directly opposite
to that on the concave side where the stimulus has been applied. Apparently
therefore perception is followed by a transmission of the stimulus. Perception,
transmission and re-
action follow each
other in this case with
a rapidity which is
very much greater
than we have seen to
be the case in any of
the tropistic move-
ments previously dis-
cussed.

If we now trace
the growth in a tendril
which has been stimu-
lated equally and
simultaneously on
both sides, we shall
find that we always
obtain the same result,
whether it be a tendril
which reacts on one
side only or on all sides
equally. The tendril

grows on as if nothing had happened to it ; increased growth, more especially, is
absent altogether. We must therefore conclude that the absence of curvature
when both sides are stimulated is not, as might be imagined, more especially
in the case of tendrils which react equally all round, to be explained by assuming
that the two stimuli induce two similar reactions on opposite sides. On the
contrary one stimulus inhibits the effect of the other, and this is the case even
when curving has already commenced. This fact shows that stimulation of the
upper side does not by any means render perception on the under side im-
possible, but whether the inhibition affects the transmission or the reaction we
cannot say.

There remains for consideration the significance of the backward curvature
which takes place after each incurving. This is due doubtless to internal causes,
for it is not a consequent of the contact stimulus, but primarily of the reaction
for it follows, in virtue of growth acceleration, after every curvature induced
by purely mechanical means. That it is a case of autotropism naturally suggests
itself, such as we have already become acquainted with in considering the counter

20'

JO'

JO'

Minutes

60'

Fig. 154. Graphic representation of the growth of a tendril of Sicyos
angulatus. *, beginning of the stimulation ; f, completion of the curvature ;
!, beginning of the readjustment.

494 TRANSFORMATION OF ENERGY

movements in geotropic curvatures. We have regarded the compression of the
concave side there as a cause of this autotropism (BARANETZKY, 1901) and there-
fore the second growth acceleration in the case of tendrils could only take place
if actual curvature had previously been effected. FITTING (1903) showed, how-
ever, that in tendrils also which are mechanically prevented from carrying
out their curvature, there are likewise two temporary growth accelerations
separated by a period of rest. It is very desirable that measurements
should be taken of the distribution of growth during the period when
curving is ceasing, since in those structures which are affected by a geotropic
stimulus, a growth acceleration manifests itself in the median zone (compare

&435) ; it is possible that the same relations may obtain in the present case,
eanwhile it is impossible to say with certainty whether the autotropism of
tendrils is of a different nature or not from that already observed in cases of geo-
tropism. If both phenomena be identical then the cause of autotropism must
lie in an effort to curve, a tension of the parts concerned, and not in the first
instance in the completed curvature (compare FITTING, p. 612).

Having now studied the movements of tendrils which result from a temporary
contact, let us turn next to problems connected with the actual encircling of the
support by the tendril in nature. The circumnutation of the tendril, exhibiting
as it does movements very similar to those seen in twining stems, must aid it
considerably in its efforts to find a suitable support ; we might almost say that
the tendril feels round about for a support to attach itself to. If the hapto-
tropically reacting region comes in contact with some solid body the further
nutatory movements enable it to place itself under conditions in which the
haptotropic stimulus may operate. As in the experiments described, the tendril
rubs itself against the support, and forthwith at once curves, whereby new
regions of the tendril come in contact with the support. Since the stimulus,
as we have seen, is transmitted for a few millimetres on either side of the point
of contact, the tendril in a very short time describes a complete circle, provided
that the support be not top thick or too thin. If the support be of suitable
diameter the tendril is still unable to carry out completely the curvature
striven after ; a tension arises which exerts a pressure on the support, a tension
which may be easily demonstrated by using compressible material, such as a roll
of paper, as the supporting structure ; such a substance will be found to have
been squeezed by the tendril. The first coil is followed by a second and a third,
if the tip of the tendril be still free. How comes it that these coils, it may be
asked, remain permanently encircling the support if each incurving be followed
by a recurving of the tendril ? The reverse curvature may indeed be observed
in tendrils which have managed to grasp a support, manifesting itself in a loosen-
ing of the existing coil. When this reverse curvature appears, however, the
movement of the tendril or of the support may bring about a fresh contact
stimulus, which again induces incurving, so finally effecting a permanent en-
circling of the support. So long as no loosening of the coil takes place, no new
contact stimulus is possible, for the pressure exerted on the support does not
act as a stimulus. As we have already seen, the stimulus is transmitted from
the point of contact both proximately and distally. For purely mechanical
reasons the base of the tendril cannot exhibit any curvature, since both ends
are firmly fixed, to the plant on the one hand and the support on the other.
But if the spiral on the support becomes slack, then curvature ensues in the
neighbouring portions of the tendril basally, so that these come in contact with
and surround the support, pushing in front of them the previously formed but
now loose coils. A glance at Fig. 155 will make this clear. When the coils
have succeeded in obtaining a permanent hold on the support further basipetal
curvature ceases.

After the completion of the permanent twining, growth in length completely
ceases, and there appears not only in these coils but also in the remainder of the
tendril a number of important changes, which are apparently induced not by

HAPTOTROPISM

495

contact but perhaps by pressure against the support. Longitudinal growth in the
basal region comes rapidly to a standstill, although the tendril may by no means
have reached the length attainable if a support be not grasped. Further,
a spiral twisting manifests itself in the basal region, which is of great service to
the plant inasmuch as it is the means of drawing the branch closer to the support.
For purely mechanical reasons this intermediate spiral changes its direction at
least once, but several such reversals of the spiral may frequently be noticed
(Fig. 153 W). That this reversal is mechanically necessary, may be readily
demonstrated by attempting to produce a spiral coiling in a cord or piece of
rubber-tubing fixed at both ends. This spiral coiling recalls the autonomous
curvatures already noted as occurring in older tendrils that have found no
supports, since it always arises by excessive growth on the upper side. There
are good reasons for believing, however, that the two phenomena are not iden-
tical, since intermediate spiral coils occur, after the clasping of the support,
in tendrils which exhibit no free senile coiling (Vitis, DARWIN, 1876 a). [FITTING
(1903 b) has shown that the incoiling of the base of the tendril after attachment
to a support is a stimulus phenomenon which takes place in consequence of a
growth acceleration in the median region.]

The following additional changes taking place in tendrils which have suc-
ceeded in finding a support, may also be noted. Not
infrequently secondary thickening accompanied by
a large development of sclerenchyma appears not only
in the part immediately in contact with the support
but also in the basal region of the tendril as well, which
renders the tendril more capable of fulfilling its function
(TREUB, 1882-3 ; EWART, 1898). Some tendrils also
give off a secretion which helps them to adhere (O.
MULLER, 1887) ; as to these and other morphogenic

results of contact stimulus compare p. 315. Finally Fi 7 tendril whose

we must note that functional tendrils remain alive for tip has just begun to coil round

a much longer time than those which have been un

successful in finding a support ; the latter soon die, tion. °n the support and on the

wither, or are thrown off.

Haptotropism manifests itself not merely in tendrils ace

ace

serving as specific climbing organs and which have somedWancefromthe
entirely relinquished their previous functions and ££££^
become adapted specially for that purpose, but it is the original coiim/; the point

Tiii • ji 1-11 j marked 4 is now in contact

manifested also in other organs which have remained with the support. (After FITT-

functional so far as their main duties are concerned, ING)-

but which add as a subsidiary function that of acting as

climbing organs. Thus, for instance, ordinary roots may become sensitive to

contact in their growing regions and may respond with haptotropic curvatures

(SACHS, 1873, 436 ; [NEWCOMBE, 1902 ; NEMEC, 1904]) ; and this capacity is so

to many different families, e.g. Clematis, Maurandia, Lophospermum, Tro-
paeolum, Solanum jasminoides, &c., grasp supports by means of their petioles,
while the leaf blades continue to act purely as assimilatory organs. Fumaria
officinalis also climbs by means of unaltered laminae. Nepenthes may also be
referred to in this connexion, where a certain part of the leaf functions as a tendril,
while of the remainder, one part assimilates carbon-dioxide and one part acts as
an insect-trap. Lophospermum may be specially mentioned as an example of a
climber whose principal axis is sensitive to contact and twines round a support
and in addition possesses both sensitive petioles and internodes. None of the
examples cited have as yet been fully investigated from a physiological stand-

496

TRANSFORMATION OF ENERGY

point, more especially as to whether they may be compared with genuine tendrils
so far as the mechanics of curvature and their irritability are concerned. Into
this question we need not enter.

A brief reference may now be made to Cuscuta. This remarkable plant
is of special interest as forming an intermediate link between tendril-bearers and
twiners. According to PEIRCE'S researches (1894), Cuscuta has two regularly alter-
nating conditions ; in the first it twines in a left-handed manner and, as a result
of rotatory movements, the shoot apex forms a number of steep spirals round some
vertical support only. After a time the second phase sets in, during which the
plant behaves like a tendril, twining round its support in much less steep and
tighter coils. These coilings are induced by contact with solids but not with
moist gelatine, so that the sensitivity of the stem is obviously quite comparable
to that exhibited by tendrils. In contrast, however, to tendrils, Cuscuta, during
the time when it is sensitive to contact, is also geotropically active as well, and
hence twines round vertical supports only. It may be noted also in passing
that Cuscuta is capable of forming haustoria as a consequence of contact
stimulus.

Haptotropic movements are also manifested freely by carnivorous plants,

Fig. 156. Leaves of Drosera rotundifolia^ that on the left viewed
from above, that on the right viewed from the side. Enlarged. After
DARWIN, from the Bonn Textbook.

although in their case the biological significance of
the movement is entirely distinct from that of
tendrils ; they enable the plant to catch and digest
small animals. Unfortunately we are still without
an up-to-date and comprehensive memoir on the
nature of the movements in carnivorous plants, so that our knowledge on many
important questions is very incomplete. We shall confine ourselves exclusively
to the study of the leaves of Drosera (DARWIN, 1876 b). In the common native
species, Drosera rotundifolia, the leaves are almost circular, attached to the axis
by long petioles. The upper side of the blade is somewhat concave and studded
with tentacles. The tentacles in the middle of the blade are glandular,
shortly stalked and stand erect, while those at the periphery are long stalked
and bent outwards. Each tentacle is tipped by a drop of sticky secretion which
sparkles in the sun like a dewdrop — hence the popular name of the plant, ' sun-
dew/ Small insects which chance to alight on the leaf are held fast by the sticky
secretion, and by further excretion from the glands digestion of the body is
effected. (Compare p. 184.) When a single tentacle has been touched by an
insect, not only that one, but other tentacles also exhibit curvatures, until finally
nearly all the glands come in contact with the prey and aid in digesting it. Let us
assume that the insect touches one or more of the shorter tentacles in the centre
of the lamina, these we shall find remain erect, but the stimulus is transmitted
from them outwards, so as to induce an inward radial curvature of every peri-
pheral tentacle. If the insect be caught by a peripheral tentacle, the latter only at

HAPTOTROPISM 497

first proceeds to carry its booty towards the middle of the leaf, but as soon as it
reaches the centre a stimulus is transmitted to the other peripheral tentacles
which then begin to incurve also. We have thus to discriminate between move-
ment induced by a direct stimulus from that resulting from a transmitted
stimulus, and we are able to establish the fact that that transmission can be
effected only by means of the central tentacles. All the disk tentacles, not
merely those which occupy the exact centre, are able to radiate impulses. If the
leaf be stimulated half-way between the centre and the margin at two opposite
points, half the tentacles bend towards one centre of stimulation, half towards
the other, so that, as DARWIN says, two wheel-like arrangements are formed, one
on either side, whose spokes are formed by the tentacles and whose hubs are the
points towards which all the glands concentrate. Let us now study the effect
of a direct stimulus on a single tentacle, so as to obtain some idea of the nature
of the sensitivity, the perceptive region, and the method of curvature.

Drosera reacts both to mechanical and chemical stimuli. The actively
mechanical stimuli are contact stimuli as in tendrils. DARWIN (1876 b) showed
that neutral liquids, such as water, produce no effect, although they are driven
against the tentacles with considerable force, and PFEFFER (1885) also proved that
a rod covered with gelatine (as in the case of tendrils) was ineffective. On the
other hand, solid, insoluble bodies, even though extremely light, induce movement,
provided only they were capable of penetrating
the secretion. The tentacles may be stimulated
also by a blow from a pencil or a splinter of wood,
although to induce a result several successive con-
tacts are necessary. In all these respects Drosera
agrees entirely in its behaviour with tendrils.
We owe to DARWIN the proof of the fact that the
sensitivity is localized in the gland exclusively,
and HABERLANDT (1901) showed that histological
structures similar to those which occur in tendrils,
i.e. sensitive pits, occur also in the epidermis of the FiR I57 Part of a transverse ^00
tentacle. Curvature is entirely confined to the through a leaf of Drosera, showing

, -,, , ., , , j -11 , •, three tentacles, one of which has under-

stalk of the tentacle and more especially to its gone curvature as the result of a
base, which bends sharply while the upper part re- stimulation. After DARWIN (1876 b).
mains quite straight. This is most marked in the

long-stalked peripheral tentacles (Fig. 157). As to the mechanics of the curva-
ture we as yet know nothing. Since, however, BATALIN (1877) has shown
that growth acceleration takes place during the curving, it is very probable that
not only the nature of the sensitivity but also the mode in which the movement
is carried out corresponds to that exhibited by tendrils.

The tentacles respond very rapidly to a mechanical stimulus. After
10 seconds DARWIN was able, with the aid of a lens, to observe the commence-
ment of curvature ; curvature visible to the naked eye was often apparent in less
than a minute. As bending proceeds the end of the tentacle in a very short time
describes a considerable curve in space, the peripheral tentacles being able not
infrequently to curve through an angle of 270° in the course of an hour, borne
time after the completion of the incurving a reverse curvature takes place and
a straightening of the tentacle, even if contact with the body acting as a stimulant
is continued. This movement certainly takes place much more slowly than m
the case of the tendril— according to DARWIN, in about 24 hours— yet the factors
concerned and the mechanics of the movement would appear to be identical
with the analogous processes in tendrils. Immediately after this straightening
—possibly even sooner— the tentacle is ready to receive a new stimulus.

It has already been pointed out that Drosera is also capable of being stimu-
lated by chemical agents. Chemical stimuli as a rule act much more vigorously
than mechanical, as is shown by the rapidity of the movement and the duration

498 TRANSFORMATION OF ENERGY

of the incurving. Apart from that, the curvature induced by chemical stimuli
corresponds exactly to that induced by mechanical stimuli. DARWIN proved
that many substances, varied in their character, acted as stimuli, when solu-
tions of these were placed on the leaf of Drosera. Among these he found that many
were neither useful nor yet injurious to the leaf, while, on the other hand, well-
known poisons, such as corrosive sublimate, and even nutrients, like many
ammonium salts or phosphates, are readily absorbed, or like proteid and pro-
teinaceous animal compounds, were first digested by the secretion given off by
the glands. Among the substances which were observed to be indifferent from
the nutritive standpoint, distilled water may be mentioned more especially,
whose value biologically as a stimulant is, according to CORRENS (1896 b), most
perplexing. It must be especially noted that in CORRENS'S experiments the
chemically purest water possible was employed ; curvature is not induced if
traces of soluble substances be present. Natural water is not a stimulant and
that because the fact that it contains lime inhibits the sensitivity of Drosera as
do such anaesthetics as ether, chloroform, &c. (compare p. 98).

Let us now glance at the curvatures induced in the tentacles by transmitted
stimuli. Very little is known as to the transmission of the stimulus itself ; it has
not been determined whether the agent in transmission is the parenchyma of the
leaf (DARWIN, 1876 b) or the vascular bundle (BATALIN, 1877). So far as we
know the stimuli are transmitted exclusively from the tentacles in the centre
to those on the outside and not vice versa. The impulse thus transmitted acts
directly on the motile zone from beneath ; that it need not first of all be trans-
ferred to the apex may be concluded from the fact that decapitated tentacles
(DARWIN, 1876 b, p. 219) react to a transmitted stimulus, although they are not
directly sensitive. This fact, in addition to its general significance, has a special
interest, inasmuch as it enables us to interpret a certain phenomenon seen in the
sap of the tentacular cells of Drosera which has been spoken of as ' protoplasmic
aggregation '. If the apex of a tentacle be stimulated its cells exhibit certain
peculiar alterations. Protoplasmic movements first of all take place (DE VRIES,
1886), and the single vacuole breaks up into a large number of small ones.
These proceed to contract by apparently ejecting some of their contents, e.g.
sugars and organic acids. These vacuoles, surrounded by their protoplasmic
membranes, lie in the extruded cell-sap ; the vacuoles contain a red colouring-
matter and hence the whole process may be readily followed. The aggregation
progresses from the stimulated gland down the tentacle from cell to cell and
appears later on in the other tentacles which have been indirectly stimulated.
We might interpret this progressive aggregation as the visible expression of
the transmission of the stimulus. More accurate investigation, however, reveals
the fact that the aggregation appearing in the indirectly stimulated tentacles
does not commence simultaneously with the curving but only when the glands
begin to secrete. Further, in the indirectly stimulated tentacles the aggregation
progresses from above downwards not from below upwards, i. e. in the path
of the stimulus. Hence it is obvious that aggregation is a process closely
connected with secretion and not with curvature, and we further know that
it is associated with very remarkable changes in the nuclei of the gland-cells
during secretion (ROSENBERG, 1899). It is also worthy of note that, according
to DARWIN (p. 220), much feebler aggregations are exhibited by decapitated
tentacles.

Apart from the fact that when the tentacle is directly stimulated the
stimulus is transmitted only from above downwards to the motile zone and in
indirect stimulation from below upwards, there is another and more important
difference between the two kinds of stimulation. In the former case it is
always a predetermined side of the tentacle (the outside) that becomes convex,
while in the latter case the course of the resulting curvature is determined by the
direction from which the stimulus comes (p. 497). Either flank of the tentacle

HAPTOTROPISM 499

may become convex, and it has yet to be determined whether the inner side can
be rendered convex under appropriate experimental conditions. It is only as
a result of indirect stimulation that a tropistic curvature is induced, the curvature
in the other case is nastic. By nastic curvatures we mean (compare p. 428)
those whose direction is determined by the plant itself. A nastic curvature may,
however, be induced just as well by a stimulus directed in a definite direction as
by diffuse stimuli. Which process takes place in Drosera is not quite established,
but it is very probable that the stimulus first affecting the apex passes with
equal intensity to all parts of the stalk, but one side (the upper or the under) in
all likelihood is more sensitive than the other, a view which is supported by a
comparison with the behaviour of tendrils. These organs behave purely tropisti-
cally after con tact stimulation, after chemical and thermal stimuli, however, they
behave nastically. The curvature which is induced by heating or cooling is,
according to CORRENS (1896 a), always in the same direction ; it is the under side
always that exhibits concave curvature, beginning from the apex in the case of
the tendril, and the result is the same whether the temperature of the tendril
is, by conduction, altered to a similar extent on all sides, or whether it be heated
on any one side by radiation. We must therefore conclude that heating of any
part of the tendril results in a uniform rise of temperature and that the conse-
quent movement is always the same, viz. that determined by the physiological
dorsiventrality of the tendril. At the same time it is very remarkable that
tendrils which react to contact equally well all round always curve so that only
their under sides become concave when subjected to heat stimulus. [The
mechanics of tendril curvature following on heating does not differ from that
resulting from contact stimulus ; the same is true of curvature induced by
wounds, save that in that case the transmission of the stimulus is very rapid
and far-reaching (FITTING, 1903 b).]

Drosera (and the tendril also) presents us with very interesting transitions
between tropistic and nastic reactions. Following on mechanical and chemical
stimulation, very delicate nastic movements, which are closely related to those
of Drosera, occur also in other carnivorous plants, as for instance in Dionaea
muscipula, Aldrovanda vesiculosa and, less markedly, in Pinguicula. Detailed
research on these forms, comparable to those which have been made on Drosera,
have not as yet been carried put, though much required, so that we had best
content ourselves with this brief reference ; still less have we space to discuss
haptotropic curvatures in lower forms (Mucorinae, ERRERA, 1881 ; TRZEBIN-
SKY, 1902 ; Algae, NORDHAUSEN, 1900).

Bibliography to Lecture XXXVIII.

BARANETZKY. 1901. Flora, 89, 138.

BATALIN. 1877. Flora, 60, 36.

CORRENS. 1896 a. Bot. Ztg. 54, i.

CORRENS. i896b. Ibid. 54, 21.

DARWIN. 1876 a. Die Bewegungen u. Lebensweise kletternder Pflanzen (CARUS).

Stuttgart.

DARWIN. 1876 b. Insektenfressende Pflanzen (CARUS). Stuttgart.
ERRERA. 1884. Bot. Ztg. 42, 497.

EWART. 1898. Annales Jard. bot. de Buitenzorg, 15, 187.
FITTING. 1902. Ber. d. bot. Gesell. 20, 373.
FITTING. 1903 a. Jahrb. f. wiss. Bot. 38, 545.
[FITTING. 1903^ Ibid. 39, 424.]
HABERLANDT. 1901. Sinnesorgane i. Pflanzenreich z. Perzeption mechanischer

Reize. Leipzig.

MACDOUGAL. 1896. Ber. d. bot. Gesell. 14, 151.
MULLER, O. 1887. Cohn's Beitr. z. Biol. 4, 97.
[N&MEC. 1904. Beihf. bot. Centrbl. 17, 52.]
[NEWCOMBE. 1902. Beihf. bot. Centrbl. 12, 243.]

K k 2

500 TRANSFORMATION OF ENERGY

NORDHAUSEN. looo. Jahrb. f. wiss. Bot. 34, 235.

PEIRCE. 1894. Annals of Botany, 8, 53.

PFEFFER. 1885. Unters. bot. Inst. Tubingen, i, 483.

ROSENBERG. 1899. Phys.-cytolog. Unters. iiber Drosera rotundifolia. Upsala.

SACHS. 1873. Arb. bot. Instit. Wiirzburg, i, 385.

TRZEBINSKY. 1902. Anzeiger der Akad. zu Krakau.

TREUB. 1882-3. Annales Jard. bot. Buitenzorg, 3.

DE VRIES. 1886. Bot. Ztg. 44, i.

LECTURE XXXIX

NYCTITROPISM

MANY plant organs, especially foliage and floral leaves, take up, towards
evening, positions other than those which they occupy by day. Petals and
perianth leaves, for example, bend outwards by day so as to open the flower,
and inwards at night so as to close it. Corresponding movements take place in
entire inflorescences as, for example, in those of theCompositae ; the capitulum
may be said to open when the ray flowers, or all the flowers, of the head curve
outwardly, and to shut when they curve inwards towards the centre. Many
foliage-leaves, also, may be said to exhibit opening and closing movements, not
merely when they open and close in the bud but also when, arranged in pairs on an
axis, they exhibit movements towards and away from each other. In other cases,
speaking generally, we may employ the terms night position and day position for
the closed and open conditions respectively ; thus, for example, the umbels of
Daucus are directed vertically downwards in the evening and stand erect by
day. The night position may also be described as the sleep position.

Day and night positions may arise by the combined action of geotropism
and heliotropism. Thus VOCHTING (1888) observed in the case of Malva verti-
cillata, that the leaves, when illuminated from below, turned their laminae down-
wards during the day, but during the night became erect geotropically. The
sleep movements in leaves and flowers referred to above cannot, however, be
explained by assuming such a combination of heliotropism and geotropism,
for, as a rule, they have nothing to do with tropisms at all, although they
are frequently occasioned by light or, in other cases, by heat. Light and heat
do not operate in these curvatures as they do in heliotropism and thermotropism
proper, where opposite sides of the plant are subjected to stimuli of unequal
intensity ; while in heliotropism and thermotropism we have to deal with a
variable regional distribution of heat and light, in the present case we have to do
with movements which are induced by these same factors operating at different
times. In other words, the plant reacts to variations in the degree of illumina-
tion and variations in temperature, and its reactions are not tropistic in character
but nastic ; we might, in fact, describe these opening and closing movements as
thermonastic and photonastic respectively, and might, at the same time,
characterize, with greater accuracy, the outward curvature as the result of
epinasty, and the inward curvature as the result of hyponasty ; that is to say, the
movements are photoepinastic or photohyponastic. These same movements
have also been described as nyctitropic and the phenomena as those of
nyctitropism. Although this phrase has now come into common use, we musl
nevertheless point out that it is incorrect from two points of view ; first, because
we are not dealing in these instances with a Iropism at all, and, secondly, be-
cause it is not darkness (™'£) that is the releasing stimulus, but the alternation
of light and darkness.

NYCTITROPISM

5°'

The reason why we associate nyctitropism with haptotropism is that
the mechanics of nyctitropic movement may, in many cases, be compared
with that exhibited by tendrils ; the likeness between these two sets of phe-
nomena is especially evident if we compare the thermonastic curvatures of
tendrils with the nyctitropic movements which we find manifested by many
flowers. The thermonastic curvatures of tendrils arise, as FITTING (1903) has
established, exactly in the same way as do haptotropic curvatures. On cooling
or heating a growth acceleration makes its appearance, which is most vigorous
on the upper side near the periphery, but which spreads to the middle zone also,
and fades away in the neighbourhood of the contracting under or concave side.
Some time afterwards, when the temperature has again become constant, the
tendril once more straightens itself as a result of an inverse growth process.
Let us now compare this with what we see in a spring flower, such as a tulip
or a crocus. When the temperature has risen sufficiently, vigorous growth takes
place on the upper side of the perianth ; this becomes convex, and the individual
leaves bend more or less outwards, in accordance with the rise in temperature.
Visible curvatures may be distinguished even after the application of feeble
stimuli ; in the crocus, for example, an elevation of temperature of one half a
degree is, according to PFEFFER (1873), sufficient to cause a movement. In the
tulip the greatest curvature takes place in the basal part of the perianth leaves,
and we may convince ourselves of this fact by carefully measuring fixed distances
of appropriate length on opposite sides of the perianth leaf. In the following
table, measurements of this kind are recorded which aim at showing the effect
of increase of temperature on the rate of opening of two flowers. The numbers
represent percentage increments per hour. The flowers were kept at a tempera-
ture of 11° C. from 5.30 p.m. one evening until 12.40 p.m. the next day, when
they were transferred to a temperature of 18° C. (JosT, 1898).

Temperature n°C.

% increase per hr. from

5.30 p.m. -9 a.m. 9-12
Flower No. i

Outside . . o-i 0-2

Inside ... 0-3 0-4

Flower No. a

Outside . . o-i o-o

Inside . . . 0-3 o-i

ist hr.
1 2-40-1 -40

o-o
6-8

3-9

Temperature i8°C.
2nd hr. 3rd hr.

1-40-2.40 2-40-3.40

7-4
o-5

4-7
o-o

1-8

3-4

o-o
0-6

4th hr.
3.40-4.40

0-4

o-i

1-2

o-3

If we consider the first flower, we find that at a constant temperature of
11° C. an extremely slight increase takes place in the course of several hours ;
but in the first hour after the higher temperature had made itself felt the
inside grows rapidly, the outside not at all. In the second hour, the relations
are entirely reversed ; the outside grows rapidly, the inside scarcely at all. The
second flower behaves in exactly the same manner. The curvature of the
perianth leaf naturally goes hand-in-hand with this distribution of growth, that
is to say, in the first hour the flower opens vigorously, and during the second
hour as vigorously closes. This phenomenon corresponds in all respects to
that exhibited by tendrils, for the stimulus results not in a movement in one
direction only, but in a backward and forward oscillation. As in the case of
tendrils, the time which is necessary for the carrying out of an oscillation varies
greatly in different flowers. The backward movement follows very rapidly
in the case of Tulipa, but it is not so in all cases ; in Crocus, two or three hours
elapse, and in other flowers an even longer period, before the backward move-
ment begins. It can scarcely be doubted that we have here to do with a case
of autotropism, and we may imagine that it is due, as is the curvature itself, to
a growth acceleration in the median zone also. It is possible that between those
two periods of more vigorous growth there intervenes a time when growth

502

TRANSFORMATION OF ENERGY

is retarded or ceases entirely ; measurements taken at shorter intervals than
have as yet been made can alone settle this question. [Measurements of this
kind have been made meanwhile by WIEDERSHEIM (1904) with results precisely
of the nature one would expect.] The growth acceleration of the middle zone
associated with the movements we are discussing is remarkably apparent in
the measurements above quoted.

Hourly percentage of growth in the median zone.

At n°C.

5.30-9 p.m. 9-12 p.m.
Flower No. i 0-2 0.3

Flower No. 2 0-2 o-o

ist hr.

3-4
2.7

After heating to 18° C.
2nd hr. 3rd hr.

4.0
23

2-7
03

4th hr
0-25
o-7

The acceleration shows itself most clearly if we compare growth in the
first and second hours after warming with that exhibited in the fourth hour
when exposed to continuous but uniformly higher temperature.

The effect of cooling may best be studied in the measurements recorded by
PFEFFER (1875, p. 125). He subjected crocus flowers, which had been exposed
for about twenty-four hours to a temperature of 17° to 18° C., to a temperature
of 7° to 74° C., and observed that the flowers closed. Increased growth set in
on the outsides of the perianth leaves, which again affected the middle zone,
but which after a short time became very considerably reduced. In support
of this statement we give below certain of PFEFFER' s measurements of the
growth of the middle zone of the crocus in percentages per hour.

At i7°-i8°C.

4 p.m. -9 a.m. 9 a.m.-i2 noon
Crocus No. i 0-64 0-70

Crocus No. 2 0-67 0-74

ist hf. hr.
4-65

6-21

At7°-7fC.

2nd hf. hr. Next 3 hrs.

1.87
3-27

0.41
o-34

It is unknown whether an opening movement, i. e. an autotropic reverse
action, takes place at lower constant temperatures still, but it is very probable
that that will be found to be the case.

The act of changing from one temperature to another increases the average
growth of the perianth leaves much above that exhibited at constant tempera-
tures ; it affects the upper and under sides differently, however, and thus induces
curvature. In all probability, a curvature may even make its appearance at such
temperatures as lie beyond the maxima and minima of growth when they affect
the plants constantly (comp. BURGERSTEIN, 1902). The analogies between these
movements and those of tendrils, especially the analogies with thermonastic
movements, are as we have already said, very great. They express themselves
in this, that during the movements due to stimulation, both incurvature and
recurvature, a new stimulus is always released owing to elevation of tempera-
ture ; the plants do not become accustomed to the stimulus, or, at least, do so only
gradually, for we can continue to induce opening movements with appropriate
elevations of temperature for many hours (Josx, 1898). All the same we must
not forget that there are differences to be taken account of. In the tendril
the most vigorous growth acceleration always takes place on the upper side
whether temperature be increased or decreased ; in the floral-leaves, on the
other hand, the upper side becomes convex when the temperature is raised
and the under side when the temperature is decreased.

Not all flowers respond with nyctitropic movements as Tulipa and Crocus
do, exclusively or especially, when the temperature is altered ; some respond
only to alterations in light, others exhibit nyctitropic movements only when
light and temperature are altered at the same time. [As to other factors in
nyctitropic movements compare HENSEL (1905).] Among plants which are
sensitive to alterations in light we must place the Compositae, where shading

NYCTITROPISM 503

causes the closure of the capitulum, as cooling does in the flowers of Crocus,
and illumination causes opening, as heating does in the case of Crocus. In
nature, as a rule, increased light is accompanied by an elevation and decreased
light by a reduction in temperature. Observations as yet available show that
curvature either in the lower tubular or in the upper flattened portions of the
corolla is effected by the same mechanical means as are thermonastic curvatures.
There are various difficulties, due partly to the nature of the movements them-
selves and partly to the fact that insufficient measurements have been carried
out in this direction, which render it impossible for us to enter into greater
detail on these questions at the present moment.

Nyctitropic growth movements occur in the foliage-leaves of a very large
number of plants of very distinct families, e.g. Alsineae, Compositae, Solanaceae,
Balsamineae, &c. (BATALIN, 1873). As a rule, simple leaves take up a more
or less horizontal position by day, while in the evening the laminae assume a
vertical position, due either to a curvature in the petiole or at the base of the
lamina. Thus the leaves either droop at night time, as in Impatiens, Polygonum
convolvulus, Sida napaea, &c., or they stand erect, so as to press themselves
against the bud, as in Chenopodium, Brassica, Polygonum aviculare, Stellaria,
Linum, &c. That the factor concerned in these movements is an alteration in
light may easily be proved in a large number of cases, for placing the leaves of
Impatiens in darkness at midday induces a very marked drooping. More exact
research is certainly required as to the sensitivity of such leaves for changes in
temperature.

Certain conclusions as to the mechanics of curvature may be drawn from
PFEFFER'S (1875) measurements, although a more exhaustive investigation of
these is urgently needed, especially after FITTING'S work on tendrils. The
following data are taken from PFEFFER'S observations : —

Impatiens noli-me-tangere.
Length of a section originally 100 units long.

Upper side. Under side.

9.30 a.m. Light 100 100

12.30 p.m. ) D , 100 100

1.30 p.m. \ jos 99.4

3'3O p.m. Light 105 102-8

Although no growth could be observed in the leaf when uniformly illuminated
for 3 hours before midday, when the leaf was suddenly darkened the upper side
increased 5 per cent, in the course of one hour and the under side decreased J per
cent. ; in the course of 2 hours, after the leaves had been again exposed to light,
the under side increased almost 3 per cent., while the upper side remained
stationary. The reversion of the curvature, however, which is associated with
this more vigorous growth of the under side may also take place without any
illumination. It would thus appear as if in this case an oscillation to and fro
might be brought about by mere darkening, each movement being accompanied
by considerable growth acceleration. The close analogy with tendrils is in no
sense supported by this result, since the movements of the leaf of Impatiens are,
as we have seen, of a more complicated character. We shall take an opportunity
later on of returning to this subject. These same complications render it im-
possible for us as yet to answer the question whether the influence of illumination
is analogous to that of darkening or not. Comparing the behaviour of flowers
when the temperature is raised, we are led to expect a temporary increase in
growth when light is intensified ; the evidence in favour of this view is, however,
by no means above criticism. [Evidence in favour of this conception has now
been adduced by WIEDERSHEIM (1904).]

Nyctitropic movements in the foliage-leaves hitherto mentioned, are

5°4

TRANSFORMATION OF ENERGY

carried out only whilst the leaf is still growing ; the amplitude of the
movements decreases proportionally with the age of the leaf. Other leaves,
however, retain the power of nyctitropic movement even in the full-grown
condition, but, as is to be expected, they are distinguished by the presence of
articulations at their bases. As to the special capabilities of the articulation, we
have already learned something when treating of heliotropism and geotropism.
Most, but by no means all, leaves which are provided with articulations ex-
hibit nyctitropic movements, such, for example, as numberless Leguminosae,
many Oxalidaceae and Marantaceae, several Euphorbiaceae (Phyllanthus),
Zygophyllaceae (Porliera), and Hydropterideae (Marsilia), and many others
(HANSGIRG, 1893, p. 131). In most cases, not one only but several articulations,
are concerned in the movement ; in bipinnate leaves, for example, we find one
pulvinus at the base of the main petiole, another at the base of each of the
secondary petioles, and yet another at the base of each individual leaflet, and

when all these pulvini
operate at the same time
the movement of the leaf
which results must neces-
sarily be complicated. In
Mimosa pudica, for ex-
ample, in the day position
the chief petiole forms with
the stem an angle of about
60° upwards ; there are
two pairs of secondary
petioles present, the basal
pair of which stands almost
at right angles to the pri-
mary petiole, whilst the two
apical ones form an angle of
about 60° with each other
forwards ; finally, the leaf-
lets spread out horizontally,
and form angles of 90° with
the secondary petioles in
the same plane. The posi-
tion of the leaf at night is
entirely different,for the pri-
mary petiole bends down-
wards as much as 80° to
100°. The four secondary

petioles bend forwards so as to place themselves almost parallel with the long axis
of the chief petiole ; the leaflets bend upwards and approximate in pairs, their
upper sides touching. At the same time they suffer slight twisting, and now form
an acute angle forwards, with the secondary petiole, the basal ones overlapping
the apical ones like the tiles on a roof. The day and night positions are sketched
at Fig. 159, in Lecture XL. In most cases it is only the articulations directly
associated with the leaflets that exhibit these movements clearly ; the leaflets
are in Mimosa turned upwards at night, but in many other plants they are turned
downwards. The behaviour of the leaflets in Acacia, in Hypocrepis, and also in
Coronilla (Fig. 158, ///) is perfectly similar to that of the leaflets in Mimosa, save
that in the last case the leaflets are directed backwards instead of forwards. In
Trifolium the terminal leaflet is simply raised up while the lateral leaflets are not
only raised, but at the same time undergo a torsion of about 90°. A single down-
ward curvature in the articulation occurs in Robinia, Amicia, Phaseolus, and in

Fig. 158. 7, Desmodium gyrans in day position. 77, The same in
night position. 777, Coronitla rosea in sleep position. After DARWIN,
(1881).

NYCTITROPISM 505

Desmodium gyrans (Fig. 158, / and //) ; in the case of Phyllanthus, torsion
occurs as well as curvature, so that in the night position the leaflets are folded
together in pairs with their upper surfaces in contact with each other. Amidst
all these variations, which the citation of further examples would only add to
(HANSGIRG, 1893), one general fact comes out, namely, that in the night position
the leaf surfaces are vertical.

It may be easily shown, say in the case of Phaseolus or Acacia lophantha,
that darkening is the cause of the sleep position, and light the cause of the day
position, but the reactions, for reasons which we shall learn later on, are not
manifested equally well at all hours of the day ; but if the plant be placed in
the dark in the afternoon and illuminated in the forenoon we always obtain a
very rapid result. Under all conditions, every movement of this kind is followed
by a reverse movement later on.

As is to be expected, sensitivity to changes in temperature is also manifested
by the articulations of motile leaves, although it cannot be demonstrated in all
cases with equal ease. There is no difficulty in demonstrating movements
resulting from changes in temperature in such leaves as those of Mimosa, Acacia
lophantha, and Phaseolus multiflorus, which react well when light is excluded, and
it may be easily shown (Josx, 1898) that an increase in temperature operates in
the same way as an increase of light, and that cooling has the same effect as
darkening. The case, however, is rendered more complicated in consequence of
the fact that a rapid rise of temperature, dependent perhaps as well on the abso-
lute height of the temperature, also brings about closure. The question now
comes to be, whether this closure, due to a rapid rise of temperature, is really
identical with the closure due to cooling or darkening, or whether it only resembles
it superficially. That the latter must be the correct view would appear to be
proved by the behaviour of plants like Robinia and Phaseolus, whose leaves, when
subjected to a considerable rise in temperature, do not reach the sleep position,
but go on rising until they succeed in orientating themselves in the same way
as they do under the influence of too intense light. At p. 465 we have drawn
special attention to this point in the case of Robinia, and described it as a helio-
tropic reaction, but we must not omit to mention here that frequently this
' profile ' position occurring in Robinia in the open does not exhibit that relation-
ship to light which, were it heliotropic, it must have ; hence we conclude that
the profile position of Robinia may perhaps be in many cases not a heliotropic
reaction at all ; it must, on the contrary, be quite independent of the direction
of light, and may be caused only by too high a temperature or too intense light.
The closure of many flowers may also be the result of too intense light, and to
OLTMANNS (1895) we owe the demonstration of the fact that one and the same
flower (e. g. of Lactuca), after being made to open by illumination, may after
a time be made to close again by increasing the light intensity as well as by
darkening. The condition brought about by too great an intensity of heat or
light is also known as ' day-sleep '.

If we now inquire into the mechanical causes of nyctitropic movements in
articulations we discover first of all that the curvature is effected without any
growth taking place, since after having performed two opposite motions the
articulation has not elongated (PFEFFER, 1875). We have here to deal, therefore,
not with nutation movements but with variation movements ; as a matter of
fact, the expansive efforts of the convex side must be due to alterations in
turgidity. In young pulvini undoubtedly growth also occurs. An elongation
in one-half of the articulation owing to osmotic activity might arise just
as well by increase of turgor pressure as by a decrease in the rigidity of the
cell-wall. Omitting for the moment alterations in the elasticity of the mem-
brane as being the less probable cause, curvature of the articulation might take
place either (i) when the turgor pressure increases in the convex half of the

506 TRANSFORMATION OF ENERGY

pulvinus, (2) when it decreases in the concave side, (3) when both these conditions
occur, or (4) when an alteration in turgidity of the same kind but of unequal
amount takes place in both halves of the pulvinus. We might imagine that it
would not be difficult to determine which of these four possibilities is the cor-
rect one with the aid of the plasmolytic method ; but HILBURG (1881) found it
impossible to demonstrate any plasmolytic difference between darkened and
illuminated articulations. This is all the more astonishing when we remember
that in articulations which are geotropically stimulated, differences in the
turgidity of the cells on opposite sides may readily be demonstrated by the
plasmolytic method. The reason for the failure in the present instance is as
yet by no means apparent, nevertheless we must assume that differences in
turgidity are the causes of nyctitropic movement, and efforts must be made in
some roundabout way to obtain definite information on the subject, as, for
example, by determining the resistance to flexion and by experiments with
articulations which have been operated upon.

As long ago as 1848, BRUCKE showed that the resistance to flexion in
the articulation was increased by darkening. In order to determine this resist-
ance he simply used the statical moment of the leaf and carried out his experi-
ment on the primary articulation of Mimosa in the following manner. He
inclined the plant carefully until the petiole of the leaf under consideration
occupied a horizontal position, that is, until the weight of the leaf exerted its
maximum effect on the articulation. After measuring the angle (a) between the
petiole and the stem, he turned the plant through an angle of 180° until the
petiole was again horizontal and once more measured the angle (a'). The
difference between these two angles (a-a') affords an estimate of the resistance
to flexion of the articulation, since as the resistance increases the value of
the remainder decreases. In two experiments with Mimosa, BRUCKE found this
difference to be quite as great in the evening as in the morning or afternoon, but
in two other experiments the difference was markedly less (namely 12° instead
of 21°, and 15° instead of 27°). PFEFFER (1875) found that in Phaseolus the
difference was 18° to 20° in daylight, 9° to 10° in the dark, and he was able to
establish the fact that in many other plants an increase in the resistance took
place in the evening. The increase in the resistance to flexion expresses
nothing more, however, than that the tension of the parenchyma of the articu-
lation against the central vascular bundle has increased ; how this increase in
tension is distributed in the articulation, whether all longitudinal areas parti-
cipate or whether the osmotic swelling takes place only at definite situations,
we do not at present know. It is even possible that turgor is reduced on one
side only when it is increased on the opposite side.

Since no conclusion can be drawn as to the mechanical cause of the bending
from the increase in the resistance to flexion, it will be necessary to experi-
ment with half articulations after removal of the other halves. With this end
in view PFEFFER removed the upper half of the articulation in the case of one
primary leaf of Phaseolus, and the under half in another leaf, and arranged the
leaves in an appropriate dynamometer so that the pressure activity of the ex-
panding articulation could be recorded. The results of these two experiments
were very remarkable ; both halves of the articulation reacted in exactly the
same way : when darkened expansion took place, when illuminated contraction,
in each half. The movements which the leaf executed, when provided with one
half -articulation only, were exactly opposite in direction according to whether
the upper or the under half of the articulation was retained. The leaf provided
with the upper half of an articulation became depressed upon darkening, and
elevated itself when illuminated ; the leaf provided with the under half of the arti-
culation elevated itself in the dark and became depressed in light. Since, in the
dark, both halves of the articulation show increased turgidity the conditions

NYCTITROPISM 507

would appear to coincide with the fourth possibility mentioned above, and a
curving is only possible if the osmotic swelling on the convex side is greater, or
reaches its maximum value more rapidly than it does on the concave side. This,
as a matter of fact, was what PFEFFER assumed, and he held that nyctitropic
curvature resulted from changes in the two half-articulations, similar in character
but of unequal rapidity. As the turgidity on the concave side gradually in-
creases the conditions become favourable for that reverse curvature which, as
we have remarked, always takes place.

This conception cannot, however, be held to be directly proved, since
PFEFFER' s results, obtained from experiments with half -articulations can by no
means be universally confirmed ; on the contrary, certain experiments have
been made (SCHWENDENER, 1898 ; JOST, 1898) on the behaviour of half -articula-
tions which gave results entirely contradictory of PFEFFER' s observations ; when
darkened, the upper half-articulation expanded whilst the under one contracted.
This would tend to show that the alterations in the degree of illumination in-
fluenced the expansive efforts of the two halves in exactly opposite ways ; the
reverse curvature would not then be an immediate result of an external stimulus
as in PFEFFER' s theory, but, as in tendrils and in nyctitropic growth movements,
would, in the first instance, be a result of the reaction arising from autotropism.
[WIEDERSHEIM (1904) is inclined to refer the dissimilar results of the removal
of half-articulations to differences in the mode of carrying out the experiments.
He maintains that his own special observations support PFEFFER' s theory, but we
are not inclined to agree with him in that conclusion.]

We must content ourselves at present with pointing out that a completely
satisfactory theory of nyctitropic pulvinus movements is not as yet forthcoming ;
such a theory can only be established after new and exhaustive experimental
research. For reasons which we will appreciate later, it will be necessary first
of all to study nyctitropic movement in leaves which have been previously
cultivated under perfectly constant external conditions, more especially where
there are no variations in light and temperature. Inquiry must then be made
whether complete extirpation of one half-articulation really moderates the reac-
tion of the other half in its original form, or whether it influences it correlatively .
Further, it is necessary to prove whether the cell-membranes remain entirely
unaltered so far as their elasticity is concerned, and whether the alterations in
expansion are due only to alterations of osmotic pressure. The increase in the
resistance to flexion of the entire articulation in the evening has not yet been
established beyond doubt, for SCHWENDENER (1897) has recently been unable to
confirm it, and it did not always make its appearance in BRUCKE'S experiments
(compare p. 506).

Should further research actually establish the fact that the alterations in
the resistance to flexion of the articulation either do not occur regularly
or have not the significance which we have ascribed to them, then the separa-
tion of genuine nyctitropic movement from so-called day-sleep movement
cannot be carried out. In the case of Oxalis, PFEFFER (1876) found that the
day-sleep position in direct sunlight went hand in hand with a decrease in the
resistance to flexion of the articulation ; this decrease must have arisen from
a relaxation in both halves of the articulation, and especially in a greater relaxa-
tion in the concave half. Whether this relaxation is a phenomenon of general
occurrence in day-sleep and whether it has essential significance in this relation
must be left to further research to determine. At the same time, in all such
experiments efforts must be made to discriminate sleep from the heliotropic
profile position more accurately than is as yet possible ; at the same time it
cannot be denied that many profile positions are conditioned by a combination
of heliotropism and nyctitropism.

While we regard such a combination of heliotropic and nyctitropic move-

508 TRANSFORMATION OF ENERGY

ments as possible, we must, at the same time, consider the possible effect of geo-
tropism on sleep movements. In fact, when we remember that a leaf when
executing sleep movements comes into entirely different relations with gravity,
the question arises whether gravity does not operate so as to bring it back again
into the old position. This effect of gravity, however, has not as yet been demon-
strated, indeed the effect is quite otherwise. In 1875, PFEFFER found that when
a bean was placed in an inverted position, the sleep movements took place in
the reverse direction so far as the plant was concerned ; that is to say, they
retained their relation to the direction of gravity. FISCHER (1890) snowed
that nyctitropic movements in the bean ceased when the plant was rotated
on a klinostat. From these observations we may conclude that we have to deal
in this case not with nyctitropic, but with geotropic movements, and that the
plant in consequence of being darkened, reacts geotropically otherwise than
before. Similar alterations in the geotropic rest position we found took place when
rhizomes and roots were illuminated ; but plants do not all behave in the same
manner. Whilst Lupinus albus agrees in all essential features with Phaseolus,
FISCHER found that in the case of Amicia, Desmodium, Acacia, Mimosa, &c.,
even after twelve days' rotation on the klinostat, nyctitropic movements retained
completely their original direction. From this we must conclude that there are
two types of plants which, following FISCHER, may be distinguished us auto-
nyctitropic and geonyctitropic. We confine ourselves merely to the statement
of this fact, but we may at least note that FISCHER'S experiments are not so
easy of explanation as one might at first sight think. NOLL (1892) has drawn
attention to the possibility that dorsiventral organs are geotropically stimulated
when rotated on a klinostat, and SCHWENDENER (1892) has shown that Phaseolus,
at least in the first days during which the plant was rotated, preserved its sensi-
tivity to changes in illumination. Further research in this subject will be
necessary before we can give a clear account of the phenomena.

A very large number of, though by no means all, plant organs which carry
out nyctitropic movements present difficulties in the explanation of the mechanics
of the process of curvature for one special reason ; the nyctitropic movements
continue after the cause, namely alteration in illumination, ceases (periodic move-
ments). This phenomenon may be remarkably well seen in Mimosa or Acacia
lophantha, when these plants are kept in the dark at a constant temperature.
For days they open their leaflets in the morning and close them in the
evening at approximately the same time as those plants which are exposed to
periodic alterations in illumination. These movements, which are of the nature
of after-effects, gradually cease, since the leaf becomes pathological when kept
in continued darkness, and changes are set up in it which lead, in the first place,
to abolition of the power of movement, that is to say, to darkness-rigor (SACHS,
1863), and in the long run even to death. The absence of light induces this con-
dition of rigor only indirectly, for we can render the leaves of Mimosa incapable
of movement when completely illuminated also, if we at the same time deprive
them of access to carbon-dioxide (VocHTiNG, 1891). On the other hand, leaves
which have been cultivated in the dark live and exhibit movements for a much
longer time in absence of light than those which have grown to maturity in light.
Obviously the rigor is due to injuries suffered by the leaf in consequence of an
interference with the chlorophyll function.

As we have mentioned above, these after-effects are also met with in leaves
which carry out nyctitropic curvatures by growth, such as the leaves oiNicotiana ;
they are also met with in many flowers. On the other hand, they occur in leaves
which, after being in the dark for a short time, cease to move, although
they still retain their capacity for movement, as, for example, Tulipa, Robinia
pseudacacia, or which carry out movements in the dark which are quite irregular
and which bear no relationship to variations in light. These latter movements,
the so-called autonomous movements, we will treat of in Lecture XLI.

NYCTITROPISM 509

The periodicity in the after-effects we have referred to are of extreme
interest from several points of view ; at the very commencement, however, it
must be evident that the studies we have hitherto made on the subject of simple
nyctitrppic movement are incomplete in one important respect. We are unable
in the individual case to distinguish what is the direct consequence of a solitary
stimulus and what is an after-effect. Since the sleep position is brought about
more rapidly by placing the plant in the dark in the evening than in the morning
we may look on that as an instance of after-effect, and if the sleep move-
ment is followed after a short time by an opening movement when the plant is
placed in the dark in the morning, we must not regard this reverse movement in
any sense as due to autotropism, but as an after-effect also. Just because after-
effects frequently manifest themselves in nature we ventured to suggest above that
nyctitropic movements should be studied in such plants as have been cultivated
under constant external conditions ; but this suggestion has not been as yet
realized. On the other hand, many authors, and more especially PFEFFER, have
shown that in certain plants which exhibit no autonomous movements, after-
effects gradually cease when the plants are exposed to continuous light. Thus
PFEFFER (1875) noticed that the periodic movements in Acacia lophantha and in
Impatiens, when exposed to continuous light, became gradually weaker, until
finally a permanent day position was reached. This result was to be expected,
for SCHUBLER (1873) had already pointed out that in the case of plants growing in
Northern Norway the periodic movements they exhibited in midsummer disap-
peared for a considerable time and reappeared again after the commencement
of the arctic night. The cessation of periodic movements as a result of exposure
to continuous darkness, presents great difficulties, as we have already pointed out
(p. 508). We do not as yet know whether leaves which become normal in
complete darkness (Josx, 1895) assume a fixed night position when the external
conditions are quite constant ; the great majority of leaves, however, assume
an abnormal rest position in darkness, because either their upper sides or their
under sides exhibit increased growth (epinasty, hyponasty) ( VINES, 1889).

The cessation of the periodic movements is especially of interest because it
shows that these movements are not hereditarily characteristic of the plant, as
one might at first imagine from their continuance in darkness. Other experi-
ments lead to similar results, for we may by illuminating plants during the night
and darkening them during the day shift the periodicity of movement as much
as twelve hours. Periodic movements are, moreover, not developed once and for
all ; on the contrary, they apparently arise gradually in the course of the develop-
ment of the plant. In order to study the question of the mode of origin of these
movements we must at present confine ourselves exclusively to plants which
have, when continuously illuminated, lost the power of performing periodic move-
ments, since as yet no leaves have been cultivated under conditions which pro-
hibited the original development of such movements. After the loss of periodic
movement these leaves are in no sense in a state of rigor. In one experiment of
PFEFFER' s (1875), a plant of Acacia lophantha closed its leaflets as soon as it was
darkened, but after a few hours began to open them once more, and in less than
12 hours the leaflets were again almost completely extended. When kept in the
dark continuously, they exhibited on the two following days, two oscillations,
whose turning points were 18 to 24 hours apart. PFEFFER concluded from these
and similar experiments that a solitary nyctitropic stimulus induced not one but
a whole series of oscillations. If that be so, then the periodic movement must be
brought about in such a way that the solitary stimulus is combined with an after-
effect. This, however, is only possible if these secondary oscillations occur in the
same rhythm as that in which the stimulus affects the plant in nature, that is to
say, the periods between opening and closing must be about twelve hours in
length. In the experiment with Acacia just described the oscillation was too

5io TRANSFORMATION OF ENERGY

rapid on the first day and too slow on the second. If, however, after-effect and
renewed stimulus do not coincide, an after-effect, as constant as we have seen
exhibited in the dark by Mimosa, cannot arise. There are two possibilities by
way of explanation of this contradiction ; either regular and long continued
periodic movement in the dark is not an after-effect at all but is, at least in part,
a result of small oscillations in temperature which were not sufficiently carefully
guarded against in many of the experiments hitherto performed, or the oscilla-
tions observed subsequent to a single darkening in PFEFFER'S experiments, in
plants which had ceased to move in light, were not the result of this solitary
stimulus but were the after-effects of a previous daily periodicity, which they
had not yet quite lost, but which were only obscured in light and which had
lost their regularity. It would be very remarkable, however, if a single darkening
induced a maximum amplitude of movement, while in the case of plants which
had lost their power of movement, e. g. Siegesbeckia, a new stimulus took five
days to induce the maximum depression of the leaf.

Seeing that objections of this kind may be advanced against PFEFFER'S
argument, it is necessary that experiments should be carried out on plants which
have never shown periodic movements. They will, we have no doubt, lead to the
result which PFEFFER has already indicated, namely, that periodic movements
arise from a summation of individual stimuli and after-effect, the single stimulus
inducing several oscillations which must be carried out with ever-decreasing am-
plitude. These oscillations following upon a single stimulus offer a subject for
detailed investigation. First of all they must be definitely proved (for SCHWEN-
DENER (1897-8) has called these facts in question, and Josx (1898) and PANTA-
NELLI (1900) have failed to find them in Robinia), and it must also be shown
how far the oscillation period depends upon the duration of the stimulus. It is
quite possible that we might succeed in inducing periodic movements of six hours'
duration or even less, but it is not outside the range of probability that the dura-
tion of the oscillation renders periods so short as these in the after-effects impos-
sible. [As a matter of fact, SEMON (1905) has succeeded in inducing a 12-hour
periodicity in the after-effect where, before illumination, the rhythm was
6-hourly or 24-hourly. All this goes to show how little indeed we do know
even now about periodic movements.]

There are still many other complicated phenomena which militate against
a thorough explanation of periodic movement ; e. g., let us look at the case of
flowers and at the tulip in particular. First of all when the flower is heatecj
a single opening and closing movement only can be recognized ; no periodic
movements at all can be observed in the tulip. If these be possible we should
perhaps obtain them most speedily by alternately heating and cooling the plant
every hour ; but perhaps they are absent altogether. In other flowers, how-
ever, such movements exhibit themselves markedly. OLTMANNS (1895) draws
attention to many points which have as yet been but little cleared up. He finds
that Bellis perennis opens its inflorescence after being 48 hours in the dark, whilst
in the case of Tragopogon 8 to 12 hours is sufficient. He certainly regarded
these opening movements not as after-effects of a previous periodicity, but looked
upon darkening itself as the stimulus which induced opening, acting especially
vigorously on night-flowering plants. A discussion of this and other results
arrived at by OLTMANNS would lead us, however, too far ; we may merely note
that all these movements have not as yet been studied in a sufficiently experi-,
mental way, and that before we can arrive at an explanation of periodicity we
must determine what part duration and intensity of light and of heat in the single
stimulus play in the process. That the origin of periodic movements from a sum-
mation of the individual stimuli of light and darkness together v&th their after-
effects is not quite so simple a matter as we might suppose, may be deduced from
the behaviour of the chief petiole of Mimosa. After a single darkening it raises

NYCTITROPISM 511

itself up, as SCHWENDENER (1897) has already shown, but in the periodic move-
ments it droops in the evening. This contradiction PFEFFER naturally did not
fail to notice, and he has also attempted to explain it. He assumed that the
drooping arose from a marked increase in the statical moment of the leaf which
was necessarily associated with the forward movement of the secondary petioles
described above, and he found that his supposition was confirmed by experiment.
When movement in the secondary petioles was rendered mechanically impossible
the evening drooping gradually ceased, but came back again when the secondary
petioles were allowed freedom of movement. In spite of this ingenious experi-
ment we have certain criticisms to advance against the correctness of the explana-
tion. In nature the erection of the petiole takes place during the night and it
may amount to from 40° to 80° in 6 to 8 hours. SCHILLING (1895) has, however,
noticed that the compression of the articulation and sinking of the petiole (the
statical moment of which had been greatly increased by hanging a weight upon
it) was compensated in 10 to 15 minutes-, and it is not obvious why the plant
should require several hours to compensate evening drooping, if this drooping
be due to purely mechanical causes. SCHWENDENER (1897) has advanced
additional arguments against PFEFFER' s explanation, so that we may regard
the sinking and rising of the petiole of Mimosa as a phenomenon of periodic
movement not yet explained, and possibly as a phenomenon which has some
special biological function to fulfil.

We cannot enter into a discussion of the mechanics of periodic movement ;
we may only notice that, according to PFEFFER, in nutation movements as in
variation movements the opposite sides behave in contrary ways during the
after-effect ; this is especially well seen in the articulation because the resistance
to flexion of the entire articulation remains unaltered owing to the expansion of
the convex side and the simultaneous contraction of the concave side.

In conclusion we may ask ourselves what is the biological significance of
nyctitropic movements ; for that they have nothing to do with the ' sleep '
of animals goes without saying. We must keep distinct in our minds the move-
ments of foliage-leaves and those of floral-leaves. The position which foliage-
leaves take up in the evening is essentially vertical ; it is a matter of less moment
manifestly, whether they bend upwards or bend downwards. The question is
what is the advantage of this nocturnal vertical position to them ? DARWIN (1881)
has drawn attention to the fact that the leaf in the vertical position is less subject
to heat radiation during the night than if it were in the horizontal position. But
this radiation as such could injure the leaf only during cold nights, and yet at
low temperatures nyctitropic movements cease ; on the other hand, they occur
during warm summer nights and are especially evident in tropical plants where
injury due to cooling is out of the question. Hence STAHL (1897) has sought to
explain the night position by considering it as a means of preventing the deposit
of dew, and he has supported this idea, as far as it was possible to do so, by means
of experiments. He looks upon the deposition of dew as injurious to the plant
inasmuch as it prevents transpiration so long as the leaves are wet.

As to the biological significance of nyctitropic movement in flowers little
trustworthy is as yet known. It is scarcely possible that we have to do here
with any transpiration,phenomena, as in the case of foliage-leaves ; in all pro-
bability quite different explanations must be given of the movement of floral-
leaves in different groups of plants. We may imagine that in the case of spring
flowers which are especially sensitive to heat stimulus, the closing of the flowers
is a protection against cold. Flowers which open in the evening are obviously
adapted to the visits of those insects which effect pollination and which are
nocturnal in habit ; insects which are active during the day would thus be
entirely excluded from such flowers. Other types again which show opening
and closing movements may thereby attain benefits with which we are as yet
entirely unacquainted.

512 TRANSFORMATION OF ENERGY

Bibliography to Lecture XXXIX.

BATALIN. 1873. Flora, 56, 450.

BRUCKE. 1848. Archiv f. Anat. u. Physiol. Ostwald's Klassiker, No. 95.

BURGERSTEIN. 1902. Bewegungscrsch. d. Perigonblatter von Tulipa u. Crocus

(Jahresbericht Erzherzog Rainer Gymnasium. Vienna).
DARWIN. 1881. Das Bewegungsvermogen d. Pflanzen (CARUS.) Stuttgart.
FISCHER, ALFR. 1890. Bot. Ztg. 48, 673.
FITTING. 1903. Jahrb. f. wiss. Bot. 38, 545 [and 39, 424].

HANSGIRG. 1893. Physiologische und phycophytologische Untersuchungen. Prag.
[HENSEL. 1005. Nebraska University Studies. V. No. 3.]
HILBURG. 1 88 1. Unters. bot. Inst. Tubingen, i, 23.
JOST. 1895. Jahrb. f. wiss. Bot. 27, 403.
JOST. 1898. Ibid. 31, 345.
NOLL. 1892. Heterogene Induktion. Leipzig.
OLTMANNS. 1895. Bot. Ztg. 53, 31.

PANTANELLI. 1900. Atti soc. dei Naturalisti. IV, 2. Modena.
PFEFFER. 1873. Physiologische Untersuchungen. Leipzig.
PFEFFER. 1875. Periodische Bewegungen. Leipzig.
SACHS. 1863. Flora, 46, 449.

SCHILLING. 1895. Einfl. von Bewegungshemmungen auf Arbeitsleistung. Jena.
SCHWENDENER. 1897. Sitzungsber. Berliner Akad. 228.

SCHWENDENER. 1898. Ibid. 176.

SCHWENDENER and KRABBE. 1892. Abh. Berl. Akad.

SCHUBLER. 1873. Die Pflanzenwelt Norwegens. Christiania.

[SEMON. 1905. Biol. Centrbl. 25, 241.]

STAHL. 1897. Bot. Ztg. 55, 71.

VINES. 1889. Annals of Bot. 3.

VOCHTING. 1888. Bot. Ztg. 46, 501.

VOCHTING. 1891. Ibid. 49, 113.

[WiEDERSHEiM. 1904. Jahrb. f. wiss. Bot 40, 230.]

LECTURE XL
MOVEMENTS RESULTING FROM SHOCK

IN the last lecture, in which we treated of nyctitropic movements, we had
frequent occasion to mention Mimosa, but we must not lose sight of the fact that
this plant is sensitive to other stimuli as well as to alterations of light and tem-
perature. In fact Mimosa exhibits another sensitivity much more prominently
than it does nyctitropism ; a gentle shaking of the leaf is sufficient to induce at
once a very characteristic change in position. This new rest position corre-
sponds exactly with the sleep position, for the primary petiole droops consider-
ably and the secondary petioles move forwards almost parallel with the long axis
of the primary petiole, the leaflets folding together in pairs and, at the same
time, bending obliquely forwards. In spite of their great superficial similarity
the two movements are in reality quite distinct and arise in quite different ways.
The reaction induced by shock differs further from the sleep movement in the
rapidity with which it is carried out ; a few seconds are sufficient for its execution.
On account of the rapidity with which the response takes place, the movement of
the Mimosa leaf is one of those with which we have been longest acquainted
in plant life, a movement which, owing to its rapidity alone, has been compared
with the responses to stimuli exhibited by animals, and which was always looked
upon as a genuine reflex action even at a time when the absence of sensitivity
in plants was regarded as one of the distinctions between plants and animals.
These movements in the ' Sensitive plant ', in addition to geotropic movements,

MOVEMENTS RESULTING FROM SHOCK

have figured prominently in the history of that department of physiology which
deals with sensitivity.

Apart from general habit, we may compare the stimulus-movements just
discussed in Mimosa, but which are manifested by other plants as well, with
nyctitropic movement, inasmuch as we have to deal in this case with the assump-
tion of a new position of equilibrium as a result of stimulus, which, however, is
temporary only ; for after a short interval we find that the stimulated leaf regains
its normal day position, and that too without being again stimulated, just as
the darkened leaf re-erects itself of its own accord in daylight. Further, just
as the day position may be again assumed in continued darkness, so also a leaf
of Mimosa may once more attain the normal day position after continuous
small shocks rapidly applied. There are other and fundamental differences,
however, between response to shock and to changes in illumination. As
already mentioned, there is a difference both in the rapidity as well as in the
mechanics of the movement in the two cases ; to both of these points we shall

K-

F'g- 159- Mimosa fiudica. On the left an unstimulated twig in the day position ; on the right the same after
having been shaken. Reduced. (From the Bonn Textbook.)

recur presently. A further point of distinction lies in the fact that the shock
stimulus under natural conditions is not periodic and that no after-affects have
as yet been determined. In all probability, even if the shock stimulus be
applied for a long time at regular intervals, no oscillations would manifest
themselves after the cessation of the stimulation. The movements at present
under consideration differ from those of nyctitropism in yet other respects, viz.
in their significance in the plant economy, and, in connexion with that, in the
nature of the releasing stimulus, and finally in their occurrence. The releasing
stimulus may be not merely a shock, but also the mechanical and chemical
influence of different bodies and the biological significance of movements due to
shock is apparently quite different in different plants. In foliage-leaves, such
as those of Mimosa, the movement is apparently intended to frighten away any
animal that is calculated to injure the plant. There are special difficulties in
proving the exact truth of this theory in this special instance, because it is only
when it immediately precedes the visit of the animal that the movement can
make any impression on the visitor ; larger animals will be puzzled by the altered

L 1

JOST

514 TRANSFORMATION OF ENERGY

appearance of the plants and smaller ones will be frightened by the movement
itself. In the great majority of cases, however, where leaves react in this way
to a shock stimulus, the response, even after a vigorous excitation, appears very
slowly. The object of the movements in stamens and stigmas is much more
apparent, for in these cases they are obviously almost always related to the
mode of pollination of the flower.

Let us now study the movement in Mimosa a little more closely. As already
stated curvatures take place in all three sets of articulations. In order to study
the changes which precede these movements we will naturally confine ourselves
to the largest pulvinus at the base of the primary petiole. Curvature in this pul-
vinus occurs not only when the whole plant is shaken but also when the articula-
tion itself is subjected to a slight shock. It may be easily shown that it is only
the under half of the pulvinus that is sensitive, for even a gentle friction of that
side with a piece of stick at once induces a response, while the upper side may
be much more vigorously stimulated without any result ensuing. Since at last
a response is obtained by vigorous stimulation of the upper side, the reason for
this lies in the fact that the stimulus has been transmitted to the lower half
of the cushion. Experimental evidence on this point was first brought forward
by LINDSAY in 1790, who showed that when the upper half of the pulvinus
was removed the leaf remained capable of carrying out these movements, but
that rigor set in when the lower half was cut off. (For the older literature
see PFEFFER, 1873 a).

The more exact conditions of curvature are to be obtained by investigating
the resistance to flexion, and by determining the alterations in volume taking place
in the two halves of the pulvinus. The resistance to flexion increases in the night
position but decreases markedly after a shock stimulus. BRUCKE (1848) found
the difference in the angles (a-a) in the two positions (compare p. 506) after
stimulation to be two or three times as great as in the unstimulated condition,
and so was able to establish a fundamental difference between the similar ' sleep '
and ' stimulated ' positions of Mimosa. It is to PFEFFER (1873 a) that we
owe accurate investigations as to the changes in volume of the articulation. He
proved by measurements, taken with the aid of the microscope, that the upper
half of the articulation showed a quite insignificant increase during the curva-
ture, but that the under half markedly decreased. Since the movement was
carried out just as well when the epidermis had been carefully removed, and
since the axial vascular bundle need not, on account of its rigidity, be taken into
account, the reduction in volume observed can be accounted for only by a con-
traction in the parenchyma on the under side. This contraction is accompanied
by a relaxation in the two halves of the pulvinus, which may be directly
estimated if the effort to droop on the part of the leaf be prevented by some
opposing force. In a more recent research PFEFFER has estimated this pressure
at between 2 J and 5 atmospheres. The expansive powers of the under half of
the pulvinus is reduced to this extent, while apparently no change of any kind
occurs in the upper half.

Whence this decrease of expansive force on the under side of the pulvinus
arises cannot be settled straight away. In order to answer this question clearly
experiments on amputated leaves are essential. Such leaves exhibit rigor at once
on amputation, but, if they be kept in a saturated atmosphere, they regain their
capacity of response to stimulus after a time, the primary articulation exhibiting
inward curvature after contact, which may have a radius of 5 mm., while
normal articulations exhibit a curve with a radius of only 3-4 mm. After
stimulation of the amputated leaf it is possible to see a certain extravasation
of liquid from the cut surface, which, if evaporation be prevented, is again
absorbed as the curvature becomes gradually relaxed. This liquid is excreted
from the parenchyma of the sensitive half of the pulvinus, and it is only after
vigorous stimulation that the upper half helps in producing the excretion, and

MOVEMENTS RESULTING FROM SHOCK 515

that tardily and feebly. There can be no doubt that it is the sensitive cells of
the under side that extrude this liquid, which then finds its way into the inter-
cellular spaces and from them to the cut surface.

As the air in the intercellular spaces becomes compressed by the excreted
liquid, another phenomenon makes its appearance which was first observed by
LINDSAY and, later, correctly interpreted by BRUCKE. At the moment of
stimulation the under side of the pulvinus assumes a darker colour, such as
follows upon injection of water under the air-pump. This dark coloration takes
place even although the articulation be mechanically prevented from curving
(PFEFFER, 1873 a), and it follows that the change in colour is not caused by the
contraction of the pulvinus and consequent approximation of the chloroplasts.
The sudden reduction in the expansive power of the lower half of the pulvinus
is obviously associated with an alteration in its turgidity, and hence arise two
possible explanations of the phenomenon (compare PFEFFER, 1890, 326) ; either
the elasticity of the membrane increases, or the pressure in the interior dimin-
ishes ; either phenomenon would result in the excretion of water from the cells.
Since, if there be no alteration in the pressure of the cell contents, the size of the
cell depends on the elasticity of the cell-wall, obviously a contraction of the cell
might be produced by a decrease in the elasticity of the membrane. This possi-
bility is not precluded in the case of Mimosa, but since it is certainly not applic-
able to the very similar movements exhibited by the stamens of the Cynareae
and, at the same time, is not very probable in itself, we need not consider it
further. When the pressure in the cell decreases we naturally assume this to
be due to a decreasing osmotic pressure, a decrease which may well amount to
2j to 5 atmospheres, and may be due either to the transformation of osmotic-
ally active substances into bodies with larger molecules, or to alterations in
the permeability of the plasma, and an excretion of materials from the cell. As
evidence of the excretion of material we may quote the fact that PFEFFER
(1873 a) observed crystals of unknown nature appearing on evaporation of the
liquid expressed from the intercellular spaces. Still there are several reasons
for doubting this conclusion. It is a remarkable fact that plasmolytic research
(HiLBURG, 1881) affords no evidence of any decrease in osmotic pressure (com-
pare p. 506). Further it is not very probable that the original turgor relation-
ships would be at once reinstated by cutting the articulation.

As PFEFFER has suggested, it is possible that a decrease of the pressure as
affecting the membrane may also be induced by alterations in the protoplasm,
more especially alterations in its capacity for swelling.

No complete insight into the mechanism of the stimulus movement in
Mimosa has as yet been obtained, although one thing is certain, viz., that there is
a decrease in expansive power on the under side of the articulation. Expansion on
the upper side arises only from the removal of the opposing pressure below :
at the same time the weight of the leaf helps to squeeze the under side.
Again, when the plant is placed horizontally or inverted, that is, when the
weight of the leaf is rendered non-effective, contraction of the sensitive half of
the pulvinus in response to stimulus still takes place, showing that the weight of
the leaf is not necessary for performance of the movement.

The articulations at the bases of the secondary petioles and of the leaflets
behave, so far as we know, in a precisely similar way to that just described for
the pulvinus of the chief petiole.

The depression of the chief petiole takes place, as has been already pointed
out, with considerable rapidity. BERT (1870) found that, in a plant laid hori-
zontally, the movement was completed in 4-7 seconds, and that it was more
rapid still when the weight of the leaf was allowed to operate at the same time.
As soon as the petiole had reached the position of maximum depression it began
again to raise itself, and in the course of 10-15 minutes the original position was
again attained, so that the leaf became once more fully capable of responding

L 1 2

516 TRANSFORMATION OF ENERGY

to stimuli, repeated at J to ^-hourly intervals. No investigations appear to have
been made as to whether or not the extent of the movement remains unaltered
according to the length of time the stimulation is continued. Before the normal
position of the leaf is reached, however, about 5 minutes after the ftiet stimu-
lation, the leaf may again react to a new stimulus, but then the amplitude of the
movement is less. It may be supposed that in this case the capacity for reaction
is still limited, because the original expansive force in the under half of the
pulvinus has not yet been recovered ; on the other hand, the sensitivity may
still be normal. Conversely the sensitivity is obviously inhibited by con-
tinuous, feeble vibrations, while the capacity for response might be restored.
We may conclude from this that the leaf returns to its normal position
during the shaking and once more regains its resistance to flexion ; although
the leaf was not sensitive to shaking continued for 2-3 hours, it became once
more fully sensitive in 5-15 minutes after the cessation of the stimuli (PFEFFER,
J873 a). From this point of view there is a great difference between the present
case and hapto- and nycti-tropism, where the organs do not become accustomed
to stimulation or do so very slowly.

There are many allied problems in this subject which yet await solution.
We know only that the sensitivity is in many respects dependent on external
conditions. High temperature, bright light and excessive moisture render the
plants extraordinarily sensitive, and under such conditions feeble stimuli operate
as well as strong ones, the responsive movement reaching its maximum ; but
when the sensitivity of Mimosa is decreased, owing to low temperature,
drought or the application of chloroform, ether, &c., feeble shocks induce a much
more limited depression of the leaf than strong shocks. Although we have no
detailed information on the dependence of the extent of the reaction on the
amount of the stimulus, we are better acquainted with the nature of the stimuli
to which Mimosa especially will respond. We have already found that feeble
contact with the sensitive part of the articulation induces a response ; it has yet
to be determined, however, whether the immediate conditions correspond to
those established for tendrils.

The fact that the reaction is induced just as well by a liquid, e.g., rain-
drops, as by solid bodies, shows in the plainest possible way the great difference
in character that exists between sensitivity as exhibited by Mimosa and by
tendrils. Both react to a shock only, for statical pressure is followed by no
response in Mimosa. PFEFFER, for example, obtained no movement when he
gradually increased the weight bearing on the sensitive part of the pulvinus up
to 30 g. Mimosa reacts, however, to every shock, if it be sufficiently intense,
and a solitary stimulus is generally enough to bring about a maximum reaction.
In the case of tendrils, however, as we have seen, such shocks are operative only
when they affect neighbouring parts with unequal intensity ; in their case also,
several simultaneous or consecutive shocks are always necessary whose indi-
vidual effects are capable of summation. In Mimosa, on the other hand,
every sudden deformation of the cells of the sensitive half of the pulvinus is per-
ceived and is responded to by a movement. Special hairs, which are, however,
not confined in their distribution to the sensitive pulvinus only, serve as
additional organs of perception ; when these hairs, formed of thick- walled cells,
become bent as a result of pressure or a blow, the deformation of these cells
must, owing to their varied individual connexion with the sensitive paren-
chyma, be greater than that due to an equally strong pressure applied to the
outer surface of the pulvinus (HABERLANDT, 1901).

The leaves of Mimosa respond not only to a blow but also to injuries, and
movements take place after an incision or after the action of a burning-glass,
though dissimilar in intensity to those which are consequent on a blow. The
stimulus effected by a wound is much more rapidly transmitted than that induced
by a blow, but the execution of the movement from a mechanical point of view

MOVEMENTS RESULTING FROM SHOCK 517

appears to be identical in both cases. Mimosa is also sensitive to chemical
stimuli (CoRRENS, 1892). Many substances which induce movement, such as
hydrochloric acid vapour, may certainly injure the plant so much as to cause
death ; but ammonia may be made to induce movements without at the same
time causing injury, provided the doses be carefully regulated, and repeated
responses may be obtained by successive applications of the stimulant. Electric
currents will also induce movements (BERT, 1870), and it is possible that the
reactions which are consequent on high temperature and intense illumination
(P- 505) are to be regarded rather as related to those resulting from a blow than
to those which are induced by change in light intensity. Detailed investigations
on this point have still to be undertaken, however.

The sensitivity above described is not confined to Mimosa ; other Legumi-
nosae, such as Neptunia oleracea and Desmanthus plenus, and some Oxalidaceae,
such as Biophytum sensitivum, are known to be very sensitive. To a more
limited extent all Leguminosae and Oxalidaceae, possibly all plants with pulvi-
nate leaves, are to be considered as sensitive to shock (HANSGIRG, 1893), only in
these cases the movements are induced by more powerful stimuli and take place
only under optimal external conditions. One blow is frequently not sufficient
to bring about a visible response, although several blows, by summation of
stimuli, gradually induce a movement (Robinia, Oxalis sp.). The sensitivity
of these plants recalls in its character that of tendrils ; sensitivity to contact is
closely related to sensitivity to shock, for between Mimosa and tendrils, as the
two extremes of the series, there are many intermediate conditions. Into the
discussion of these, however, we need not enter.

We have yet to consider the transmission of the stimulus, which has been
most thoroughly studied in Mimosa and Biophytum. Let us look at the phe-
nomenon as it manifests itself in Mimosa. When this plant is grown under
suitable external conditions, not only does the leaf droop when friction is applied
to the primary articulation, but after a brief interval the pinnae also assume the
folded rest position. If, on the other hand, one of the outermost leaflets be stimu-
lated, not only does that leaflet move but the movement spreads to the opposite
leaflet and onwards to the pinnae inserted lower down the petiole, all of which
fold together in pairs. It has already been noted that a wound operates much
more effectively than mere contact If the terminal pinnae be scorched with
a glowing splinter or with a burning-glass, the stimulus is rapidly conveyed to
the base of the secondary petiole and on to the three other secondary petioles,
the pinnae of which also fold together from the base outwards. The secondary

Eulvini also take up the sleep position and shortly afterwards the primary articu-
ition follows suit. But the process is not yet complete. After a short time
the primary petiole of the leaf next above or next below suddenly droops and
the stimulus soon affects the rest of the pulvini also. The stimulus may also
be transmitted from the stem itself. If a deep cut be made in the stem, care
being taken that it be not at the same time shaken, after a short interval move-
ments appear in the neighbouring leaves. The stimulus may, when the con-
ditions are favourable, be transmitted to a distance of half a metre, and the
transmission is effected with a rapidity which, though indeed feeble when com-
pared with the conduction of impulses in the nerves of animals, is very
considerable as compared with other cases of like nature in plants. Various
values have been recorded by different authors for the rapidity of transmission
of the stimulus, and this is not to be wondered at, seeing that the plant is not
always in the same physiological condition and that the stimulus may also vary ;
still, allowing for the personal equation, there are many differences which are as
yet inexplicable. At least it is certain that the stimulus is transmitted at the
rate of several millimetres (2-15) per second ; by way of comparison we may
remember that the speed of transmission in the nerves of the higher animals is
about a thousand times as great, while the most rapid heliotropic response

5i8 TRANSFORMATION OF ENERGY

observed as yet, viz. 0-3 mm. per second (Brodiaea congesta ; ROTHERT, 1894,
137), is 10 to 50 times slower than the rate of transmission in Mimosa.
Apparently the transmission of the stimulus in tendrils comes next to that
of Mimosa, so far as rapidity is concerned.

One must not, however, compare the transmission of stimuli j in the
animal nerve with transmission in Mimosa, seeing that in the former con-
duction is effected by living protoplasm which is not the case in the latter. As
a matter of fact, the stimulus in Mimosa may travel by way of tissues which have
been killed by narcotics (PFEFFER, 1873 b) or by heating (HABERLANDT, 1890),
and hence the conception of a transmission by living cells, and especially by
intercellular protoplasmic strands, is excluded from consideration. DUTROCHET
(1837) assumed that the stimulus travelled by way of the vascular bundle, and
PFEFFER has endeavoured to confirm this view. The chief argument used
was derived from an experiment of DUTROCHET' s in which vigorous stimuli
were propagated through a portion of a stem which had been deprived of its
cortex, and in which the pathway available for the transmission of the stimulus
was exclusively the vascular elements. PFEFFER endeavoured to show that the
movement of water in the vascular tissue might serve as the medium of trans-
ference of the stimulus. Some of the water extruded from the pulvinus on
stimulation might, he thought, enter the vascular bundle, and this sudden move-
ment of water might be propagated along the vessels and so release a stimulus in
other pulvini. It is immaterial how this movement of water arises provided only
it be rapid, for movements of water connected with transpiration are well
known to produce no effect at all. If an incision be made in the stem, the stimu-
lus, as we have seen, is transmitted to neighbouring pulvini, but only when the
vascular bundles are touched and when the extravasation of liquid from them
demonstrates directly the movement of water taking place in them.

HABERLANDT'S (1890) elaborate researches have completely upset this
theory. This author showed that the sap referred to in PFEFFER' s experiment
did not by any means come out of the vessels, but from tubular cells in the
phloem, corresponding to the tannin canals of other Leguminosae, but differing
from them in the possession of numerous fine pores in their transverse walls.
These pores allow of easy transference from place to place of the whole of the
cell-contents, and every injury inflicted on such a cell permits an abundant
extravasation of sap, just as in the case of sieve- tubes, and which may be recog-
nized by its characteristic constituents. Movement also takes place when the
stem is cut only when an alteration in pressure follows in these tubular cells, and
such a change in pressure may be transmitted just as well in the dead as in the
living tube. HABERLANDT has further shown that in DUTROCHET' s decortica-
tion experiment the whole of the tissue, right into the wood, was not removed,
but that the entire phloem along with the conducting hyphae was retained.
These tubular cells HABERLANDT regards as the specific transmitters of stimuli
in Mimosa. The one criticism, however, which militates against HABERLANDT'S
theory, is derived only from the result of the experiment where the cortex has
been completely removed. This experiment showed that after the removal of
the whole of the conducting tissue the stimulus could be transmitted by the xylem
only. Although HABERLANDT has very ingeniously fitted this fact into the tail
end of his theory by a subsidiary hypothesis, we are inclined to regard it some-
what in the light of the heel of Achilles ! Further experiments specially directed
to this point must show how transmission of the stimulus is effected when the
xylem is interrupted but when the conductive tissue remains intact.

If this experiment should show that the xylem is indispensable for the
transmission of the stimulus then we must fall back on the DUTROCHET-
PFEFFER theory. One further point must, according to our idea, be cleared up
in this theory, viz., how the liquid which exudes from the cells of the sensitive
half of the articulation succeeds in entering the vessels. PFEFFER himself was

MOVEMENTS RESULTING FROM SHOCK

519

unable to observe any pressure worthy of the name in the intercellular spaces,
when these were injected during stimulation. BONNIER (1892) even established
a slight decrease in pressure after the stimulation by inserting a manometer
into the pulvinus, arranged so as to show pressure of the air in the inter-
cellular spaces. It is not easy to understand how, under such circumstances,
the extruded liquid can enter the vessels under pressure. HABERLANDT'S
views, on the other hand, are quite intelligible. On direct stimulation of
an articulation, flaccidity ensues in the sensitive parenchyma and, owing to
the deformation of the cells, a pressure will be induced on the conductive tissues
which is propagated along them and which, wherever it reaches a new pulvinus,
is capable of stimulating it just as if a blow had been inflicted on it from without.
In that case we should have to consider this really as a genuine instance of
transmission of a stimulus and not of an excitation as in other cases.

Other parts of the plant as well as the pulvini may be affected by a
primary stimulus ; for example, a pinna may be stimulated without touching
its articulation (BERT, 1867), and yet it, too, is
able to appreciate the stimulus. In this case, also,
the cells conducting the stimulus must primarily
become deformed, and an increase of pressure
effected which is transmitted. Stimulus conduct-
ing cells are, as a matter of fact, also found in
the pinnae, following the course of the larger
vascular bundles. By employing the method of
stimulation by wounding, BERT (1867, 17) showed
that the leaf parenchyma was insensitive and that
response took place only when the stimulus
affected the veins. In the case of all stimuli,
whether of the nature of an incision, scorch-
ing, or corrosion by acid, there was always a
decrease in pressure in the conductive tissues,
never an increase, but which was propagated
just like increased pressure, and which led to
mechanical stimulation in the pulvini. It would
appear that increase and decrease of pressure
might take place more readily in HABERLANDT'S
thin-walled but turgescent tubular cells than
in the rigid- walled vessels. [FITTING has (1903)
carried out some new experiments on the trans-
mission of stimuli in Mimosat but this author
has not been successful in solving the question
in dispute.]

As far as regards sensitivity and the
mechanics of movement in response to stimulus,

a perfect comparison may be instituted between those phenomena as exhibited
by Mimosa and those manifested by certain stamens, although the move-
ments in the latter have an entirely different biological significance. Let
us study the stamens of the Cynareae, more especially those of Centaurea
(Fig. 160) which have been minutely investigated by PFEFFER. The five
syngenesious anthers form a tube round the style ; from these arise five free
filaments, bent slightly outwards, the basal ends being inserted on the corolla
lower down. When the filaments are touched (A ) they contract and at the same
time straighten themselves (B) ; in this way the anther tube is pulled down-
wards and the pollen is thus swept out by the hairs on the style ; the movement
is thus obviously an adaptation to pollination by insects. The filaments react
only directly to the contact ; there is no transmission of the stimulus. The
experiment may also be performed on a single isolated filament, and it may be

If

Fig. 160. Stamens of Centaurea
jacea, after removal of corolla. A, before,
J3, after stimulation. c} base of the
corolla ; s, filaments ; a, anthers ; g, style ;
/>, pollen. Enlarged. From the Bonn
Textbook.

520 TRANSFORMATION OF ENERGY

noticed that the contraction, after stimulation, may amount to 10, 20, or even
30 per cent, of the original length. The filament has a very simple anatomical
structure, consisting merely of a delicate vascular bundle surrounded by paren-
chyma, which latter is the only part that is sensitive. When stimulated, this
tissue becomes less turgid and water enters the intercellular spaces, as in
Mimosa, while the filament decreases markedly in volume. If water be pre-
viously injected into the intercellular spaces, the water exudes by the cut surface
when the filament is stimulated, although in ordinary cases the intercellular
spaces are sufficient to accommodate the liquid which has been extruded. In
the Cynareae it may be definitely shown that a diminution in the expansion of
the cell-wall is not concerned, since its elasticity is quite as great in the con-
tracted filament as in the extended one, a fact which may be shown by
previously subjecting it to the action of chloroform and so preventing stimu-
lation from shock. Further, no change takes place in the condition of the
membrane during the stimulation, for a weight capable of stretching a contracted
filament out to its original length is also sufficient to prevent any contraction on
stimulation (PFEFFER, 1873 a ; 1890, 326). On stimulation, therefore, there
must be a decrease in the pressure of the cell- con tents, apparently a decrease
in osmotic pressure, which, according to PFEFFER, must amount to from i to 3
atmospheres. The plant therefore does not make use of the whole of the elastic
play of the membrane, for the filament, after becoming contracted on stimu-
lation, may be still further markedly shortened by plasmolysis, while, on the
other hand, the filament when extended may still be stretched by mechanical
means, within the limits of its elasticity. The cell-walls, when the tension is
completely relaxed by plasmolysis, do not exhibit a permanent elongation,
if they be extended 100 per cent, by mechanical means ; but in their exten-
sibility, the filaments of the Cynareae stand quite alone in the vegetable king-
dom (p. 420).

Sensitive filaments occur in other sub-orders of the Compositae besides the
Cynareae (HANSGIRG, 1890), and curvature movements of filaments in response
to a shock stimulus have been noted in Cactaceae, Cistaceae, Mesembryanthema-
ceae, Tiliaceae, Portulacaceae, and Berberidaceae as well. The filaments in
these plants are sometimes sensitive on one side, sometimes on all sides, and
the curvatures they exhibit are sometimes inwards, sometimes outwards;
but in all of them the nature of the sensitivity and the method of movement
appears to resemble the phenomena as exhibited by Mimosa. Transmission
of stimulus has also been recognized in Sparmannia by HANSGIRG (1890). In
most cases these movements have some obvious relation to the cross-pollination
of the flowers.

In addition to single sensitive styles (Arctotis ; MINDEN, 1901) there are stig-
mas whose lobes approximate when stimulated by contact ; they occur especially
in the Scrophulariaceae, Acanthaceae, Pedalineae, Bignoniaceae, and Cappari-
daceae ; these movements have been as yet very little investigated from the
physiological point of view. We are at liberty to assume, however, that the
approximation in them also is the result of a decrease in osmotic pressure.
In certain cases a transmission of stimulus takes place from one stigmatic
lobe to another, but the conduction is obviously effected in a way altogether
different from that seen in Mimosa. OLIVER (1897) supposes that the stimulus
is transmitted by intercellular protoplasmic communication, at all events the
transference of the stimulus is effected just as well after the vascular bundle
has been cut through. After a simple contact, the stigma remains closed only
for a short time, and after opening again it is ready to receive a new stimulus.
BURCK has made the interesting observation that the stigma of Mimulus luteus
remains closed if the contact be caused by pollen-grains ; Torrenia fournieri
closes its stigma permanently if it comes in contact with pollen from the
larger stamens, although it opens again if pollen from the smaller stamens or

MOVEMENTS RESULTING FROM SHOCK 521

from foreign plants be placed upon it. Closure of the stigma must thus be of
service in excluding foreign pollen.

HABERLANDT (1901) has drawn attention to certain anatomical adaptations
both in sensitive stigmas and stamens which are obviously correlated with recep-
tion of the stimulus, but we must refer our readers to his own descriptions of
these.

Bibliography to Lecture XL.

BERT. 1867. Mem. Soc. des sc. phys. Bordeaux, 1866. Paris.

BERT. 1870. Ibid. February.

BONNIER. 1892. Revue gen. de bot. 4, 513.

BRUCKE. 1848. Archiv f. Anatomic u. Physiologic. Ostwald's Klassiker, No. 95.

BURCK. 1901. Botan. Centrbl. 1902, 89, 645 Review.

CORRENS. 1892. Flora, 75, 87.

DUTROCHET. 1837. Mem. pour servir a 1'hist. d. veget. et d. animaux. Paris.

[FITTING. 1903. Jahrb. f. wiss. Bot. 39, 424.]

HABERLANDT. 1890. Das reizleitende Gewebesystem d. Sinnpflanze. Leipzig.

HABERLANDT. 1901. Sinnesorgane im Pflanzenreich etc. Leipzig.

HANSGIRG. 1890. Botan. Centrbl. 43, 409.

HANSGIRG. 1893. Physiologische und phycophytologische Untersuchungen. Prag.

HILBURG. 1 88 1. Unters. aus d. bot. Inst. Tubingen, i, 23.

MINDEN. 1901. Flora, 88, 238.

OLIVER. 1887. Ber. d. bot. Gesell. 5, 162.

PFEFFER. 1873 a. Physiolog. Untersuchungen. Leipzig.

PFEFFER. 1873 b. Jahrb. f. wiss. Bot. 9, 308.

PFEFFER. 1890. Plasmahaut etc. Abh. K. Gesell. d. Wiss., 1 6, 185. Leipzig.

ROTHERT. 1894. Cohn's Beitr. z. Biologic, 7, i.

LECTURE XLI

SUMMARY OF PARATONIC MOVEMENTS. AUTONOMOUS

MOVEMENTS

IN our very first lecture we drew attention to the fact that it was not
possible, off-hand, to recognize movement in all plants. Nevertheless, in so
far as we have studied the changes in form and position exhibited by fixed
plants, not to speak of the locomotory phenomena seen in non-fixed forms, we
are bound to admit that the popular view that plants have no power of movement
is entirely erroneous. Careful observation has made us acquainted with abun-
dant instances of movement, although these are less noticeable than movements
in the animal world, simply because they are not so rapid. Still, from the
scientific standpoint, the rapidity of a movement is not the most important of its
features ; its nature, its causes, the means by which it is accomplished, and its
significance in the organic economy are the points to which our attention is most
prominently directed. With regard to most of these points a remarkable parallel
has been drawn in recent years between the movements in response to stimulus,
described in Lectures XXXIII to XL, and the reflex movements of animals, and
when, in Lecture XLI 1 1, we come to consider locomotory directive movements
the analogy will become even more remarkable.

In Lecture XXXIII we termed the various movements hitherto dealt with
paratonic, induced, or receptive movements, and contrasted them with autonomous
or spontaneous movements, and in that lecture also we briefly pointed out the
grounds on which that distinction was based. Before studying autonomous
movements more in detail we must endeavour to bring out the contrast more
sharply than we have as yet done.

Movements in response to stimuli arise only as a result of the continuous

522 TRANSFORMATION OF ENERGY

influence of the environment, and this influence is twofold ; in the first place,
it provides the general (formal) conditions, in whose absence no movement in
response to stimulus — nor indeed any other vital phenomenon — can take place,
and it also provides specific stimuli. Both these effects of the environment require
further elucidation, and we may start with a consideration of specific stimuli.

We have already described these movements in response to stimuli as
released movements (PFEFFER, 1893 ; Physiol. I, 9 and II, 80) ; it should be
added that the factors which act as stimuli are merely the inducing causes of the
movements which the organism carries out by its own inherent energy ; the
stimulus itself never provides the energy actually required for carrying out the
movement. It follows that no definite relation exists between the energy
of the stimulus and that manifested in the response. The response is always
accompanied by an expenditure of energy, but the stimulus may be effected just
as readily by a withdrawal as by an application of energy. An example of the
former is seen in the stimulus induced by a reduction in temperature (compare
p. 5°2)- It may be said that the stimulus releases the reaction, it causes or
induces it, and hence we speak of induced movements ; we further ascribe to the
plant the power of perceiving the stimulus or of being sensitive to it, hence the
term ' receptive movements '. The stimuli we have previously dealt with have
been external stimuli, such as light, heat, electricity, gravity, chemical and
mechanical effects of certain substances, &c. ; but, just as in the differentiation
of the plant so in its movements there are internal stimuli concerned, some of
which we have to study in this lecture.

Since the essential characteristic of a stimulus lies in the fact that it acts
as a releasing agent, it follows that such stimuli are not confined to organisms ;
indeed we constantly make use of such releasing agents in machines of various
kinds, and since the causal connexion is incomparably more apparent in them
than in organisms, owing to the simplicity of the conditions, we can explain the
principle of a releasing agent in a machine far more easily than we can in an
organism (PFEFFER, 1893). Let us take the case of an electric bell. The apparatus
consists of a bell, the electric battery, and the conducting wire. The bell is the
mechanism which is stimulated by an electric current emanating from the battery.
Ordinarily, however, the conductor which runs from the source of electricity
to the bell is interrupted and a sound is produced only when the current is
' closed '. This ' closing ' or joining of the wires is the point of interest at present,
since it is the closing that ' releases ' the bell's capacity for making a noise. In
order to make and break the electric circuit a key is used, and the necessary
metallic connexion between the terminals is made by means of a gentle pressure
on a metal plug. It may be seen at once that the amount of pressure employed
stands in no relation whatever to the loudness of the resulting clang. According
to the way in which the key is constructed it may require the slightest touch
of the finger or the whole of a man's strength to close the circuit, but, so long as
the pressure is sufficiently great to effect closure, the bell responds in the same
way, provided the electric current passing along the wire remains unaltered.
When we remove the pressure from the button the key returns to its rest position,
the current is interrupted, and the bell ceases to ring.

There would be no great difficulty in manufacturing other kinds of ' keys '
by which the current could be closed by magnetic energy, electricity, heat, or
light, instead of by mechanical means. It is quite unnecessary for us to spend
time in describing how such an apparatus could be made, it will be sufficient
for our purpose to note that a new type of key will be required for each type of
releasing agent. No matter what the structure of the key, the bell reacts only
to a single releasing agent ; the mechanism responds to this influence, or, as we
might say, is sensitive to it. It is also obvious that the application of pressure
to any part of the system other than the key, i. e. the application of the releasing
agent to any other place than the sensitive apparatus, must be quite ineffective.

SUMMARY OF PARATON1C MOVEMENTS 523

This simple illustration will help us to understand many of the phenomena
of stimulation. Pressure on the button no more provides the energy for moving
the clapper of the bell than does contact in the case of a tendril, a shock in the
case of the leaf of Mimosa, or gravity or light in geotropic and heliotropic move-
ments respectively, carry out the work that is ultimately accomplished. It is
quite true that when geotropism was first studied, attempts were made to prove
that gravity was the actual source of the energy expended in the curving which
ensued. What in the case of geotropism took many years and much hard work
to demonstrate, was obvious on the face of it in the case of heliotropism. No one
dreamt of suggesting that sunlight pulled the stem towards it and pushed the
root away. Even the original explanation of heliotropic curvature given by
DE CANDOLLE, and which we have described as a mechanical explanation, is not
mechanical in the sense we now mean, for it always assumes a ' stimulating
effect ' of light.

Looking first at heliotropism by way of illustration, the heliotropic curvature
or released response might be compared to the clanging of the bell in our mechanical
illustration. The bell's capacity may also be regarded as mechanical ; we can
readily understand that such an apparatus, on account of its structure, is able to
function in this way and in no other when an electric current affects it ; on the other
hand we do not know how it is that a plant curves when unilaterally illuminated,
although we must assume that this phenomenon is the necessary result of the
' mechanical structure ' of the plant, just as a clanging noise is the necessary
consequence of the structure of an electric bell. That the active force in the
case of the plant is turgor or growth and in the case of the bell electricity has
nothing to no with the matter. The releasing force in the one case is pressure
on the key, in the other case sunlight ; with the sensitive apparatus in the
machine we are fully acquainted but we know nothing of it in the organism.
We can only say where the sensitive apparatus is in the plant, for we know that
in many instances it is closely associated with the reacting region and in others
that it is situated at some distance from it. Its structure apparently lies outside
the limits of microscopic investigation and hence we can do nothing more than
guess at its mode of operation. The same is true of all other stimulus phenomena ;
as to the structure and mode of action of the sensitive apparatus we are quite in
the dark, although the conditions under which it carries out its functions have
been more or less accurately determined.

Very many plants exhibit curvatures due to the influence of other stimuli,
which differ from heliotropic curvatures in no respect or only in trivial details.
Still, according to the nature of the stimulus, we have distinguished these move-
ments as geotropic, chemotropic, thermo tropic, and so on. These curvatures
owe their origin to all appearance to the same mechanical structure as the helio-
tropic movements, but we must assume that the perceptive apparatus must
have a different structure in each case, adapted to the reception of the particular
stimulus in question, just as a different kind of key will be required in the
electric bell apparatus according as pressure, electricity, light, &c., is the
releasing agent employed.

Just as we meet with similar reactions in the plant accompanied by dis-
similar perception, so the converse, also, holds good, for we find dissimilar response
resulting from similar external influences. Thus the root responds positively,
but the stem negatively, to gravity ; again the same external factor may induce
curvature in one organ and torsion in another, and the same differential illu-
mination which induces curvatures may influence the symmetry of a plant, so as
to induce the formation of new organs on one side rather than another, as, for
instance, in the development of roots, roothairs, or sexual organs on the shaded
side. There are two possible suggestions which we might offer by way of
explanation of these phenomena, i. We might assume that the plant possesses
only one kind of perceptive organ by which it appreciates each individual

524 TRANSFORMATION OF ENERGY

stimulus, but which is in connexion with different kinds of apparatus. This
would correspond to the case where the electric wire, closed by the key, was
connected up, in one case with a bell, in another case with a glow-lamp, and in
a third with, let us say, a voltameter ; the work done as a consequence of the same
releasing stimulus would be quite different in each case. 2. It is also possible,
however, that the difference in the reaction depends upon the varying struc-
ture of the perceptive organ. NOLL (1892) makes an assumption somewhat
like this, and has accounted for the different forms of geotropic response in this
way. There are many arguments against such a hypothesis, however, and we
are inclined to think the first view is the more probable one.

Let us now inquire whether there is or is not only one kind of perceptive
apparatus for each and every stimulating agent. The same stimulus can
indeed induce a response in quite different ways. Light, for example, gives
a stimulus to the plant when it affects the plant equally in every part, and
the response given by the plant is indicated by alteration in its rate of growth.
The response is totally different when the light falls with unequal intensity
on opposite sides, say, of the shoot, for then the plant responds by exhibiting
helio tropic curvatures. In contrast with these regional light stimuli we have
the periodic variations in light intensity which lead to nyctitropic movements.
The preliminary phenomena of stimulation in the case of heliotropism and
nyctitropism are doubtless different from those which lead to etiolation.
We have been compelled, for good reasons, to discard the ' etiolation theory '
of heliotropism, and on similar grounds we are entitled to look with scepticism
on any hypothesis which would seek to explain thermotrppic or chemotropic
curvatures by asserting that the organ in question exhibited growth in each
longitudinal area with a rapidity proportional to the temperature to which it was
exposed or to any definite concentration of the chemical medium employed. The
facts are in striking opposition to such a hypothesis, for under certain circum-
stances the side that is more remote from the influence of the optimum tem-
perature grows more rapidly than the others (compare p. 480). Although
the phenomena precedent to stimulation in etiolated growth are doubtless
different from those of heliotropism the first effect of light may be the same in
both cases ; we may assume, for instance, that in each cell a quantity of
a certain material, proportional in amount to the intensity of the light, dis-
appears with the discontinuance of illumination. This change, consequent on
the action of light, may be compared with the closure of the key as a result of
pressure, it indicates a preliminary chemical (or physical) action of the stimu-
lant, and may represent what we term perception. If thereafter an acceleration
or retardation of growth takes place in each sensitive cell, we must look upon
that as the released movement. We have already especially noted that, in the
case of etiolation, not all cells which perceive the stimulus proceed to react to
it, otherwise all organs must elongate in darkness ; experience teaches us that
organs behave differently in this respect ; correlations between the individual
units prevent a similar reaction of all. All the same we may assume in this case
that perception and reaction striven for are similar throughout ; secondary
influences, however, which we need not consider here, but which may be of
a relatively simple character, may interfere with the reaction in certain regions.

The phenomena of heliotropism are different and more complicated. When
perception, variable in its intensity, is brought about in different cells in con-
sequence of unequal intensity of light, reaction does not follow it directly. On
the other hand, the varying degree of sensation on opposite sides operates as
a new stimulus and it is this that induces movement. Assuming that unequal
sensitiveness in different regions acts as a new stimulus, we must grant to the
plant the power of comparing the primary light effects in different situations.
The term comparison suggests that we have here to deal with a psychical capacity
in the plant. Although psychical capacity suggests consciousness, still we

SUMMARY OF PARATONIC MOVEMENTS 525

must dismiss such an idea at once from our minds, for we are acquainted with
plenty of examples of movement in our own bodies which take place without
our being conscious of them, i.e. reflex actions, which, obviously, strongly
resemble these responsive movements in plants. In the higher animals,
however, an afferent conductive apparatus to a central organ and an
efferent one to the motile organ are also required for the carrying out of the
reflex action, in addition to the organ which perceives the stimulus (sense
organ) ; the existence of such a central organ in plants has been suggested
by CZAPEK (1898), but no reliable evidence in favour of the idea has been
adduced, still less do we know as to its localization.

At present it is not easy to illustrate by means of a mechanism those move-
ments at least which arise as a result of a comparison of differential primary
stimuli. This much is clear, however, viz. that, in principle, such a machine
must differ from that we have employed above, inasmuch as it must show
two or more consecutive releasing movements before the chief or final response
is observed. The electric current set up by closing the key must be made to release
another current, or even another energy altogether, which in its turn does the work.

We may conclude, therefore, that the primary effects of light in etiolation
and in heliotropism may be the same, and hence that it is sufficient to
assume one type of perceiving organ for every stimulant, and this leads us to the
conception that very frequently quite a chain of releasing movements may inter-
vene between the first application of the stimulus and the final response.

Simple machines, like our electric bell arrangement, may still serve to render
intelligible many phenomena of stimulation, such for instance as the relation
between the intensity of the stimulus and the amount of the response. Looking
at an ordinary electric key we see that a certain amount of releasing energy must
be expended before perception follows. Every pressure which fails to make
contact, fails also to reach the liminal value of the stimulus and induces no re-
sponse, no matter how long it may be continued. Every pressure, on the other
hand, which succeeds in making contact, induces a maximum reaction. The same
thing occurs in Mimosa. In other stimulation phenomena, such as geotropism
(pp. 439 and 457), we found that the response varied according as the releasing
energy increased or decreased. It would not be difficult to construct a key of such
a character that as pressure on it was increased, more and more electric elements
became involved and added their currents to the total, so that the activity of
the apparatus would thus bear a certain quantitative relation to the releasing
force. The primary positive curvature of a heliotropic plant, followed by
a negative curvature as the intensity of the light is increased, may be demon-
strated by means of an electric model, where the key is pressed upon with ever-
increasing force first on one side and then on the other. Such an apparatus also
shows that the regulating arrangements seen in all stimulation phenomena, and
which as a rule have a definite purpose to fulfil, are not characteristic of organic
nature only ; they apply to machines as well. We will not pursue these analogies
further, however, for they are apt to mislead beginners into the belief that plant
phenomena are simpler than they really are. We may, however, once more
draw attention to the long chain of phenomena which is as a rule interpolated
between the reception of the stimulus and the response (compare p. 440), and
which in the machine is reduced to a minimum.

If we do not recognize, after all these remarks, at least one essential differ-
ence between the releasing stimulus in a machine and that in an organism, then
the idea of a stimulus is to all intents and purposes superfluous. We retain it,
in the first place, on historical grounds, but also because we are able, by its means,
to recognize the organism at once as the scene of certain phenomena, while, at
the same time, admitting that we are entirely in the dark with regard to the
releasing process, as indeed to the entire chain of events right up to the final
response (PFEFFER, 1893).

526 TRANSFORMATION OF ENERGY

Just as in the case of every machine, so in the case of the organism, several con-
ditions must be fulfilled if its various functions are to be carried out satisfactorily.
In addition to internal factors there are also a number of external or formal con-
ditions (PFEFFER, Physiol. II, 76) which must be taken into consideration, the
significance of some of which is quite obvious, although the part played by others
is as yet far from being understood. The need for water and certain constructive
material is apparent on the face of it ; oxygen also is required in order that
respiration may be carried out, a physiological process without which, as a general
rule, no movements can take place. In addition to these material conditions,
heat and light must be taken into account, for a definite amount of heat, a de-
finite degree of temperature, is one of the most fundamental formal conditions
of plant life, while some, though by no means all reflex movements require
light of appropriate intensity for their manifestation. Finally, injurious
external influences must be absent, such as poisons and narcotics, which either
merely delay the response or prevent it taking place altogether, or even may
bring about the death of the organism, according to the degree of concentration
in which they occur. Every insufficiency in any of the formal conditions tends
to bring on a state of rigor, cold rigor, heat rigor, drought rigor, &c., and each of
these rigors, if long continued, tends to become fatal.

None of these facts are new to us ; we have summarized them once more
merely that we may add to them a few general remarks. Let us inquire first
as to the significance of the intensity of these factors. We have abundant data
at our disposal on the subject, for in almost all vital processes we have estab-
lished the fact that the influence of formal conditions is, in the most intimate
way, dependent on their intensity, and that that dependence may be expressed
graphically by means of a curve in which three cardinal points may be recog-
nized, viz. a minimum, an optimum, and a maximum. It is often said that this
dependence is limited to organisms, but that is by no means the case. It must
be remembered that in the inorganic world also ' optima ' may be recognized (com-
pare ERRERA, 1896). Since water attains its maximum density at 4° C., we may
express this dependence of density on temperature by means of a curve which
exhibits one optimum point, but which certainly shows no minimum or maximum.
There are also, however, processes, entirely apart from those occurring in or-
ganisms, which exhibit well marked minima, optima, and maxima in their relation
to temperature. Thus the solubility of sodium sulphate has its minimum at o°C.,
its maximum at 100°, and its optimum at 33° C. Betol (compare TAMMAN, 1898)
reminds one even more of the organism, for it melts at 96° C. and, after cooling,
remains liquid as long as it is maintained above + 25° C. and under — 5° C. Tem-
peratures above the minimum (— 5°C.) and below the maximum ( + 25°C.) do
not act in the same way, for at 10° C. we reach an optimum, inasmuch as far more
crystals make their appearance at that than at other temperatures.

Temperature influences the various processes in the plant in a variety of
different ways ; the graphic curve for assimilation is quite different from that
for respiration or for growth, and we cannot be wrong in assuming that what
is true of these processes is true of others. Naturally the behaviour of different
organisms varies to a much greater extent still. Take the case, for example, of
the dependence of responsive movements on oxygen only ; we cannot wonder
that typical anaerobes require no oxygen and may even be brought into a state
of rigor by its presence, while the same pathological condition is induced in
aerobes by its absence. It is a very remarkable fact, however, that among
genuine aerobes, also, very great variations are met with in relation to the
minimum percentage of oxygen required ; CORRENS (1892) has shown that
at least 6 percent, of the normal amount of oxygen present in the atmosphere
must be present before Passiftora can carry out haptotropic movement, while
Mimosa leaves and the tentacles of Drosera will respond to contact stimulus
when oxygen is entirely absent. The various phases in the reflex action also

AUTONOMOUS MOVEMENTS 527

exhibit dissimilar degrees of dependence on external formal conditions, and
this fact enables us in certain cases to distinguish between perception, con-
duction, and reaction (compare p. 440) in cases where other criteria are
not available. For example, geotropic response is inhibited by chloroform
although its perception is not ; in other cases the same anaesthetic may con-
versely destroy the power of perception of a stimulus and yet not interfere
with the power of movement to any great extent (compare ROTHERT, 1903).

In conclusion, let us inquire how these formal conditions really operate.
Do they operate in virtue of their own inherent energy or do they act as releasing
agents only ? In by far the majority of cases an exact answer cannot be given
to this question ; there can be little doubt, however, that certain essential sub-
stances, by bringing to the plant a store of energy or of constructive material,
act as energizing bodies, while temperature and, generally speaking, the
majority of the formal conditions are doubtless to be regarded as releasing
factors only. How then are we to distinguish between formal conditions and
specific stimuli ? In many cases the distinction is undoubtedly possible, as, for
instance, if the formal condition, e.g. temperature, is to be considered as
a general stimulus for many or all of the vital processes, and the factor, regarded
merely as a stimulus, may be proved to induce a single change or movement. In
other cases no such distinction can be drawn, for when several external con-
ditions are operative at the same time it is exceedingly difficult to say which
of them are to be considered as formal conditions and which as specific stimuli.
An example will make this clear. A lateral root in darkness assumes a different
geotropic rest position from what it does in light. If the roots be first of all
grown on a klinostat and then allowed to carry out a geotropic curvature, we
must look on gravity as the stimulus and say that the result of this stimulus is
different according as it operates on an illuminated or a darkened root. If we
allow the root, however, to attain its geotropic rest position in the dark and then
illuminate it, we must conclude that the light is the stimulus which brings about
the ensuing curvature. Similar cases are of very frequent occurrence.

If we regard all the formal conditions as operating with optimum intensity,
and if care be taken that they remain constant for a long time, and that other
external influences are entirely excluded, the movements in response to stimuli
hitherto studied cannot take place ; but it would be quite incorrect to assume
that the plant under these conditions was incapable of movement. In the first
place, it is obvious that the conditions which we have assumed are operating
are just those which are favourable to growth, and all growth is necessarily
accompanied by movement. Although many plant organs when exposed to
uniform external conditions grow more or less in a straight line, there are other
organs also which exhibit growth curvatures without any specific external stimuli
being applied, which are very similar in character to curvatures in response
to stimuli already studied. Variation curvatures, however, do not, in general,
cease when specific stimuli leading to curvature are absent. Growth and varia-
tion movements, which cannot be referred to definite external stimuli, but
which are dependent on formal conditions just as are the reflexes, are spoken
of as autonomous or spontaneous movements. Each movement must naturally,
all the same, have its own specific cause and the term ' spontaneous ' must not
be taken as synonymous with ' causeless '. If external factors be excluded
from consideration as the agents to which these movements are due, internal
factors must be assumed in their place. When we further investigate what
these internal causes in turn really are, it would appear in the highest degree
probable that we are dealing here also not with energizing but with releasing
agents ; in other words, spontaneous movements are to be considered also as
stimulus-movements, although the stimuli inducing them are not external but
internal and unknown. Although we have contrasted spontaneous movements

528 TRANSFORMATION OF ENERGY

with movements induced by stimuli previously studied, this comparison does
not by any means lead us to the root of the matter. We cannot prove it, but
we think it is extremely likely, that autonomous movements are also induced,
but by internal rather than external stimuli. It will now be our task to
attempt to gain some acquaintance with these autonomous movements. As
already noted, we may distinguish them into variation movements and nutation
movements, according to the media by which they are carried out.

When speaking of nyctitropic movements of articulations we had occasion
to note that these periodic oscillations continued for a long time in darkness
when the temperature was kept constant, with almost the usual daily rhythm ;
in that case we were dealing with after-effects, which must not be confounded
with autonomous movements. These oscillations are very prominent in Mimosa
and Acacia, but they are not manifested by all leaves provided with pulvini.
When we study a plant of clover kept in darkness, we may observe very marked
to and fro oscillations in the leaflets which, however, show no relations to daily
periods (PFEFFER, 1875). This is a case of genuine autonomous movement
which is indeed itself autonomously periodic. These movements occur in light
also, though frequently masked, owing to the greater effect of the paratonic
(nyctitropic) movements. In Averrhoa bilimbi (a member of the Oxalidaceae)
these movements may be seen very clearly. When temperature and illumination
are kept constant the pinnate leaves of this plant con-
tinually perform backward and forward oscillations (DAR-
WIN, 1881), suddenly drooping and then slowly rising
again. [Very remarkable autonomous movements are
also exhibited by Oxalis hedysaroides (MoLiscn, 1904).]
PFEFFER'S (1875) researches have shown that while
the autonomous movements are in progress, just as in
the case of after-affects, the resistance to flexion of the
pulvinus remains unaltered. We may, therefore, assume
that the expansive force of the cells on the concave side
of the articulation decreases proportionally as it increases
on the convex side.

After PFEFFER from DET- The leaves of Averrhoa and of the majority of nycti-

tropic plants perform simple autonomous pendulum oscil-
lations, but in the well-known Desmodium gyrans (DAR-
WIN, 1881, p. 304) the movements are more complicated still. The leaves of
this plant (Fig. 161) are tripartite. The terminal leaflet is large and performs
well marked nyctitropic as well as less noticeable autonomous movements, while
the two smaller lateral leaflets, on the contrary, show no nyctitropic movements,
but do show autonomous oscillations, which at a certain temperature (minimum
22° C. ; optimum, 40° C.) are so rapid that they may be readily followed with the
naked eye. The whole backward and forward movement is complete in about
ij minutes. The alteration in the expansion of the pulvinus does not take place
alternately, first on one side and then on the other, but it proceeds in a circular
manner, one longitudinal area after the other being affected. The result of that
is that the tip of each leaflet describes approximately an ellipse whose long axis
is parallel with the main petiole. The movement is, however, not uniform but
is jerky in character, and on the whole more rapid downward than upwards.
The jerks are especially prominent if the efforts to move on the part of the leaf
are prevented for a long time by external resistance, so leading to tissue tensions.
According to STAHL (1897), such tensions, arising by inhibiting movement in the
terminal leaflet, as they become equalized, lead to vibrations and hence to
increased transpiration in the terminal leaflet. Whether other autonomous
variation movements also have a biological significance may be left undecided.
In the flowers of certain Orchidaceae and Stylidiaceae we also meet with many

AUTONOMOUS MOVEMENTS

529

cillating
occur in

remarkable autonomous movements which are perhaps of the nature of vari-
ation movements. In Stylidium adnatum (GAD, 1880) it is the gynostemium
which oscillates and which occasionally presses itself so vigorously against one
definite perianth leaf that tensions arise which in the long run may lead to a
sudden release of the co-
lumn from the leaf ; the
sudden backward curva-
ture thereby arising re-
sembles in its character
a reflex action. [Com-
pare HOSSENS, 1903.]
Among Orchidaceae os-
movements
Megadinium

LREN,l842)

which are in this case
carried out by a narrow
basal part of the label- / ^
lum, but of whose mode
of operation we know
nothing. Possibly it
may be a growth move-
ment, at least autono-
mous periodic movements are much more frequently due to growth than
to changes in turgidity.

The whole cycle of growth under constant external conditions, i. e. the grand
period so called (Lecture XXIII), may with perfect correctness be regarded
as an autonomous movement. During that period the apex of the root or
the stem does not follow a perfectly straight course (circumnutation ; DARWIN,
1881). Where such apices do appear to grow straight, looked at casually, the
microscope discloses inequalities in growth in certain longitudinal areas which

Fig. 162. Projection curve of
Phycomyces ni/ens, after FRITZSCHE
(1899). X 200.

Fig. 163. Projection curve of
Zea mats, after FRITZSCHB (1899).

X IO.

v ,

Fig. 164. Yucca filamentosa% showing two axes
inflorescence. 7, May 27, 1900, noon. //, May 28, 9.30 a.m.
7/7, May 28, 2.30 p.m. From a photograph. Reduced.

Fig. 165. Spirogyra j>rt'ncej>s.
Positions taken up by a filament at

short intervals.
1874.

After HOFMEISTER,

are sometimes regular, at other times follow no obvious rule (FRITZSCHE, 1899),
In Fig. 162 the movements of the conidiophore of Phycomyces are recorded, as
observed from above by means of the microscope. If the conidiophore grows
rectilineally its apex must always occupy the same position in the field of the
microscope, but in reality readings taken even after 7 J minutes show that it has
moved away considerably from that position. Similar records may be obtained

JOST

M m

530 TRANSFORMATION OF ENERGY

by observations in seedlings (i.e. Zea mais\ Fig. 163). The nutations performed
by many of the higher plants are still more complicated and more irregular if
several zones of growth come into operation at the same time. The movements
of the inflorescence of Yucca, for example, are very remarkable ; indeed during
the process of unfolding the inflorescence looks as if it were pathological, but by
and by it assumes its normal straight character. Fig. 164 shows these changes in
position. Curvatures also arise in the cellular filaments of the Zygnemaceae,
due to the irregularities of growth which are always taking place and which
cause the filaments to assume different positions. Fig. 165 shows a filament of
Spirogyra drawn at short intervals (compare E. WINKLER, 1902).

It is impossible for us to enumerate all the cases of autonomous nutation re-
ferred to in the literature on the subject ; we must confine ourselves in what
follows to examples of especially uniform nutations.

The best known are the revolving nutations (NoLL, 1885 ; WORTMANN,
1887), where the oblique or horizontal apices of plants move forward in circles
or ellipses. This type of nutation corresponds exactly to that shown by Desmo-
dium, but it arises from the fact that one flank grows more vigorously than the
other and that this increment of growth affects new longitudinal areas in regular
succession. Circumnutation may be compared also with the outwardly similar
movements of the apices of twining plants, and here, as there, we find a twisting
of the apex of the shoot on its long axis in order to avoid torsions. The differ-
ence between the two sets of phenomena lies in this, however, that whereas in
the case of twining, gravity plays an active part, we have here to do with purely
autonomous movements. Such movements occur very prominently in tendrils
and are obviously of the utmost importance as aiding them in finding a support.
Circumnutatory movements occur also in seedlings, stola, &c., and, in these
cases, when the curve is a much elongated ellipse, they approach the pendulum
movements in character, as may be especially well seen inAllium scorodoprasum.
Careful observation certainly shows that in the case of pendulum movements the
oscillations are by no means always in the same plane, any more than, in cir-
cumnutation, are true circles or ellipses described by the growing points. Hence
it would be difficult to separate these two types of nutation from each other.

The same is true of a further distinction which has been drawn between
periodic and transitory nutations. Typical transitory or solitary nutations occur
in many organs which are curved at an early stage in their growth and after-
wards become straight. The cotyledons, the hypocotyl. and the root of the
embryo frequently exhibit special curvatures characteristic of the type, and
which appear to be entirely autonomous. With these may be associated the
curvature manifested by embryonic organs in the bud. Stamens, floral and
foliage leaves are very frequently curved inwards owing to excessive growth on
the under side (hyponasty), and become straightened later on, owing to increased
growth on the upper side (epinasty). The bud form is thus due to hyponasty
of the lateral appendages. Not infrequently it happens that the epinastic out-
curvature is not completed in one movement, but that a more vigorous de-
pression alternates with a feebler erection, thus constituting a periodic nutation.
Transitions between transitory and periodic nutations occur in those cases where
epinastic growth, so to speak, shoots beyond the mark, as, for example, when the
leaflets of Aesculus, converging upwards in the bud position, bend downwards on
the opening of the bud and then, during unfolding, owing to renewed hyponastic
growth, spread out approximately horizontally. Epinasty and hyponasty co-
operate with diageotropism and diaheliotropism in bringing about the definite
rest position of dorsiventral organs. External factors often aid in bringing
about this result, or they may operate antagonistically to it. It is impossible
for us to discuss the action of epi- and hypo-nasty in greater detail ; the matter
becomes especially complicated, inasmuch as in addition to autonomous move-
ments induced nastic movements also come into play (e. g. photonasty).

AUTONOMOUS MOVEMENTS 531

Peculiar nutations are met with in the leaves of ferns and of other plants whose
leaves retain the power of apical growth for a long time (e. g. Drosophyllum ; com-
pare GOEBEL, Organographie, p. 508, Fig. 336). Such organs have their apical
regions coiled in a circinate manner. In ferns this circinate condition is hypo-
nastic and, as the leaf straightens itself, a less vigorous epinastic curvature sets
in, ultimately bringing about the expanded condition. Similarly, in the case of
the nutations of many seedlings, where more vigorous elongation takes place on
one side of the plumule, usually spoken of as the posterior side, a second
curvature occurs on the incurved apical region at the zone of maximum growth
which acts antagonistically to the original curvature. WIESNER (1878) speaks
of an ' undulating ' nutation in this case, and of a ' simple ' nutation when the
region of the plumule behind the hooked end is straightened at once (Linum).

In addition to autonomous movements in one plane we have also autono-
mous movements in space, such as torsions and twinings. Examples of this
type of movement are met with in the peduncles of Vallisneria and of many
species of Cyclamen after fruiting, leaves of the garden variety of Juncus known
as Juncus spiralis, leaves of Typha and many other narrow-leaved Monoco-
tyledons, the labellum of Himantoglossum, the internodes of Chara, and finally
the senile coilings of tendrils previously mentioned. We must limit ourselves to
the mere enumeration of such instances for they have not as yet been studied
in detail. It is not improbable that it may be found necessary to remove many of
these examples from the category of autonomous to that of induced movements,
as has already been done in the case of the nodding flower-bud of Papaver and
the apex of the shoot of Ampelopsis ; such cases of ' nodding ' would certainly
have been regarded as autonomous from their likeness to the nutation of plumules
had it not been that VOCHTING (1882) and SCHOLTZ (1892) have shown that
they are geo tropic in their nature.

Glancing back over what has been said, we recognize in autonomous move-
ments phenomena which, as yet, are very imperfectly understood both from
the physiological and from the biological point of view — hence the brevity of our
treatment of them.

Bibliography to Lecture XLI.

CORRENS. 1892. Flora, 75, 87.

CZAPEK. 1898. Jahrb. f. wiss. Bot. 32, 175.

DARWIN. 1881. Das Bewegungsvermogen d. Pflanzen (CARUS). Stuttgart.

ERRERA. 1896. L'optimum (Revue Univ. Bruxelles, i.)

ERRERA. 1900. Generation spontan. (Revue Univ. Bruxelles, 5.)

FRITZSCHE. 1899. Beeinfl. d. Circumnutation d. versch. Einfl. Diss. Leipzig.

GAD. 1880. Bot. Ztg. 38, 216.

HOFMEISTER. 1874. Wurtemb. naturw. Jahreshefte.

[HossENS. 1903. Beeinfl. d. auton. Variationsbewegungen d. e. auss. Faktoren.

Diss. Leipzig.]

[MOLISCH. 1904. Ber. d. bot. Gesell. 22, 372.]
MORREN. 1842. Mem. Acad. Bruxelles, 15.
NOLL. 1885. Bot. Ztg. 43, 664.
NOLL. 1892. Heterogene Induction. Leipzig.
PFEFFER. 1875. Periodische Bewegungen. Leipzig.

PFEFFER. 1 893. Reizbarkeit d. Pfl. (Gesell. d. Naturf . u. Aerzte. Verhandlungen).
ROTHERT. 1903. Jahrb. f. wiss. Bot. 39, i.
SCHOLTZ. 1892. Cohn's Beitr. z. Biol. 5, 373.
STAHL. 1897. Bot. Ztg. 55, 7 1.
TAMMAN. 1898. Quoted by Errera, 1900.
VOCHTING. 1882. Bew. d. Bliiten u. Friichte. Bonn.
WIESNER. 1878. Sitzunsgber. Wiener Akad. 77, i. Abt.
WINKLER, E. 1902. Kriimmungsbewegungen von Spirogyra. Diss. Leipzig.

WORTMANN. 1887. Bot. Ztg. 4$, 49.

M m 2

532 TRANSFORMATION OF ENERGY

LECTURE XLII
AUTONOMOUS LOCOMOTORY MOVEMENTS

HAVING discussed the movements exhibited exclusively by fixed plants,
we must now turn to movements in those plants or plant parts which are capable
of locomotion from place to place. At the first glance it would appear as if we
had to deal in such cases with an entirely different category of phenomena from
those we have hitherto been considering. ' More careful study leads us, however,
to the conclusion that it is only the nature of the reactions, i. e. the change of
place and the apparatus by which these reactions are carried out, that alone are
novel. The general and special conditions under which locomotion is effected
are on the contrary essentially similar to those we have met with already in the
phenomena of growth and movement in fixed plants. The external stimuli
which influence the nature of the response are precisely the same immaterial and
material agents referred to in previous lectures on movements in the higher
plants. Indeed many authors have advocated the treatment of locomotory
movements simultaneously with those of curvature. While keeping these two
series of phenomena apart in our present discussion on the subject, attention
must be drawn to the numerous analogies which exist between them, further
instances of which we shall meet with frequently enough later on.

We will first of all study autonomous locomotory movements which stand in
close relation to the autonomous curvatures considered in the last lecture ;
induced locomotory phenomena will be treated of in the next lecture.

Autonomous locomotory movements are exhibited by the protoplasts of
almost all plants, but they are naturally limited in extent by the rigid cell- walls.
In many lower organisms, on the other hand, these movements are not so cir-
cumscribed, for such organisms have the power of creeping over the substratum
or of swimming through the watery medium in which they live. We will take
the latter case first.

Natatory movements occur in many Flagellata, very lowly organisms which
may with equal accuracy be classified either among plants or among animals.
These movements may continue during the entire life-history of the organism.
In Algae, Fungi, and Bacteria, certain cells, at least temporarily, possess the
power of locomotion ; such cells are known as swarmspores or zoospores, and
their function is to carry out asexual propagation and thus conduce, more espe-
cially, to the wide distribution of the species concerned. Further, the sexual
cells are frequently adapted to a motile existence, both male and female cells
among the lower forms having that power, while among more highly developed
types motility is confined to the male cell. Motile sperms occur not only among
mosses and ferns, but the corresponding cells even in Gymnospermae may exhibit
the same characteristic (of motility) more or less distinctly. All such motile
cells are provided with filamentous appendages, flagella, or cilia, whether they
be surrounded by cell-walls (Bacteria, Flagellata) or not (swarmspores, sperms).
These cilia effect the movement of the cell-body by rapid bendings, beating the
water and driving the cell forwards, as a boat is propelled by its oars. They
are developed from the ectoplasm and are themselves protoplasmic in character.
In order to carry out their function they must be surrounded by water, into
which they project through pores in the cell-membrane. As a general rule
alterations in form do not come under consideration in natatory movements.

Let us take as our first example of an organism propelled by cilia the swarm-
spores of Algae (NAGELJ, 1860). The swarmspores are naked, and formed,
several at a time, in a mother-cell, and each exhibits all the essential constituents

AUTONOMOUS LOCOMOTORY MOVEMENTS 533

of the typical cell, i. e. protoplasm, nucleus as well as chloroplasts. They are
almost always elongated, ovoid or pear-shaped, always markedly polar, but by
no means always exhibiting radial structure in relation to their long axis. One
pole, the anterior one in any movement, is generally free from chlorophyll and
provided apically, or not infrequently laterally, with two, four, or more cilia.
The posterior end is generally more rounded and contains chloroplasts. The
movements are by no means simple, consisting as they do not only in a forward
movement in the direction of the long axis of the cell but also a torsion on it.
This is at least true in the case of certain cells ; in other instances the move-
ment is more complicated still. The forward movement may, instead of being
in a straight line, be in the path of a long drawn out spiral, the body of the cell
rotating on its axis at the same time, the axis remaining parallel to that of the
spiral. Finally a third type of movement is met with, when the anterior end of
the swarmspore advances in a spiral manner while the posterior end maintains
a straight course. If there be no mechanical or stimulating interference to its
movements the swarmspore continues to follow the direction taken at the com-
mencement of its movement, which on the whole is approximately a straight line,
but in other cases the spores swim in curves or move about quite irregularly.

When the swarmspore meets with a mechanical obstacle it is able without
moving from the spot to institute a twisting movement, frequently recoiling or
moving backwards, twisting on its axis in the opposite direction. The butt end
is anterior in this movement but very soon the original forward movement is
resumed. Apart from the backward movement, the torsional movement changes
only in certain free swimming cells ; in the majority of cases the direction of
torsion is constant and characteristic of the species.

All these phenomena may be observed in swarmspores only if these
movements be retarded, and this is best effected by. replacing the water-
culture by a weak solution of gum. The absolute rapidity of movement of
the swarmspores, which is markedly dependent on external conditions, is by
no means great. It appears to be considerable only when looked at under the
microscope, but then it must be remembered that the distances are also magnified.
According to HOFMEISTER (1867) the movements are most rapid in the swarm-
spores of Fuligo varians, viz. about I mm. per second ; the swarmspores of Ulva
attain a speed of 0-15 mm. per second (STRASBURGER, 1878), but the antherozoids
of the fern move much more slowly— 0-015 to 0-030 mm. per second — accord-
ing to PFEFFER (1884).

The fern antherozoids, which we shall study more in detail in the next
lecture, differ from the zoospores of Algae in their form only but not in their
movements. They consist of a spirally twisted body with 2-4 coils (Fig. 107,
P- 359), tapering gradually from base to apex, the cilia being inserted on the
thinner anterior spirals. No alteration in the form of the cell takes place here
either during the movement.

That the cilia are the agents concerned in the movement may be easily
proved. If a swarmspore be cut in two, only the part bearing the cilia remains
capable of movement. If, by mechanical means, the cilia be removed, all move-
ments cease and the body of the swarming cell sinks to the bottom. How the
cilia bring about the forward movement, and, at the same time, the torsion, has
not been investigated in detail, but we can scarcely be mistaken if we assume
that the lashing of the cilia are carried out in the same way as VERWORN (1901)
has demonstrated in the case of the ciliate Infusoria (Fig. 166). The apex of
the cilium is seen at first to lie parallel with the forward path of the animal and,
in the left-hand diagram (Fig. 166), the successive positions taken by the cilmm
at short intervals are figured ; the withdrawal (right-hand figure) is effected by
other curvatures which are much slower, otherwise a forward movement would
be impossible. The rotation of the body is due to the fact that the curving of the

534 TRANSFORMATION OF ENERGY

cilia does not take place in one plane. When several cilia co-operate in effecting
the forward movement (e. g. Oedogonium and Vaucheria) they must obviously
beat at the same rate, otherwise the movements would be quite irregular.

In addition to natatory movements the lower organisms also exhibit creeping
modes of locomotion, necessitating at all events a partial adherence of the body
to the substratum. In some cases these movements are due to the exudation
of slime from the cells (Desmidiaceae, STAHL, 1880 ; ADERHOLD, 1888 : Oscil-
laria, CORRENS, 1897 : and possibly also Diatomaceae, O. MULLER, 1897 :
compare also LAUTERBORN, 1896, andScHurr, 1899). Putting on one side such
movements as these, which have not as yet been examined in detail from the
physiological standpoint, we have left for consideration the locomotory move-
ments of naked protoplasmic masses, which creep over the substratum by altera-
tions in their form. Such phenomena are spoken of as amoeboid movements, since
they were first exactly studied in Amoebae ; in the vegetable kingdom they are
exhibited almost solely by the slime Fungi (Myxomycetes). The swarmsporer
after escaping from the spore-wall, moves partly in an amoeboid manner partly by
means of a flagellum ; later on many of these bodies fuse together into the so-called
plasmodium, which continues to. exhibit amoeboid movement until the recurrence
of the reproductive stage. Owing to its large size, the plasmodium is eminently

adapted for the study of amoeboid movement, for
all the phenomena can often be observed with the
naked eye without calling in the services of optical
instruments. Such plasmodia, especially -those of
the Physareae, (Fig. 167), occur on decaying leaves
and old tan in the form of much-branched, reticu-
late filaments of very varied thickness, the more
delicate anastomoses being visible only with the
aid of a microscope. DE BARY (1864, p. 37) gives
the following account of the general appearance
and movements of the plasmodium : — ' In one
region, the anterior or advancing side of the plas-

166. Movements of single modium, the chief branches are especially richly
f Vdliatei I"[us?r;an- After subdivided and the terminal twigs are swollen at

N iLrcnera.1 * nysioJOfify, lens,, ji • i i i • /• ^*i

1901). their ends and spread out in a f anhke manner over

the substratum, and united by very numerous ana-
stomoses. The individual branches and anastomosing threads of the anterior
peripheral network are either thick, hemispherical or circular in section, with
club-shaped, swollen and often ragged ends, or they may be flattened out so that
the advancing border is thin and perforated, with indented margin and somewhat
lobed, the whole being permeated by the stouter branches, like the swollen veins
in a mesentery. The plasmodium is soft and slimy in its texture and may be
readily smeared out with the finger, and yet it is firm enough to be sectionized
with a sharp knife, so as to show the cut edge. Most commonly it adheres firmly
to the substratum, but if placed under water large portions separate off, without
suffering injury, in soft, elastic but by no means liquid masses.'

' It may be readily seen with the naked eye that the plasmodium is con-
tinuously altering its shape, new branches being pushed out and others gradually
retracted, so that the whole body slowly creeps forward.' This may be seen even
better, however, under the microscope. ' The chief branches are constantly in-
creasing and decreasing in thickness, every here and there broad processes appear
on their upper surfaces which slowly or rapidly develop into new branches, while
branches already in existence become gradually reduced until they are reabsorbed
into the main body. Two branches may be seen growing towards each other,
until their ends touch, and the next moment the anastomosis is complete. A
lacelike network may thus arise at any point, composed of fine, threadlike uniting
strands. If the anastomosis be broken, the two branches are slowly reabsorbed

AUTONOMOUS LOCOMOTORY MOVEMENTS

535

into the mam axes. These movements are much more prominently exhibited
by the microscopic branches than by those visible to the naked eye ; such fila-
ments are being perpetually pushed out and pulled in, just like fine tentacles, and
the shape of the plasmodium is thus constantly altering ; branches shoot out and
are withdrawn again, forming and breaking anastomoses, often swelling up to
a great size and gradually taking on the characters of the stouter main branches
These alternative movements may be observed in all parts of the plasmodium
but they are readily seen to be more vigorous on the advancing side than behind,
and that, anteriorly, the characteristic feature is the protrusion of new branches
while, posteriorly, the
reabsorption of older
strands is predomi-
nant ; hence arises the
forward creeping
movement of the plas-
modium.' The direc-
tion of the motion
is, however, not infre-
quently changed.

In addition to this
change in outward
form associated with
regional changes of the
whole plasmodium,
other active move-
ments in the interior
of the plasmodium
may also be observed.
The plasmodium con-
sists of a colourless,
hyaline ground sub-
stance, the protoplasm
proper, through which
are scattered
numerous granules,
some of them com-
posed of carbonate of
lime, others of pig-
ments. The stream-
ing movements of the
protoplasm are easily
followed by the passive
migration of these
granules. Thus we may observe, first of all, in the centre of each branch an active
streaming taking place, while the peripheral region, on the contrary, is at rest,
not only the hyaline edge but layers still further inwards in which granules occur.
The movement takes place as though in a tube, for a time in one direction and
later in the opposite direction. In the marginal prominences for the most part
numerous streaming movements may be noticed, and not infrequently contiguous
portions move in opposite directions. Very often movements arise in regions
previously at rest, so that one may assume that canals in which the streaming
occurs did not exist there. This is shown even more clearly by the streaming
motion spreading laterally, the previously firm hyaline wall becoming fluid
and mobile. Rapid movements of the granules always take place, especially
in the active anterior regions of the branches, as though this streaming were
the cause of the forward movement of the tip of the branch. The smaller

Fig. 167. Plasmodium of Fuligo varians, creeping over a piece of filter
paper. From a photograph. Slightly reduced.

536 TRANSFORMATION OF ENERGY

projections, however, consist frequently of hyaline protoplasm only, destitute
of granules, so that the interdependence of internal granular streamings and
external alterations in form is somewhat doubtful.

The simplest form of amoeboid movement is met with in the Amoebae.
Pelomyxa, for instance, consists of a flattened, elongated mass of protoplasm,
which creeps over the substratum without any great change in form. Centrally
we may observe a single axial stream of granules spreading outwards towards the
advancing regions of the body of the organism, and the granules in the rear end
appear to converge into this axial current, while a zone surrounding the body
where the granules are at rest appears to separate the region of confluence behind
from the region of effluence in front. Amoeba behaves exactly in the same way
(Fig. 168) ; a forward movement appears not only at 7, but also in the laterally
advancing branches at L and R, so that the granules are seen streaming in three
directions. Between V and R a fourth, but subsidiary, current may even be
noted, so that there are five zones (indicated by crosses) which are at rest.

The detailed description of amoeboid movement given above renders the
phenomena of movement seen in the protoplasts of the higher plants, which are en-
closed in cell- walls, easily comprehensible (HOFMEISTER,
1867). We might, in fact, compare such protoplasts with
Myxomycetes enclosed by a membrane. A stationary
layer of protoplasm of greater or less thickness always
lies immediately in contact with the wall ; next comes
a layer of motile protoplasm lying between the vacuole
and the peripheral layer, and from this motile layer
arise the anastomosing strands and filaments, similar to
those seen in a plasmodium, which permeate the cell-sap
as with a network and which exhibit continual altera-
tions in form and position, although these, owing to the
confined character of the cell cavity, are necessarily
limited in extent. Just as in the case of the plasmo-
dium, so here also we observe a streaming of granules
both in the individual strands and in the peripheral pri-
mordial utricle. The direction of these currents varies
from time to time and, even in contiguous regions of a
strand, may be not infrequently in opposite directions ; in

the Bonn Textbook. other cases the granules may accumulate all on one side.

In addition to these more irregular protoplasmic

movements, another type of movement has been distinguished from those just
described (circulation) under the name of rotation. In this type of movement
the peripheral protoplasm (save an external layer of varied thickness) moves
in a constant direction, following, in elongated cells, the long axis of the cell, and
often showing obvious torsion if the cell be exceptionally long. The move-
ment is most rapid nearest the vacuole, which becomes moved about passively,
showing that the cell-wall and the vacuole are to the protoplasm of the cell
what the substratum is to the plasmodium. The vacuole acts as a pivot for
the movement, and hence the current set up in it is in the reverse direction to
those in the protoplasm.

It is frequently the case that, in addition to setting the minute non-living
particles included in the protoplasm in motion, rotation and circulation also
bring about, passively, alterations in position of the organs of the cell, i. e. the
nucleus and the chloroplasts, and these changes in situation are often of great
importance in the plant economy.

Owing to the widespread occurrence of the movements described and to
their obvious importance, attempts have for long been made to discover the fac-
tors concerned in them, and not only to correlate amoeboid with the obviously
related rotatory and circulatory movements, but to include under the same

AUTONOMOUS LOCOMOTORY MOVEMENTS 537

category ciliary and muscular movements as well. From the well-known
characters of muscular motion we might feel inclined to refer all these proto-
plasmic movements to contractility of the outer layer. When such an assump-
tion was found to be indefensible, HOFMEISTER (1867) attempted to show that
protoplasmic movement was due to a change in the attraction for water of the
smallest protoplasmic particles, while ENGELMANN (1879) held that it was due
to alterations in their form. All these attempts to explain the phenomena attri-
bute to the protoplasm an unexplained character, which is taken for granted,
although it must apply to living protoplasm only and not at all to lifeless bodies ;
moreover, they remove the problem into the realms of the invisible. More recent
explanations (compare JENSEN, 1902), such as those of BERTHOLD (1868),
BUTSCHLI (1892), and QUINCKE (1888), must be dealt with more carefully,
because they attempt to refer protoplasmic movement to purely physical causes.

In general these theories assume that protoplasm is a liquid and that its
normal shape is a sphere. Variations from the spherical form and the move-
ments themselves are thus accompanied by alterations in the surface tension
[compare EWART, 1903]. As a matter of fact, the protoplasm, after receiving
a wound or other injury, may frequently be observed to round itself off into a
sphere, and hence it cannot be doubted that certain parts of it at least are liquid
in character. Surface tension is undoubtedly a very important principle, but
we must not expect to solve by its aid, once for all, every problem connected
with protoplasmic movement. Moreover the authors referred to are not in
accord as to the details of the explanation.

In order that we may obtain a superficial acquaintance, at least, with such
physical theories of protoplasmic movement, we will look somewhat more
closely at amoeboid movement only, and leave on one side ciliary motion and
the streaming that takes place within the cell, since these present greater diffi-
culties. We need not discuss QUINCKE'S (1888) views, since they assume certain
conditions that are certainly not realized in the organism. We will therefore
limit ourselves to a consideration of BERTHOLD'S (1886) and BUTSCHLI'S (1892)
theories as to the movements exhibited by such a form as Pelomyxa. BERTHOLD
compares this amoeba with a drop of liquid spreading itself over, or round, a solid
body or a drop of another liquid with which it cannot mix. Looking more especi-
ally at the former case, and considering a drop of liquid lying on a plate of glass,
the space which it covers will depend in the first instance on the surface tensions
between the glass and the liquid, the glass and the air, and the liquid and the air,
and this will vary especially according to the chemical composition of the liquid
and with the temperature. A homogeneous liquid spreads over the substratum
equally in the form of a lens ; if the glass be not quite clean, or if the drop be
heterogeneous, or if it be of a different temperature in different places, the form
of the drop is irregular and more extended in one direction than another. In
Amoeba, owing to chemical differences between anterior and posterior ends,
polarity is induced ; the anterior end alone extends in a thin layer, adhering
to the substratum, while the posterior end detaches itself from the substratum
when adhesion is reduced, and, owing to surface tension, endeavours to round
itself off. The extension of the anterior end takes place, according to BER-
THOLD, with a certain amount of energy, it is pushed out, not pulled out and
the material required for this extension can be provided only out of such parts
as lie behind. ' Thus there arises a suction action of the centrally directed
current. As far as the anterior margin is concerned, the movement is
fan-shaped, because the extension is more vigorous in the centre than it is at
the sides.' Another factor of moment to be taken account of is the pressure
from behind, associated with the efforts on the part of the posterior margin to
round itself off.

BUTSCHLI advances first of all certain physical objections to this theory.
He shows that QUINCKE'S views as to extension, on which BERTHOLD bases

538 TRANSFORMATION OF ENERGY

his views, are not quite correct ; he denies the existence of any close adhesion
of the advancing margin of Amoeba to the substratum, and shows that BER-
THOLD'S hypothesis necessitates internal currents in the plasma which must run
in the exactly opposite direction to that observed. For this and other reasons
BUTSCHLI holds that BERTHOLD'S theory cannot be accepted, and proceeds to
replace it with one of his own. According to BUTSCHLI amoeboid movement
resembles that seen in emulsions, as may be seen when one side of an oil drop
comes in contact with a soap solution. An oil-soap emulsion suitable for the
purpose may be obtained by triturating thick olive oil with potassium carbonate
and adding water to the mixture. The soap originally dissolved in the oil passes
rapidly into the water, which in turn diffuses into the oil, and the watery soap
solution dividesup into particles, giving the appearance of minute vacuoles in the
oily ground substance. According to BUTSCHLI' s observations, such a foam
shows a strong analogy to protoplasm, which also, according to this author,
generally, exhibits a frothy appearance (compare FISCHER, 1901). When
some of the vacuoles in such a foam burst unilaterally, the oil at that place
becomes covered over with a soap layer and the same conditions arise as when
a homogeneous oil drop surrounded by water is allowed to come in contact with
a soap solution on one side (Fig. 169). Under these circumstances, the drop as
a whole exhibits a progressive movement, and currents are set up in its interior
which recall very vividly those seen in Amoeba.

The explanation given by BUTSCHLI of this phenomenon, cannot be dis-
cussed in detail here ; we will only note that,
in consequence of the lowering of the surface
tension at the point of contact with the soap,
a disturbance is set up in the equilibrium in
surface tension previously existing. One im-
portant point in BUTSCHLI' s explanation must
be drawn attention to, viz. that the strongest
currents occur immediately on the surface of
the oil drop (indicated by the larger arrows in

Fig. ,69. Oil drop in contact with soap the ^^ and that> Owing to these, COlTe-

solution. The arrows indicate currents, sponding currents are set up in the surrounding

water. BUTSCHLI has drawn our attention to
another point in amoeboid movement, viz. that

currents in that case in the water are absent or run in the opposite direction,
and he himself rightly concludes that his theory cannot be entirely correct in
all its details.

From another point of view also important objections may be raised to the
foundation on which both BERTHOLD'S and BUTSCHLI'S theories are based ; these
we may just glance at, without mentioning the special difficulties which arise
in comparing the oily foam with protoplasm. PFEFFER (1890) has shown that
protoplasm enclosed in a cell-wall is generally in a liquid condition, but that in the
plasmodia of Myxomycetes a very obvious cohesion occurs in the passive external
layer. Stout strands of Chondrioderma may be subjected to a weight of 60 mg.
per sq. mm., but these strands recoil to their original length on removal of the
weight ; no permanent stretching takes place. Since, obviously, the external
passive layer has to withstand this pull practically by itself, PFEFFER calculated
that its tensile strength amounts to 300 mg. per sq. mm. When we remember
that, in order to tear asunder a lead filament of similar transverse area, a weight
of about 2 kg. is necessary, we see that the protoplasm of the Myxomycetes must be
a very delicate substance. All the same the cohesive force thus demonstrated
proves to us that we are not dealing with a genuine liquid. The cohesion in
the peripheral regions is further shown by PFEFFER' s (1890) observation that
vacuoles, when carried by the current through narrow channels in the plas-
modium, become deformed in consequence.

AUTONOMOUS LOCOMOTORY MOVEMENTS

539

This cohesion, which is at least noticeable in the surface layer, renders it
questionable whether we ought to regard it as liquid at all, and whether we may
therefore refer alterations in its form to surface tension only. The assumption
of an alternation in the condition of the protoplasm from a semi-solid to a liquid
state expresses most accurately our present knowledge of the subject. If
amoeboid movement is chiefly occasioned, as we are bound to believe, by surface
tensions, it must be pointed out at the same time that these tensions are doubt-
less initiated not by the environment but by the protoplasm itself. When the
medium in which a plasmodium lies is made perfectly homogeneous, movements
still go on in the plasma, and, conversely, a quiescent plasmodium remains un-
changed even when alterations in the medium are effected which are calcu-
lated to modify its surface tension very greatly (PFEFFER, 1890, p. 275).

As in the case of growth phenomena, so also in locomotory movements,
the environment plays a great part [compare EWART, 1903]. Many of these
external factors are the essential formal conditions, without which locomotion
cannot take place. These, or other factors, influence the direction of the move-
ment, so that we may divide locomotory phenomena into autonomous and induced.

The presence of a certain amount of water is one of the most important of
the formal conditions of locomotion, for it must be at once apparent that water
often acts as the medium in which the movement is carried out. Furthermore,
the protoplasm must also itself contain a certain amount of water of imbibition
in order that streaming or ciliary motion may take place. Rotation and circula-
tion, it is true, do not cease at once when the cell is plasmolysed, but the resting
condition of the peripheral layers may be observed with special clearness in such
plasmolysed cells. Ciliary movement also still continues in plasmolysed Bacteria,
but if a 5 per cent, to 10 per cent, solution of potassium nitrate be employed to
bring about plasmolysis a rigor sets in, which has been termed by A. FISCHER
(1894, p. 75) ' drought-rigor ', and which disappears when the water is again
replaced. Similar rigor phenomena have been observed by FISCHER in flagella,
when these are treated with certain substances, e.g. acids, or when there is
a deficiency in nutrients ; narcotics such as ether, as might be expected, produce
similar results. Similarly, observations on amoeboid movement show that it,
too, ceases when the protoplasm is affected by narcotics, weak ammonia, &c.

Among all the substances which influence movements the action of oxygen
is perhaps the most interesting. In many cases the presence of oxygen is an
absolutely essential condition for the performance of such movements, but that
applies only to aerobic organisms. The anaerobes referred to above cease to
move in presence of traces of oxygen, while facultative anaerobes exhibit move-
ments of very varying intensity when oxygen is withdrawn. There is thus no
essential interdependence between movement and growth, for some facultative
anaerobic Bacteria, according to RITTER (1898), grow very well without oxygen,
forming flagella which, however, move only when oxygen is present. Other
facultative anaerobes move, for a time at least, without oxygen and, if well
nourished, their capacity for movement is maintained for a much longer period
than if starved. Doubtless the energy required for this movement arises from
intra-molecular respiration, for the continuance of which the presence of sugar is
necessary. According to CELAKOWSKI (1898), Pelomyxa continues to move for
72 hours in absence of oxygen, Oscillaria for 24 hours, Char a for 18 hours, and
Elodea for 1-4 hours, while the protoplasmic movements in the staminal hairs
of Tradescantia come to a standstill at once when oxygen is withdrawn.
KUHNE'S researches (1898) on Characeae prove that even closely related organisms
behave very differently in this respect, for some species continue to exhibit move-
ments, employing intra-molecular respiration, only for hours, others for weeks.
A few organisms, e. g. the chromogenic Bacteria investigated by EWART (1897),
have the special power of fixing oxygen loosely and making use of this reserve

540 TRANSFORMATION OF ENERGY

when oxygen is absent from the environment. Whether or not organisms like
Thiothrix must be added to this category appears doubtful (WiLLE, 1902).

• . Since the dependence of movement on oxygen is so variable, it is needless
for us to attempt the determination of the limits of partial pressure of oxygen
permissive of movements in relation to individual organisms ; it is manifest
at once that each organism must have an optimum, a maximum, and minimum
<CLARK, 1888).

The influence of temperature is very striking and many estimates have
been made on the subject. The rate of rotation under different temperatures
has been accurately studied more especially. The following table, based on the
observations of VELTEN on the leaves of Vallisneria and condensed by SCHAFER
{1898) will illustrate this :—

i° 5° 10° 15° 20° 25° 30° 31° 32° 33° 34° 35°
002 0-06 0.12 0-20 0-26 0-32 0.42 0-43 0-40 0-30 0-17 o-ir

It may be clearly seen that the rate of movement increases at first rapidly
and then more slowly, until the optimum is reached at 31°, and that, thereafter,
it as rapidly falls; above 35 °C. heat rigor sets in. The minimum does not
always, as in this case, lie somewhere near zero ; in many terrestrial plants it
obviously lies much higher. The optima and maxima also may be considerably
higher, e.g. 40° and 50°, or even above these temperatures (HAUPTFLEISCH,
1892). The phenomena in this respect resemble those of growth, so that we
need not pursue the matter further. Similar results have been obtained from
the study of the motile swarmspores.

As far as light is concerned, it may be briefly said that its influence on
locomotion is indirect only, inasmuch as many organs develop very imperfectly
or not at all in the dark and hence, naturally, protoplasmic movement does
not take place in them. Further we know that intense sunlight is fatal both
to ciliary and protoplasmic movement. If, however, swarmspores or cells ex-
hibiting streaming protoplasm, previously exposed to diffuse light, are placed
in the dark, the movements do not appear to suffer any change.

..Recently JOSING (1901) has made some remarkable observations on this
point. Protoplasmic streaming ceases at once in the dark when the cells are
treated with ether or chloroform, with carbon-dioxide or non- volatile organic
acids, or salts, but the movements continue in presence of these substances
if the cells be illuminated. The same author has recorded additional observa-
tions as to the action of ether not less remarkable, but he was unable to give a
causal explanation of them.

Still less comprehensible is the well-established fact that protoplasmic
streaming may arise in cells which have been separated from the plant though
they show no such movements when incorporated in it (KELLER, 1892). [Com-
pare KRETSCHMAR, 1903.] In other cases an already existing protoplasmic
movement may be accelerated merely by wounding the cells. It is equally well
established that movement may occur in many cases where no injury whatever
has been inflicted (HAUPTFLEISCH, 1892). DE VRIES (1885) has pointed out that
this streaming effectively aids in establishing a uniform distribution of materials
in the cell, and that, too, much more rapidly than by simple diffusion, so facili-
tating the circulation of nutrients, &c., in the plant. Since, however, proto-
plasmic movement is not so widespread in the uninjured plant as DE VRIES
assumed, obviously such a circulation must be effected without the aid of proto-
plasmic movement. The further services rendered by streaming, such as the
shifting of the position of chloroplasts, we shall treat of in our next lecture. It is
impossible at present to pronounce an opinion as to whether the movements which
follow after or are accelerated by wounding are to be considered as purposeful

AUTONOMOUS LOCOMOTORY MOVEMENTS 541

reactions or not. The use of locomotion in free organisms is obvious on the
face of it ; where a fixed plant strives to attain a suitable environment and
to escape an injurious one by curvatures, free organisms do so by changing
their habitat. They frequently attain their object precisely in the same way
as in the higher plants, their direction of movement being determined by ex-
ternal factors (Lecture XLIII). Amoeboid movements in protoplasts enable
these organisms to absorb solid bodies, which they envelop, and so in their
case this movement may be regarded as of importance in nutrition.

Bibliography to Lecture XLII.

ADERHOLD. 1888. Jen. Zeitschr. f. Naturw. 22, 310.

DE BARY. 1864. Die Mycetozoen. 2nd ed. Leipzig.

BERTHOLD. 1886. Studien uber Protoplasmamechanik. Leipzig.

BUTSCHLI. 1892. Unters. uber die mikrosk. Schaume. Leipzig.

CELAKOWSKI. 1898. Bull, de 1'Acad. d. sc. de Boheme.

CLARK. 1888. Ber. d. bot. Gesell. 6, 277.

CORRENS. 1897. Ber. d. bot. Gesell. 15, 139.

ENGELMANN. 1879. Hermann's Handb. d. Physiologic, i. Leipzig.

EWART. 1897. Journal of the Linn. Soc. Botany, 33, 123.

[EWART. 1903. On Protoplasmic streaming. Clarendon Press, Oxford.]

FISCHER, A. 1894. Jahrb. f. wiss. Bot. 27, i.

FISCHER, A. 1901. Archiv f. Entwicklungsmechanik, 13, i.

HAUPTFLEISCH. 1892. Jahrb. f. wiss. Bot. 24, 173.

HOFMEISTER. 1867. Die Lehre v. d. Pflanzenzelle. Leipzig.

JENSEN. 1902. Ascher-Spiro, Ergebnisse d. Physiologic, i. Wiesbaden.

JOSING. 1901. Jahrb. f. wiss. Bot. 36, 197.

KELLER. 1890. Ueber Protoplasmastromung. Diss. Zurich.

[KRETSCHMAR. 1903. Jahrb. f. wiss. Bot. 39, 273.]

KUHNE. 1898. Zeitschr. f. Biol. 36, 425.

LAUTERBORN. 1896. Unters. ttb. Bau, Kernteilung u. Beweg. d. Diatomeen.

Leipzig.

MULLER, O. 1897. Ber. d. bot. Gesell. 15, 70.
NAGELI. 1860. Beitr. z. wiss. Botanik, 2. Heft, p. 96. Leipzig.
PFEFFER. 1884. Unters. bot. Inst. Tubingen, i, 363.
PFEFFER. 1890. Pasmahaut (Abh. Kgl. Ges. d. Wiss. Leipzig, 16, 185).
QUINCKE. 1888. Annalen d. Physik, N. F. 35.
RITTER. 1899. Flora, 86, 329.
SCHAFER. 1898. Flora, 85, 135.
SCHUTT. 1899. Jahrb. f. wiss. Bot. 33, 594.
STAHL. 1880. Bot. Ztg. 38, 393.

STRASBURGER. 1878. Wirkg. des Lichtes u. d. Warme auf Schwarmsporen. Jena.
VERWORN. 1901. Allg. Physiologic. 3rd ed. Jena.
DE VRIES. 1885. Bot. Ztg. 43, i.
WILLE. 1902. Biol. Centrbl. 22, 257.

LECTURE XLIII
LOCOMOTORY DIRECTIVE MOVEMENTS

IN treating of locomotory directive movements we will confine our atten-
tion at first exclusively to non-fixed free-moving organisms (Flagellata, Bacteria,
Myxomycetes) and consider protoplasts enveloped in cell- walls later. Just as
the directive movements in fixed plants have been spoken of under the general
term of ' tropisms ', so the word ' taxis ' has been employed to indicate locomo-
tory directive phenomena ; thus we speak of geotaxis, phototaxis, chemotaxis,
&c., according as the direction of the movement is induced by gravity, light,

542 TRANSFORMATION OF ENERGY

chemical substances, &c. Among these varied phenomena those of chemotaxis
have at present been undoubtedly studied most thoroughly (PFEFFER, 1884 and
1888), and hence it will be most appropriate to begin with them.

One special example of chemotaxis, viz. aerotaxis, has long been known,
and we have already become acquainted with the directing influence of small
amounts of oxygen on motile Bacteria in discussing the elimination of oxygen
by green plants during carbon-dioxide assimilation. Bacteria react to many other
substances as well as to oxygen, seeking, so to speak, an optimum concentra-
tion of them and retreating from them when their concentrations are too high
or too low. Chemotactic movements are thus often purposeful, since they are the
means of bringing the organism under optimum vital conditions, but reactions
which are apparently purposeless are not unknown, reactions, for example, to
substances which the organism is not in the habit of encountering in nature,
and to which it is unable to adapt itself. ROTHERT (1901, p. 382) has observed
a case of this sort, where two Bacteria were attracted in a remarkable manner
by ether, a substance which could only be injurious to them. Again, note
must be taken of the fact that, as PFEFFER has observed, many chemotactic
organisms are often unconscious of the presence of such poisons as corrosive
sublimate or strychnine nitrate, while they run away from such injurious agents
as acids, alkalis, &c. Such exceptional behaviour, naturally, does not enter
into the question of the biological significance of chemotaxis, which we must
assume has always to do with the attainment of optimum environmental con-
ditions. Chemotaxis, however, fulfils a far more special role than this in relation
to the sexual cells of differentiated lower organisms and of higher plants also.
The marked chemotaxis of the male cells leads them to seek the female cell, and
fertilization is secured by the attraction exerted on the sperm by certain sub-
stances present in the ovum or excreted by it or by parts in its immediate
neighbourhood. It had been long suspected that the sperm did not reach the
ovum merely by chance but by the attractive influence of certain substances,
but exact proof of this was first afforded by PFEFFER (1884), who showed that it
was probable that, in the case of the ferns, malic acid excreted from the archego-
mum was the attractive agent in bringing the sperm into its vicinity.

If antherozoids of ferns be observed under the microscope in a drop of
water, they will be seen to move generally in straight lines ; whenever they come
into the neighbourhood of an archegonium, however, they twist sharply round,
so as to direct their anterior ends towards the mouth of the archegonium ; they
thus rapidly approach it, enter its neck and fuse with the ovum in the interior.
In his experiments PFEFFER replaced the neck of the archegonium by a capillary
glass tube of about o-i mm. bore, which he filled with various substances, placing
it at the side of the cover-glass. When the tube was filled with a 0-01-0-5 Per
cent, solution of malic acid, which was neutralized by appropriate means, PFEFFER
found that the sperms rushed towards it and entered it in great numbers. He
estimated that 60 sperms had managed to enter such a tube in 30 seconds and
600 of them in 5 minutes. If the movement of the sperms be retarded by using
a weak solution of gum it may be clearly seen that the sperms curve round
sharply the moment they come within the sphere of influence of the malic acid
diffusing out of the tube, and place their long axes parallel with the course of the
diffusion current. Without any acceleration of their movements they then
steer their way towards the more concentrated solution, straight for the opening
of the capillary tube. Since the sperms distribute themselves equally in a homo-
geneous solution of malic acid, just as they do in water, we are bound to regard
the unequal distribution of the acid as the directive stimulus. The fact that
numerous other substances have no power to induce such directive movements,
however, leads us to the conclusion that the diffusion current, as such, is not
the actual stimulus.

LOCOMOTORY DIRECTIVE MOVEMENTS 543

Taking into account the facts we have already learned as to stimuli, we
are led in the present instance to seek for the liminal intensity of concentration
of malic acid which will give an obvious attractive result. In PFEFFER'S experi-
ments that concentration was found to be o-ooi per cent. ; still weaker solutions
had only a casual effect. As might be expected, the liminal value altered with
the age of the organism, just as in the case of other factors, e. g. temperature
( VOGLER, 1891). In fact, extraordinarily minute quantities of various substances
may operate as chemical stimuli. In one of PFEFFER'S experiments the tube
contained not more than 0-0000000284 mg. of malic acid, and of that naturally
only the minutest fraction could come in contact with a single sperm. Such
an amount is, however, by no means so insignificant when the weight of the
sperm is taken into account ; still if the weight of the sperm be estimated at
about 0-00000025 mg- it is only ten times as great as that of the malic acid which
was used. Quite as minute quantities of other chemicals are able to induce
reactions, for DARWIN (1876, p. 246) found that 0-00000328 mg. of ammonium
phosphate excited a response in Drosera, and ENGELMANN (1883) showed that
a trillion th part of a milligram of oxygen could act as a stimulus to Bacteria.

It has already been pointed out that fern antherozoids distribute them-
selves uniformly in a homogeneous solution of malic acid, but it would be quite
incorrect to suppose that the sperms were not stimulated by such a solution ; it
renders them, as a matter of fact, less sensitive to a unilateral action of the acid
and the liminal intensity of the stimulus is higher for sperms swimming in such
solution than for those swimming in pure water. Moreover, the increase in
the liminal intensity is quite proportional to the concentration of the solution in
which they lie. In water o-ooi per cent, of malic acid is required to induce
a stimulus.

In 0-0005 % malic acid, 0-0015 % is required to effect a stimulus
„ oooro% „ o.o3o% „ „ „

>> °-01 % » °-3 % ?> >» »

>> 0>°5 % » J-5% » » »

It will be seen that the liquid in the capillary tube must always be 30 times as
concentrated as the culture liquid, and hence the absolute difference in concentra-
tion necessary to induce a response is much greater when the culture solution
is concentrated than when it is weak. We have already become acquainted
with the law which governs the relation of sensation to stimulation, a law which
was first established with reference to certain human sensitivities and is known
as WEBER'S law or the law of psychophysics. So far as regards our own special
sensitiveness to weight, for instance, a weight of I mg. must be increased J,
a weight of 10 mg. must be increased ^ before we can appreciate a difference
between them. This law, it would appear, is of very general, but by no means
universal, application. It is limited to certain concentrations, for instance,
in the case of fern antherozoids, and does not apply to very high or very low
concentrations. A capillary tube containing a 0-0003 per cent, solution of malic
acid can attract sperms swimming in a o-ooooi percent, solution, and yet, as we
have seen, this solution, after diffusing into water, is unable to act as a stimulus
on the sperm ; on the other hand, PFEFFER was unable to attract sperms float-
ing in a 0-04 per cent, solution by concentrations either 30, 40, or 50 times as great.
In the last mentioned experiment PFEFFER found that the sperms were quite in-
different to a 3-4 J per cent, solution. No end was to be gained by raising the
concentration higher than that, for 5 per cent, solutions were found to act quite
obviously in a repellent manner. Of this phenomena we shall speak later on.

According to PFEFFER'S experiments, fern antherozoids react only to malic
acid and its salts as also to maleic acid (which does not occur in nature), but not
to its stereoisomer fumaric acid. The reaction is so specific that antherozoids

544 TRANSFORMATION OF ENERGY

it was thought might be employed for determining the presence of malic acid.
More recent investigations (BuLLER, 1900) have certainly introduced important
limitations. Not only malic acid but also a large number of organic and in-
organic salts operate attractively, but to all the non-dissociating bodies, such
as carbohydrates, glycerine, alcohol, asparagin, the sperms are quite indifferent.
The attractive power of the salt is manifested to the best advantage generally
when it is in a solution isosmotic with a o-i G.M. of potassium nitrate ; i G.M.
for the most part is injurious and o-oi G.M. is only rarely effective. Malates,
on the other hand, act in much weaker concentrations, from o-oi G.M., down to
o-oooi G.M., and free malic acid from o-ooi G.M. to o-oooi G.M. Since it is in
the highest degree improbable that solutions as concentrated as these are dis-
charged from the archegonium (that is to say equal to a i per cent, solution
of potassium nitrate, or of a 1-24 per cent, solution of potassium oxalate), it is
very likely, but not absolutely certain, that the antherozoids of ferns are at-
tracted by malic acid in the ovum itself, where it probably exists, as PFEFFER
has given us reason for believing, not as free malic acid but as a salt of that acid.

As already remarked, there are also substances which act repulsively on
antherozoids. If the repulsion is brought about by high concentrations, as
in the case of malates, we have to deal rather with an osmotic than a chemical
influence, and hence we may draw a comparison between osmotaxis and chemo-
taxis (compare p. 547). Many substances, however, such as alcohol, acids,
alkalis, &c., doubtless repel owing to their chemical peculiarities, and, when
they do operate, they always do so repulsively only. Other substances, such
as free malic acid, attract when in weak concentrations (o-ooi G.M.) and repel
when in somewhat stronger concentration (o-i-o-oi G.M.). It is not yet clear
whether the individual ions operate in a different way, as BULLER thinks.

The antherozoids of Selaginella react to malic acid exactly in the same way
as do those of ferns, but the specific attractive stimulant has not as yet been
determined in the higher Pteridophyta ; although such a body is doubtless
present there also. [We now know that in all the Pteridophyta the chemotactic
stimulant is malic acid (SHIBATA, 1905). Still, interesting differences among
these have been discovered, for the sperms of Equisetum are sensitive to malic
acid only (SHIBATA, 1905 c), while those of the Filicinae (SHIBATA, 1905 b) respond
to maleic acid as well, but not to fumaric acid ; the antherozoids of Isoetes,
on the contrary, are sensitive to fumaric but not to maleic acid.] Among
Bryophyta we know as yet of such attractive chemicals only in the mosses.
The remarkable thing in this case is that the attractive substance appears to be
cane sugar, a non-dissociating compound. [LiDFORS (1905) affirms that pro-
teids are the chemotactic agents in Marchantia.] The antherozoids of mosses
are in no sense inferior, so far as sensitivity is concerned, to those of ferns, for
the liminal stimulus intensity was determined by PFEFFER in Funaria as
a o-ooi per cent, for sugar solution.

Chemotactic movements have also been recognized in Bacteria, Flagellata,
and in the swarmspores of Saprolegnia (PFEFFER, 1888 ; STANCE, 1890), as the
result of the action of various, but not all, materials nutrient to these organisms.
Among inorganic bodies, potassium salts and phosphates are effective, among
organic substances, peptone and asparagin but not glycerine. At the begin-
ning of this lecture we drew attention to the fact that oxygen had also a
chemotactic effect ; we may now add that it acts in a strongly repulsive
manner to certain anaerobes, although, it must be remembered, other bodies
also act in the same way. Many specific differences may be noticed in this
relation ; certain sulphur-Bacteria, for instance, are attracted by sulphuretted
hydrogen, a substance which, under ordinary circumstances, has never that
effect on ordinary motile cells (MiYOSHi, 1897). WEBER'S law, as might indeed
have been expected, has been found to apply to certain other cases also. There

LOCOMOTORY DIRECTIVE MOVEMENTS 545

would have been for all that no need for us to enter into any further discussion
of chemotaxis in such organisms, if ROTHERT (1901) had not recently drawn
attention to a feature which had escaped previous observers.

Observation of the slow movements of large Bacteria (such as B. solmsii) in
the vicinity of a capillary tube filled with meat extract shows that although the
Bacteria pggregate round it, they exhibit no alteration in the direction of their
movement when they reach the zone of diffusion ; on the contrary, they often pass
close to the opening of the tube, maintaining their original direction of move-
ment, and not being apparently stimulated to enter it. At a certain distance from
the mouth of the tube they suddenly stop and swim backwards (posterior end
foremost). Once more they pass the opening of the tube, unaffected by its
contents, and again halt at the same distance from it as in the first instance,
and again proceed to carry out a forward movement. They are to be met with
in a definite zone opposite the centre of the opening of the tube, but their pre-
sence there is obviously purely accidental, and the effect of the stimulus is not
one of attraction but of repulsion, induced by the transition to lower concentra-
tions, the response consisting, not, as in fern antherozoids, in a reversal of the
anterior end, and in a consequent change in the direction of the movement, but in
locomotion backwards. After more careful investigation, ROTHERT found that
there were greater differences between these two kinds of response than might
at first sight have been imagined, for they have only one feature in common,
viz. the nature of the stimulant (a chemical compound), while not only the
response but also the motive cause of the stimulus and the sensitivity as well is
different. It is advisable, therefore, that these two movements should be known
by different names, and hence ROTHERT describes the movement where the
body of the organism is inverted as strophic chemotaxis, and that where the
organism swims backwards as apobatic chemotaxis.

Wherein then lies the difference in the motive cause of the stimulus in
these two series of phenomena ? PFEFFER and ROTHERT look upon the strophic
chemotaxis as due to differential distribution of the stimulant on different sides
of the plant, as in the case of chemotropism, heliotropism, &c. ; in other words,
the organism measures and compares the intensity of the stimulant as effecting
different parts of its outer surface. It is impossible, however, that a fern
antherozoid can appreciate the inequality of the stimulus on opposite sides,
since, owing to its rotation on its own axis, any unilateral influence of the
stimulant is excluded, just as when a higher plant is rotated on a klinostat.
(This criticism, which has not as yet been published, has been communicated
to me by OLTMANNS.) The organism must also be able to compare the inten-
sity of the stimulant at its anterior and posterior ends, and, on the analogy
of a dorsiventral body, must not be in a state of equilibrium, when both ends
are subjected to the same intensity of stimulus, but only when the intensity of
the stimulus is greater at the anterior end ; this will be the case at least as
long as positive chemo tactic movement follows, in negative chemotaxis the
inverse relations hold good.

In apobatic chemotaxis it is possible that the motive cause of the stimulus
lies at least in differential concentration of the stimulant at the two poles of the
organism, but it is more probable that we have here to do rather with differences
in time than differences in place, and that response occurs when the organism
has remained for a definite length of time in a solution of the stimulant less
concentrated than that in which it was a short time previously. A homo-
geneous solution, therefore, must in this case also act as a stimulus, and the
bacterium will move backwards if it be transferred from a 10 per cent, solution
of meat extract to one of 5 per cent. JENNINGS (1897 and onwards) has, as a
matter of fact, shown this to be the case in motile Infusoria (Paramoecium), but
Bacteria present experimental difficulties too serious for accurate investigation.
OST N n

546 TRANSFORMATION OF ENERGY

This difference in sensitivity between apobatic and strophic organisms,
discovered by ROTHERT, does not surprise us, since we have already met with
similar phenomena in studying the movements of organs in fixed plants. Ob-
viously, the sensitivity of strophically reacting organisms corresponds exactly
to that associated with tropisms (in the true sense of the word), while apobatic
reactions resemble nastic movements (heat curvatures of tendrils, sleep move-
ments, &c.).

It is to be hoped that ROTHERT' s observations on the difference between
apobatic and strophic chemotaxis may soon lead to detailed experimental
investigations on the subject ; the field for such inquiry is a wide one, and there
are still many related problems which are very obscure and of which we can
present no explanation here. There is only one point to which we may draw
attention, viz. the difference between positive and negative taxis, for in this
respect also strophically and apobatically reacting organisms are not alike. So
far as strophic organisms are concerned the difference between positive and nega-
tive taxis lies only in the reaction, the motive cause of the stimulus is the same
in both cases and is due to the diffusion of the stimulant into the water. The
case is different with apobatic organisms, where the reaction is always the same
(a retreating movement) ; whether a positive or negative taxis takes place,
depends only on the motive cause, being positive when the concentration
of the stimulant decreases and negative when it increases. The position of the
optimum of the stimulant probably determines whether the decrease or the
increase in concentration will induce stimulation. Strophic organisms actively
seek this optimum, for they turn their bodies sometimes in one direction, some-
times in another, towards it ; apobatic organisms, on the other hand, are sensi-
tive, not to the approach to but only to the withdrawal from the optimum and,
in the latter case, retreat from it. It is not impossible that the same organism
may exhibit both types of taxis at the same time. [This has been shown to
be the case in the sperms of Isoetes by SHIBATA (1905 b), and it must be true
of fern antherozoids also.]

The existence of the optimum is especially strikingly manifested if the
concentrations in any one preparation are greatly diversified, for then the
motile organisms congregate at a definite place where the optimum concentra-
tion prevails. Aggregations such as these are shown by certain Bacteria (Spi-
rillum : ENGELMANN, 1881 ; BEIJERINCK, 1893 ; Beggiatoa : WINOGRADSKY,
1887), which seek regions where there is low oxygen tension and which are
negatively aerotactic to high tensions and positively aerotactic to low tensions.
Many instances of aggregation of motile organisms at definite distances from
the mouth of the capillary tube have been demonstrated by PFEFFER and others
(ROTHERT, 1901, p. 402). PFEFFER draws special attention to the case of Spi-
rillum undula, which may frequently show both positive and negative response
to the same stimulant, and, in so far as the action of this stimulant is purely
chemical, the difference in the results can be due naturally only to differences in
concentration. If, however, a definite concentration can act at the same time
both attractively and repulsively, negative osmotaxis must be allowed to be
always operating in addition to positive chemotaxis. The fact that not every
stimulant induces positive and negative chemotaxis leads us to conclude that the
optimum for many substances is close to zero, while in the case of others it is very
high ; in the former case we observe that the stimulant has always a repulsive
influence, but, in the latter case, the organisms are always attracted or are
indifferent.

We have hitherto spoken of chemotaxis in general and considered the
directive influence both of substances in solution and of gases. The question
we have now to answer is whether a gaseous stimulus operates in the same
way as a solution of a solid body and whether the sensitivity to different gases

LOCOMOTORY DIRECTIVE MOVEMENTS 547

or solutions depends on like or unlike alterations in the protoplasm. When we
remember that aerotactic organisms often exhibit no chemotactic sensitivity,
and that fern antherozoids are not aerotactic, we must obviously regard aero-
taxis and chemo taxis as perfectly distinct sensitivities. Similarly, organisms
which are sensitive, for instance, to potassium salts are not necessarily sensi-
tive to oxygen. In short, it is probable that we must assume that there are
as many types of chemotaxis as there are chemical substances, or groups of
substances, distinguished by the organism. ROTHERT ( 1901), in fact, has shown,
in the case of a species of Amylobacter, that chemotaxis in relation to two
different substances may indicate a difference in sensitivity in the organism, for
he found that this species was chemotactically sensitive both to ether and to
meat extract. If both types of taxis are dependent on the same sensitivity,
according to WEBER'S Law, the liminal intensity of the stimulus for unilateral
action of meat extract will be raised by a homogeneous solution of ether. That
is not the case, however. Further investigations on this subject will lead us, no
doubt, to extremely important and interesting results as to the powers possessed
by organisms for distinguishing between different chemicals and the limits of
these. [SHIBATA (1905 b) has, with the aid of this method, been successful in
showing that it is possible to decrease the sensitivity of the antherozoids of
Isoetes to malic acid by using homogeneous solutions of fumaric acid. The
effects produced by these two substances on the perceptive apparatus are the
same, in other words, the antherozoids could not distinguish between these two
substances. On the other hand, the sensitivity of the antherozoids to salts of
the potassium group, e.g. of potassium, rubidium, &c., is quite different (BuiXER,
1900 ; SHIBATA, 1905 b).]

What is the exact nature of the first effect of the chemotactically active
body and on what the chemotactic perception depends, is as yet entirely un-
known. We are ignorant whether the cilia only are the perceptive organs — as
is possible ; if so, then, in a strophic reaction, it is obviously due to dissimilar
concentrations of the solution affecting opposite sides of the cilia. Since, how-
ever, the cilia must by their movements neutralize differences of concentration
in the fluid, this view does not appear to us to be correct and we prefer to adhere
to the hypothesis already formulated, viz. that it is the difference in concentra-
tion, anteriorly and posteriorly, that is appreciated. Further, we are ignorant,
whether it is necessary that the stimulant must actually enter the organism
before a chemo tactica stimulus can be produced, since, as PFEFFER (1888) has
shown, it may also operate by contact, i. e by merely striking against the pro-
toplasmatic layer. It is more probable, however, that these bodies enter the
cell and induce chemical changes in its interior.

It has been already several times pointed out that the osmotic pressure
of the solution acts as a stimulus on motile organisms (osmotaxis). Proof of this
fact has been advanced by MASSART (1889). If Spirillum undula and Bacterium
megatherium be submitted to the attractive influence of a very dilute (0-0005 G.M.)
solution of potassium carbonate placed in a capillary tube, the positive chemo-
taxis may be counteracted by the addition of various substances, and it would
appear that the repulsive effect then depends only on the osmotic pressure of
these bodies and not on their chemical constitution. Materials with an isosmotic
coefficient = 3, such as ammonium chloride, sodium chloride, potassium chloride,
&c., initiate a repulsive reaction when the concentration =0-07 G.M., while sub-
stances with 4 as their coefficient induce it at a concentration of 0-05 G.M. to
0-06 G.M. Exceptions are undoubtedly known, but these may be readily ex-
plained. Since, e.g., potassium and sodium oxalates or potassium cyanide act
repulsively at all concentrations which have been experimented with, the action
is not osmotic but chemical. The same result takes place when certain good
nutrients act as attractive agents, even when in a high state of concentration,

N n 2

548 TRANSFORMATION OF ENERGY

yet the absence of any repulsive reaction in the case of such substances as
glycerine or urea may be due to their known power of penetrating the pro-
toplasm rapidly. An osmotactic effect is absolutely correlated with the imper-
meability of the protoplasm to the substance under consideration, while its entry
is perhaps essential to a chemotactic response. We shall not go far wrong if
we look upon the withdrawal of water as the main agent in inducing perception
in osmotaxis.

WEBER'S law applies to osmotaxis almost as well as to chemotaxis. In
experiments with Spirillum undula it may be clearly shown that as the osmotic
pressure of the culture fluid increases the liminal intensity of the stimulus
necessary for inducing osmotactic repulsion rises. Osmotactic repulsion is
induced,

in a normal solution by 0-07 G.M. of sodium chloride

„ „ +0-03 G.M. of sodium chloride by 020-0-25 „ ,,

„ „ +006 „ „ „ 0-25-030 „ „

„ „ +009 „ „ „ 0-40-045 „ „

In addition to negative osmotaxis there is also a positive osmotaxis in
organisms whose natural habitat is a concentrated medium, and to which they
are adapted. This has been shown to be the case by MASSART (1891 a) in cer-
tain marine Bacteria which exhibit positive osmotaxis. The significance of
osmotaxis is closely related to that of chemotaxis, for both sensitivities serve
to bring the organism under optimum vital conditions or to retain them there.
Many lower organisms are known, however, which are positively chemotactic in
highly concentrated fluids, and they collapse at once after being placed in them,
as a result of osmotic activity ; osmotactic phenomena are not manifested by
these forms at all.

Chemotaxis and osmotaxis are also well illustrated in the plasmodia of
Myxomycetes. The fundamental facts were first established by DE BARY (1864)
and STAHL (1884), still a systematic revision of the phenomena from the point
of view gained by similar studies on Bacteria and sperms is as yet non-existent.
Since in this case we have to deal not only with alterations in directive move-
ments pure and simple under the influence of chemical stimuli, but at the same
time with alterations in the form of the plasmodium, we will not discuss the
phenomena presented to us further, but merely note that sensitivity in the
Myxomycetes is special in character and dependent on their peculiar habit and
relation to a solid substratum. The plasmodia are hydrotactic, i. e. they seek
situations where a certain amount of moisture prevails or avoid dry substrata.
Hydrotaxis is probably very closely allied to osmotaxis. In both series of
phenomena it is the withdrawal of the water that leads to perception, and
perhaps it is immaterial to the organisms whether the withdrawal is brought
about by osmotic activity or by transpiration.

Hydrotaxis in Myxomycetes is also known to be closely related to rheotaxis
QONSSON, 1883 ; STAHL, 1884), a fact which may be easily shown by allowing
water to flow off from a vertically placed medium, e. g. filter paper, when the
plasmodium moves upwards in the opposite direction to the current. Analogy
with rheotropism suggests to us that the motive cause of the stimulus is to be
sought for in the mechanical action of the water, that is to say, in the impact
of the fluid on the plasmodium. This fact leads us to mention that ' hapto-
taxis ' (thigmotaxis), a movement induced by contact stimulus, is also to be
recognized as occurring among lower organisms.

Light and heat, like the chemical and physical properties of bodies, also
induce directive movements in motile plants, i. e. we must also recognize both
phototaxis and thermotaxis. We shall not go far wrong in regarding such
types of taxis as entirely analogous to chemotaxis in their nature, inasmuch as

LOCOMOTORY DIRECTIVE MOVEMENTS 549

they have for their object the placing of the organism in the most favourable
relationship to light and to heat. When the intensity of the heat or the light is
graduated, the organisms respond, becoming positively photo- and thermo-tactic
at infra-optimal intensities and negatively so at supra-optimal intensities.

There are but few data available as to thermotactic phenomena, so that we
may dismiss the subject in a sentence. Thermotaxis has been shown to occur
in certain Infusoria, Amoebae ( VERWORN, 1901, p. 473), and specially in Myxomy-
cetes. In the case of the last-named organisms STAHL (1884) has demonstrated
positive thermotaxis by placing one side of a plasmodium of Fuligo in water at
a temperature of 30° and the other in water of 7°. He believed that at higher
temperatures positive thermotaxis would change into negative, and this idea has
been confirmed by WORTMANN'S experiments (1885). The optimum tempera-
ture for Fuligo lies about 36°, above which negative thermotaxis ensues. Whether
or not a difference in the temperatures of the two sides of as much as 20° is
necessary has not been determined, but it is probable that it need not be quite
as much as that.

Phototaxis, and especially the negative form of it, has also been shown to
occur in plasmodia, but the phototactic influence of light has been much more
thoroughly studied in relation to swarmspores, whose aggregation on the bright-
est sides of vessels placed in diffuse light has long been known. Phototaxis is
best seen in the swarmspores of Algae, but it occurs also in the colourless swarm-
spores of Chytridium, Polyphagus, &c. ; it is not exhibited, however, by fern
antherozoids. STRASBURGER (1878) has also shown that in many cases the
phototactic reaction is dependent on the intensity of the light. If a vessel con-
taining swarmspores be placed at a certain distance from a window, it will be
seen that as a rule they arrange themselves so that their long axes are parallel
with the path of the incident rays, and with their anterior ends facing the light.
Further, the spores proceed to make for the most illuminated spot, moving in
straight lines. If the vessel be brought gradually closer and closer to the window
the light becomes at length so intense as to cause the swarmspores to retreat
from it. Obviously a certain optimum intensity exists between these two inten-
sities, the attainment of which explains the phototactic movement. OLTMANNS'
(1892) experiments on this subject are very instructive. He placed motile
colonies of Volvox in light of very varied intensity and noticed that they always
strove to place themselves where they would be subjected to light of a definite
intensity. Not all the colonies, however, behave precisely in the same
manner ; the light requirements of each colony, or — as one might say — its
disposition with regard to light varies with its developmental condition. The
female colonies, in OLTMANNS' experiment, placed themselves under much feebler
illumination than the asexual specimens, where they effected movements which
were extremely peculiar but as yet inexplicable. External influences play also
an important part in this light disposition. When the illumination is con-
tinuous and bright, and when the temperature is at the same time raised, the
light requirements are greater, and the colonies migrate to where the light is
more intense (STRASBURGER, OLTMANNS).

The crowding together of swarmspores at one point inside a vessel exposed
to light of gradually changing intensity is to be ascribed to positive and nega-
tive phototactic movements, and also to indifference to that intensity to which
the region where the crowding occurs is exposed. Although in the case of
many swarmspores the region of indifference is by no means sharply defined, in
other cases it would appear that it is extremely restricted ; for STRASBURGER
observed positive phototactic swarmspores becoming negatively phototactic
quite rapidly as the light was increased without remaining for any length of
time in the indifferent condition.

As in the case of heliotropism, so in phototaxis, the question frequently

550 TRANSFORMATION OF ENERGY

arises whether it is the direction or the intensity of the light that has the
greater influence. There can be no doubt that, in general, phototactic move-
ments are carried out in a state of nature so that swarmspores place their
long axes parallel with the incident beam, and further we have every reason to
believe that the plant aims not at orientating itself in a definite direction to the
path of the rays but at placing itself under an optimum light intensity. The
only question is whether it is possible that experimental conditions may be
arranged under which no light rays pass from the brighter to the darker region
of the apparatus in which the experiment is conducted. According to OLTMANNS
it is possible (Fig. 170) to arrange that parallel rays of sunlight may fall at right
angles on the lateral wall-of a glass trough in which the swarmspores are moving ;
immediately in front of the wall exposed to the light he placed a prism filled with
gelatine in which indian ink had been dissolved. Under these circumstances
the light rays will fall on the glass trough parallel to each other (as indicated by
the arrows), but their intensity will gradually decrease from one end of the trough
to the other. If now phototactic organisms are uniformly distributed in the
water in the vessel they will all collect together on the illuminated wall only,
and there also distribute themselves uniformly. If they seek, however,
a definite intensity of illumination they must obviously move at right angles
to the direction of the incident ray. The result of this experiment is that we

invariably find an aggregation of swarmspores
_„ at a point where the light is of definite in-
tensity, viz. the optimum. Several criticisms
may be advanced, which tend to shake our
confidence in this experiment ; first of all,
OLTMANNS has not arranged that the sunlight
should fall horizontally and at right angles to
the prism, but has allowed it to fall on the
Fig. 170. nan of OLTMANNS' experi. Darkened side wall in its natural direction, and

mental apparatus. 67, vessel containing henCC the distribution OI light intensity and

r fS ;ar££ ISSJIK the direction of the rays in the culture vessel

direction in which the light rays fall on the have been Somewhat Overlooked J but 6V6n if
vessel, and their size indicates the intensity .-, • , i_ • j , • j r j

of the light. the experiment be carried out in the way de-

scribed all difficulties would not thereby be

removed. If the light-absorbing prism be quite homogeneous, and if there be
an empty space behind it, then certainly our supposition as to the direction of
the rays and the distribution of the light is correct, but in the prism itself, in
the glass, and finally in the water in which the organisms are distributed as
well, we always find reflection of light, and hence the experiment becomes
perfectly useless for the purpose intended (TowLE, 1901) (compare p. 472).

There are not only physical difficulties to be considered but physiological
difficulties as well. ROTHERT'S (1901) observations on strophic and apobatic
tactic movements apply naturally not to chemotaxis alone but to all forms of
taxis. Apobatic phototaxis we have long been acquainted with ; ENGELMANN'S
(1882) experiments with Euglena are in the highest degree valuable, for they
leave no doubt in our minds that these organisms recoil when passing from light
into darkness. An isolated spot of light with a dark surrounding operates at
once upon them ; other and earlier experiments carried out by COHN (1852)
and FAMINTZIN (1867), and confirmed by STRASBURGER (1878), are unintelligible
without the assumption of apobatic phototaxis. In these experiments Euglena,
Stephanosphaera, Haemato coccus, &c., were placed in shallow dishes which were
exposed to direct sunlight, and which were therefore illuminated uniformly ;
if a narrow plate is then laid transversely across the vessel the motile organisms
rapidly assemble in the half-shaded places, leaving the regions of greatest
shade and also those on which the direct sunlight falls. It is quite impossible

LOCOMOTORY DIRECTIVE MOVEMENTS 551

that swarmspores illuminated on all sides by light of uniform intensity can have
any knowledge of the fact that there exists a region at a certain distance from
where they happen to be at the moment where the intensity of the light is more
adapted to their requirements ; it is only by chance that they reach that situa-
tion, where they remain on account of apobatic phototaxis. It appears to us
in the highest degree probable that Volvox also is apobatically phototactic,
although at the same time we are not prepared to deny that it is also strophically
so. Further investigations must be carried out upon the subject so as to deter-
mine in how far light intensity and the direction of the rays affect phototactic
movements.

From what we have learnt in regard to heliotropism we have the right to
expect that all wave lengths have not the same value in phototaxis ; in fact,
experiments designed for this very purpose have shown that the more re-
frangible rays have obviously much greater phototactic influence than the less
refrangible.

As to the primary physical or chemical effect of light leading to perception
nothing at all is known ; nor is the region where light perception occurs suf-
ficiently accurately determined. As in the case of chemotaxis so in the case
of strophic phototaxis we must assume that the organism reacts to differential
illumination of the anterior and posterior ends, but that in apobatic taxis locali-
zation of light perception is possible at the anterior end. It is known that in
many swarmspores, &c., there appears in the otherwise colourless anterior ends,
a red spot which has been termed the 'eye-spot,' and to which sensitiveness to
light has been attributed, but there are certain phototactic swarmspores which
possess no such spot, and hence it would appear very improbable that it has
any significance in relation to light perception.

There are also certain free-moving organisms which exhibit a directive
response to the galvanic current (galvanotaxis) ; this has been shown to be true
especially of Amoebae and Infusoria, although there is evidence of it also in the
higher animals (VERWORN, 1901, 476) ; it is probable that similar phenomena
will be discovered in typical plants. Amoebae and Infusoria, general speaking
place themselves so that their long axes lie in the direction of the current, creep-
ing or swimming towards the negative pole ; certain Flagellata behave exactly
in the contrary way, aggregating round the positive pole. It is very probable
that galvanotaxis is not due to a special sensitiveness on the part of the organism
to the electric current itself, but rather results from the chemical decomposi-
tion which the current gives rise to. According to LOEB and BUDGETT (1897)
free alkali arises at the anode end of the organism, and this induces negative
chemotactic movements towards the kathode. How it is that some Flagellata
move towards the anode remains yet unexplained.

Finally, we have still to speak of geotaxis, which has been demonstrated as
occurring in Bacteria, Flagellata, &c., by SCHWARZ (1884), ADERHOLD (1888),
and MASSART (1891 b). Many organisms, when other attractive forces are
excluded, move upwards or, in other words, are negatively geo tactic. MASSART
found in the case of two species of Spirillum which were equally sensitive tono-
tactically and aerotactically, that the one was positively and the other nega-
tively geotactic. Whether, in the case of geotaxis, we have to do with some
sensitivity which may be associated with geotropism appears to us very proble-
matical ; for geotactic upward movement cannot bring the organism under
conditions where the influence of gravity is different from what it was before,
while a phototactic movement is capable of placing the organism in other light
intensities, just as a chemotactic or osmotactic movement brings the organism
into liquids of different concentration.

Still it must be extremely useful to an organism which lives under definite
vital conditions to be able to reach more superficial or deeper layers of a fluid

552

TRANSFORMATION OF ENERGY

medium. We are led to believe, therefore, that such an organism must possess
the power of appreciating how deep it is in the medium ; such a sensitiveness
cannot arise in any way from the direct action of gravity but it may be due to
its power of appreciating the pressure of the liquid upon it. In fact JENSEN
(1893) has endeavoured to refer geotaxis to perception of this pressure, although
he was unable to offer exact proof of the truth of his hypothesis.

Tactic movements are not limited only to free organisms ; they occur also
in protoplasm enclosed within cell-walls, and especially in certain organs of the
cell, such as chloroplasts and nuclei. In the former case we are acquainted with
remarkable phototactic movements, in the latter the movements are trauma-
totactic arising especially after wounding. The movement of chloroplasts may
be best studied in Mesocarpus ; only one chloroplast, which takes the form of a
flat, rectangular band, is present in each of the cylindrical cells of this alga.

In Fig. 171, /, the band is shown, in a transverse section of the cell, in the
position which it takes up when subjected to light of medium intensity ; the
chloroplast behaves itself under these conditions just like a heliotropic leaf
placing itself at right angles to the incident ray, and hence, presenting its greatest
surface to it. If the intensity of the light increases, however, the chlorophyll
plate twists round through an angle of 90° and finally presents its edge to the
light (correspondnig to the profile position of the foliage-leaf). Experiments in

illustration of this fact may be readily carried out ;
they have been favourite lecture experiments ever
since the phenomenon was first described by STAHL
(1880). Nevertheless there are many important
details with regard to these movements which have
not yet been explained, for instance, is the chloro-
phyll itself active ? in answer to which it must be
admitted that motile organs in connexion with the
chloroplast have not yet been discovered. Does
it move passively ? How comes it about then that
the active protoplasm twists the chlorophyll plate
only until it reaches the desired position ? Were
it simply the case that the protoplasm lying on
the side previously receiving the most light moved away from that side as the
intensity of the light increased and passively carried the chloroplast with it,
then the whole phenomenon would be perfectly intelligible. But as a matter
of fact the protoplasma touching one edge of the chloroplast must perform
a movement exactly opposite to that touching the other edge. It is quite
inexplicable how it is that after sufficiently long illumination the twisting of
the chloroplast is carried out in the dark only to the same extent as it would
be if continuously illuminated (LEWIS, 1898). How does the plate know when
it has turned through an angle of 90° ?

The phenomena concerned in the movements of many small chloroplasts in
a cell are more easily understood. Here also we meet with a surface and a profile
orientation but these arise not by simply twisting the chloroplast in situ but
by transferring them to more appropriate situations. The surface orientation
is attained when the chloroplasts arrange themselves over the illuminated wall
of the cell, the profile position when they lie parallel with the path of the incident
ray, and therefore at right angles to the surface of the cell exposed to light.
Fig. 172 represents a transverse section of the frond of Lemna trisulca, the arrows
indicating the path of incidence of the light rays. At T the chloroplasts are
represented in the superficial position which they assume in diffuse light, at S
they have taken up the profib position in direct sunlight. It must be noted,
however, that a third, or night orientation may be observed in which some of
the chloroplasts are in the profile, some in the superficial position, but where

Fig. 171. Diagrammatic trans-
verse section through a cell of
Mesocarpus, The chloroplast is re-
presented by the shaded band in the
middle; the arrows indicate the
direction of the light.

LOCOMOTORY DIRECTIVE MOVEMENTS

553

the external walls are always free of chloroplasts. This night position is not
found in all plants ; in many shade plants and aquatics all the chloroplasts are
superficially arranged in darkness. In plants which distinctly prefer bright
sunlight, the nocturnal profile position may be assumed on subjecting them to
light of relatively slightly diminished intensity. The day profile position also
occurs in various plants when subjected to light of varying intensity, an in-
tensity which is feeble in shade-loving plants and high in those which prefer
bright sunlight. Even in the cells of the same leaf differences manifest them-
selves, for the cells of the under-side of Elodea arrange their chloroplasts in the
superficial position before those of the upper side (MooRE, 1887).

The significance of the profile position in bright light is generally intelligible,
for this position enables the chloroplasts to ar-
range themselves in such a manner that they
may receive the exact amount of light they re-
quire, just as a leaf can by changing its position.
The nocturnal profile position has, however, yet
to be explained. [It is obvious that in produc-
ing the nocturnal position chemotactic move-
ments co-operate, and carbon-dioxide more es-
pecially must play a part in determining the
position of the chloroplasts, for it must accumu-
late on the inner and lateral walls of the cells and
be less apparent on the outer walls (SENN, 1904).]
How the two positions are arrived at in Meso-
carpus has already been noted ; we have no
information, however, as to whether the move-
ments are active or passive, although if they be
passive they are more readily intelligible in the
present instance.

It is impossible for us to enter more fully
into the discussion of certain other phenomena
presented by chloroplasts closely related to those
we have discussed, such as the aggregation of
the chloroplasts in the angles of the cells under
high intensities of light and their change of form
in palisade cells ; we must content ourselves
with observing that the external brighter or
darker green colour of the plant is often due to
changes of position of the chloroplasts.

Passive changes in position of the nucleus
may generally be observed whenever the proto-
plasm shows signs of vigorous movements, and
these movements are autonomous in their
nature. Induced movements and movements
towards a definite region take place after wound-
ing. TANGL (1884) was the first to show that injury to the epidermis of the
scales of the onion induced a movement of the nuclei towards the surface
which had been wounded ; in that region also the protoplasm tended to collect.
To NESTLER (1898) belongs the credit of having demonstrated the very general
occurrence of traumotaxis, but it is to NEMEC (1901) that we owe the most
thorough investigations on the subject, and especially the determination of the
rapidity with which the stimulus due to injury travels and the induced position
is replaced by the normal. For all the new facts on this subje t we must refer
our readers to NEMEC'S papers. MIEHE (1901) and KORNICKE (1901) have also
recorded remarkable observations as to the migration of nuclei. Under certain

Fig. 172. Transverse sections through
the frond of Lemna trifulca. ZJ position
of chloroplasts in diffuse light; 5", in
bright light ; N, at night. After STAHL,
1880. From the Bonn Textbook.

554 TRANSFORMATION OF ENERGY

not very clearly defined but still anomalous conditions these authors observed
the transference of nuclei through apparently intact cell-walls into neighbouring
cells after injury had been inflicted. We mention these observations chiefly
because we have previously had occasion to refer to the subject. If it can be
shown that this transference of nuclei into quite normal cells is a physiological
reaction, the fact is one of great importance in many respects. Proof of this
is, however, not forthcoming, and it is, on the contrary, very probable that such
a transference of nuclei is quite pathological and perhaps passive in its nature.

Bibliography to Lecture XLIII.

ADERHOLD. 1888. Jen. Zeitschr. f. Naturw. 22, 310.

DE BARY. 1864. Die Mycetozoen. 2nd ed. Leipzig.

BEIJERINCK. 1893. Centrbl. Bact. 14.

BULLER. 1900. Annals of Botany, 14, 543.

COHN. 1852. Zeitschr. f. wiss. Zoolog. 4, in.

DARWIN. 1876. Insektenfressende Pflanzen (CARUS). Stuttgart.

ENGELMANN. 1881. Pfliiger's Archiv, 26.

ENGELMANN. 1882. Ibid. 29.

ENGELMANN. 1883. Bot. Ztg. 41, 4.

FAMINTZIN. 1867. Jahrb. f. wiss. Bot. 6, i.

JENNINGS. 1897 and onwards. Quoted by ROTHERT, 1901.

JENSEN. 1893. Pfliiger's Archiv, 53.

JONSSON. 1883. Ber. d. bot. Gesell. i, 512.

LEWIS. 1898. Annals of Botany, 12, 418.

[LIDFORS. 1905. Jahrb. f. wiss. Bot. 41, 65.]

LOEB and BUDGETT. 1897. Pfluger's Archiv, 65.

KORNICKE. 1901. Sitzungsber. niederrh. Gesell. Bonn.

MASSART. 1889. Archives de Biologic (van Beneden and Bambeke) 9, 515.

MASSART. 1891 a. Bull. Acad. Belg. 22.

MASSART. 1891 b. Ibid. 22, 158.

MIEHE. 1901. Flora, 88, 105.

MIYOSHI. 1897. Journal of College of Sc. Tokyo, 10, II, 143.

MOORE, SP. LE MARCHANT. 1887. Journal Linn. Soc. 24, 200.

NEMEC. 1901. Reizleitung u. reizleitende Strukturen. Jena.

NESTLER. 1898. Sitzungsber. Wiener Akad. Math. nat. Kl. 107, 1, 706.

OLTMANNS. 1892. Flora, 75, 183.

PFEFFER. 1884. Unters. bot. Inst. Tubingen, i, 363.

PFEFFER. 1888. Ibid. 2, 582.

ROTHERT. 1901. Flora, 88, 371.

SCHWARZ, FR. 1884. Ber. d. bot. Gesell. 2, 51.

[SENN. 1905. Dunkellage d. Chlorophyllkorner. Verh. d. schweiz. naturf. Gesell.

Winterthur.]

[SHIBATA. 1905 a. Bot. Mag. 19, 39.]
[SHIBATA. 1905 b. Jahrb. f. wiss. Bot. 41, 561.]
[SHIBATA. 1906 c. Bot. Mag. 19, 79.]
STAHL. 1880. Bot. Ztg. 38, 297.
STAHL. 1884. Ibid. 42, 145.
STANCE. 1890. Bot. Ztg. 48, 107.
STRASBURGER. 1878. Wirkung des Lichtes u. d. Warme auf Schwarmsporen.

Jena.

TANGL. 1884. Sitzungsber. Wiener Akad. Math. nat. Kl. 90, 1, 10.
TOWLE. 1900. Journal of Physiol. 3, 360.
VERWORN. 1901. Allgem. Physiologic. 3rd ed. Jena.
VOGLER. 1891. Bot. Ztg. 49, 641.

WlNOGRADSKY. 1887. Bot. Ztg. 45, 493.

WORTMANN. 1885. Ber. d. bot. Gesell. 3, 117.

INDEX

Absorption by insectivorous plants, 186.

AbsoiptiOii, in the soil, 92, 93 : of light, by
chlorophyll, 129; of heat, 398.

Acceleration of growth, in geotropism, 434 ;
in hap otropism, 493 ; in nyctitropism,
501 ; by stimuli, 300.

Accumu.ation of materials in cells, 20.

Acetic acid. Bacteria, 216 ; as a product of
fermentation, 216,218; oxidation of, 216,
218.

Acids, organic, see Organic acids.

Acquired characters, inheritance of, 391.

Acropet.l, 274.

Action, physiological, at a distance, 485.

Activities of the organism, 247, 397.

Adaptation, capacity for, 389, 391 ; here-
ditary fixation of, 389.

Adaptations, 389 ; active, 389 ; to external
factors, 3^.9 ; inheritance of, 391 ; to con-
centration, 179 ; to locality, 253 ; direct,
254; functional, 251, 331, 391; inheri-
tance ol functional, 392 ; passive, 390 ;
inherite I, 254.

Adaptive characters, 386.

Adventitious origin, of growing points, 284 ;
o embryos, 370.

Aerial roots, heliotropism in, 461 ; growth
in, 289.

AeriicTous system, 37.

Aerobionts, 213, 526.

Aerotaxis, 105, 542, 547.

Aerotropism, 484.

After-effect of external factors in geotropism,
437 ; in adaptations, 390, 392 ; in periodic
phenomena, 343 ; in periodic movements,
508.

Aggregation of protoplasm in Drosera> 498.

Agriculture and nutrient materials, 101.

Air-bubbles, counting of, in assimilation,
104.

Albumins, 140.

Albumoses, 139.

Alcohol, in fermentation, 208; in intra-
molecuiar respiration, 203.

Alcoholase, 212.

Alcohols, higher, in fermentation, 214.

Aleu one, 160.

Alinite, 235.

Alkaloids. 4, 176.

Alpine plants, 319.

Alternation of generations, 358.

Aluminium, 86.

Amide- organisms, 181, 241.

Amido-compounds, 4, 139; assimilation of,
143 ; formation of, 173 ; as reserves, 163;
transformation of, in seedlings, 173 ;
translocation of, 167.

Ammonia, assimilation of, 135; formation
of, from urea, 224; formation of, from
peptone, 200 ; oxidation of, 228 ; occur-
rence of, in nature, 135 ; occurrence of,
in the soil, 137.

Amoeboid movement, 534.

Amphibious plants, 253.

Amphimixis, 371.

Amylase, 152.

Anaerobes, 213 ; biological significance of,
215; oxygen requirements of, 215; in-
jurious influence of oxygen on, 215.

Anaesthetics, see Narcotics.

Analysis, see Composition.

Anaphase in nuclear division, 268.

Aniline dyes, osmosis of, 20.

Anisophylly, 313.

Annual periodicity, see Periodicity; an-
nual rings, 350.

Anthers, dehiscence of, 416.

Anticlinal, 280.

Antienzymes, 156.

Apex, 273, 331.

Apical cell, 279 ; growth, 261.

Apogamy and apospory, 359.

Apposition, see Cell-wall, Growth.

Archegonium, 359, 361.

Arginin, 140, 160, 174.

Arrangement of lateral organs, 274.

Articulations of leaves, 454.

Asci, ejection of spores from, 422.

Ash, 77-102 ; non-essential constituents of,
86 ; origin of, 77 ; amount of, 79, 80 ;
dependent on substratum, 79 ; dependent
on transpiration, 79; essential constituents
of, 80, 8 1 ; nature of, 78.

Asomatophytes, 273.

Asparagin, 143, see Amido-compounds ;
accumulation of, in darkness, 174 ; in-
fluence of, on diastase, 152 ; as a nutrient,
144, 182.

Assimilation, products of, transformation of,
147 ; of materials of the ash, 83, 178 ; of
carbon by autotrophic plants, 102, see
Carbon-dioxide ; by heterotrophic plants,
178; of nitrogen, see Ammonia, Aspara-
gin,Amido-compounds, Proteid, Nitrogen,
Peptone, Nitric Acid ; proper, 259 ; pro-
ducts of, in carbon-dioxide decomposi-

556

INDEX

tion, 1 10 ; products of, in nitrogen assimi-
lation, 138; amount of products of, 114,
116.

Atavism, 373.

Autonomous movements, 428, 521, 528 ;
induced by internal stimuli, 527 ; induced
by variation, 528 ; induced by growth,
529.

Autotrophic nutntion, 177.

Autotropism, 448 ; in geotropic curvature,
432, 448 ; in haptotropic curvature, 493 ;
in mechanical curvature, 493 ; in nycti-
tropic curvature, 507.

Auxanometer, 287.

Averrhoa, autonomous movements in, 528.

Axillary bud, 278.

Bacteria, movements, see Natatory move-
ments, Chemotaxis, Aerotaxis, &c. ; nu-
trition of, see Heterotrophic plants.

Bacterium radicicola, 237.

Bacteroids, 237.

Base, 273 ; in regeneration, 330.

BasidioboluS) development of, 248 ; depen-
dence of, on nutrition, 248.

Beggiatoa, 221 ; organic nutrients of, 223,
229 ; oxidation of sulphuretted hydrogen
by, 221 ; oxidation of sulphur by, 222;
oxygen requirements of, 222.

Benzol-derivatives as nutrients, 180.

Bilateral, 276.

Bleeding, 50 ; significance of, 56; conditions
of, 53; duration of, 51 ; pressure in, 52 ;
local, 54 ; mechanics of, 54 ; sap, 51.

Bordered pits, 68.

Branches, amputation of, 351 ; autotropism
in, 449; essential angle of, 449 ; excentric
thickening in, 3 1 4 ; plagiotropism of, 449 ;
factors in the direction of growth of, 449 ;
change in, to orthotropy, 449.

Branching, 26 ; of the leaf, 278 ; dichoto-
mous and lateral, 274 ; of the shoot, 278 ;
of the root, 283.

" Budding " in horticulture, 333.

Buds, propagative, 362, 365.

Butyl-alcohol, as a product of fermentation,
214.

Butyric acid, as a product of fermentation,
217, 218.

Caesium, 84.

Calciphilous, 99.

Calciphobous, 99.

Calcium, 84 ; oxalate, 141.

Callus, origin of, 328 ; formation of mem-
bers from, 329.

Cambium, 294, 350.

Cane sugar as a reserve, 162.

Capacity, 341 ; selective, 20.

Capillarity, in water conduction, 72.

Carbohydrates, 4 ; as products of assimila-
tion, 115; as respiratory material, 196;

as fermentable material, 208 ; as re-
serves, 162, 163; relation of, to fats,
158.

Carbon-assimilation, in autotrophic plants,
see Carbon-dioxide ; in heterotrophic
plants, 177 ; products of, 103, no.

Carbon compounds, nutritive value of, 178,
179-

Carbon-dioxide, assimilation of, see Assimi-
lation ; dependence of, on external factors,
119, 123, 124; on chlorophyll, 103; on
light, 103; absorption of, 118-22; ex-
cretion of, by the root, 95 ; source of, 103 ;
occurrence of, 118, 119; decomposition
of, 103, up.

Cardinal points of temperature, 124, 201,
299, 526.

Carnivorous plants, see Insectivorous
plants.

Catalytes, 152.

Catasetum, slinging movements in, 426.

Cell aggregates, 272.

Cell-nucleus, 6, 268 ; movements of, 553.

Cells, rounding off of, 296 ; mature, 296 ;
structure of, 6 ; as fundamental units,
258 ; embryonic, 295 ; form of, 296 ;
contents of, 297 ; osmotic characters of,
13 ; regeneration of, 380 ; growth of, 258;
fusion of, 297 ; division of, 268.

Cell-sap, 6, 7.

Cell- wall, 297 ; formation of, 260 ; stretch-
ing of, due to osmotic pressure, 419 ;
lamellation of, 266 ; growth of, by apposi-
tion, 262, 266 ; cessation in growth of,
267 ; significance of the nucleus in
growth of, 268 ; osmotic pressure in
growth of, 265 ; growth in thickness of,
265 ; growth in surface of, 260 ; inter-
calary growth in, 261 ; growth by intus-
susception of, 262, 264 ; growth by cap
formation in, 263 ; growth by interpola-
tion of protoplasm in, 266; growth by
plastic stretching of, 263 ; at the apex,
261 ; reduction of tension in, 421.

Cellular plants, 258.

Cellulose, as a reserve material, 158 ; disso-
lution of, by cytase, 158 ; by Fungi, 183 ;
fermentation of, 218.

Centrifugal force, 430.

Centrosome, 368.

Chain, Jamin's, 71.

Chain of releasing stimuli, 525.

Chemical stimuli, in relation to spores of
Fungi and to pollen, 317 ; influence of,
on form, 317, see also Chemotropism,
Chemotaxis, Drosera, Galls, Poisons,
Mimosa, Tendrils.

Chemotaxis, 541 ; by malic acid, 542 ; apo-
batic, 545; significance of, 542; by gases,
546 ; negative, 545 ; positive, 545 ; sti-
mulative agents in, 542, 545, 547 ; per-
ception of stimulus in, 547 ; liminal in-
tensity in, 543 ; repulsion in, 544 ; stro-

INDEX

557

phic, 545 ; occurrence of, 542 ; Weber's
law, in relation to, 544.

Chemotropism, 481 ; by gases, 484; by
dissolved bodies, 482 ; in Fungi, 482 ; of
pollen-tubes, 483 ; Weber's law in rela-
tion to, 483.

Chlorine, 83.

Chloroform, see Narcotics.

Chlorophyll, chemistry and physics of, 107 ;
effect of darkness on, 308.

Chloroplast, 6 ; as the organ for decomposi-
tion of carbon-dioxide, 107.

Chlorosis, 85.

Cholesterin, 4 ; in the plasmatic membrane,
22.

Chromatin, 268.

Chromosomes, 268 ; formation of, 377 ; as
hereditary agents, 377 ; number of, 368,
378.

Cilia, as motile organs, 533.

Circulation of protoplasm, 536 ; of carbon
and nitrogen, 243.

Circumnutation, 529.

Citric acid, 197.

Climbing plants, 455.

Closing of flowers, 500.

Clostridium pasteurianum, 233.

Cobalt-paper, 36.

Coefficient, isosmotic, 16 ; economic, 191.

Cohesion of water, significance of, in water
conduction, 65, 72 ; in dehiscence, 415 ;
in imbibition, 417.

Cold rigor, 300.

Collective species, 385.

Colloidal solutions, 153.

Colonies, 272.

Colouring matters, 4, 176 ; formation of, in
darkness, 308 ; diosmosis of, 20.

Combustion, chemical and physiological,
205.

Comparison of intensities, 524.

Compass plants, 466.

Compensation, 330.

Composition, chemical, 3, 5.

Concentration, influence of, in osmotic pres-
sure, 15 ; on growth, 316; as a directive
stimulus, 546.

Constructive material, 178.

Contact stimulus, 490 ; movements in con-
sequence of, 487 ; organogenetic results
of, 314.

Continuity of the embryonic substance,
284.

Contractile layer, 411.

Contractile threads, 268.

Contraction ellipsoid, 410.

Contraction in secondary growth, 295.

Copper, 87, 88.

Cork, 327.

Correlations, 252, 326 ; demonstration of,
327,331,332; quantitative and qualitative,
330; as regulating growth, 252; in move-
ments due to stimuli, 524 ; causes of, 335 ;

between leaves and buds, 330 ; between

leaves and conductive strands, 330, 335 ;

between buds, 329 ; between shoots, 330 ;

between parts of the cell, 335.
Corrosion, of starch, 155 ; by the root, 95.
Corrosive sublimate, 87, 542.
Culture on solid substrata, 80 ; in aqueous

solutions, 80.
Curvature, 406; geotropic, 431 ; work

accomplished in, 437 ; course of, 432-5 ;

heliotropic, 460-75; in shrivelling, 409;

due to turgor and growth, 421 ; due

to weight, 448.

Cuscuta, carbon assimilation in, 188; twin-
ing of, 496.
Cuticle, permeability of aerial, 33, 38, 120;

permeability of subterranean, 33.
Cydanthera, ejaculatory movements in, 425.
Cynareae, movements of the stamens in,

due to stimulus, 519.
Cytase, 152, 158, 183.
Cytoplasm, 6.

Daily periodicity, 508.

Darkness, see Etiolation.

Darkness-rigor, 302, 508.

Darwinian theory, 384.

Day position, 500.

Day sleep, 505.

Death, 351,362.

Definite branches, 277.

Denitrification, 232.

Descent, theory of, 383.

Deserts, 97.

Desiccation, 34, 318 ; respiration during,
341.

Desmodium, autonomous movements in,
528.

Destructive metabolism, 207.

Development, 250 ; of branches, see Bran-
ches, Growing point ; inhibition of, in
sexual cells, 371 ; stimuli inducing, 369.

Dextrin, formed from starch, 150.

Dextrose, see Carbohydrates.

Diageotropism, 446.

Diaheliotropism, 464.

Diastase, 149, 152; nature of, 150; co-
ordinate formation of, 183 ; effect of tem-
perature on, 151 ; effect of accelerating
agents on, 151 ; effect of poisons on, 151;
occurrence of, 149, 164, 183.

Diatomaceae, movements of, 534.

Dichotomy, 274.

Differentiation, 251 ; internal, 284, 296.

Diffusion, 13; of carbon-dioxide, 120; as
an agent in transport of materials, 167 ;
as an agent in directive movements, 481,
542, 547-

Diminution, harmonious, 315.

Directive movements, 428.

Disaccharides, hydrolysis of, 164.

Dissimilation, 190.

Dissociation, 16.

INDEX

Dissolution of reserves, 147-65.

Division, of the nucleus, 268 ; of the cell,
269, 270 ; amount of, 271.

Dorsiventral, 277.

Drosera, protoplasmic aggregation in, 498 ;
movements in, 496 ; chemical stimuli as
affecting, 497; as an insect trap, 185,
496 ; contact stimulus in, 497 ; mechanics
of curvature in, 497 ; nastic and tropistic
curvature in, 499 ; direct stimulation in,
498 ; indirect stimulation in, 498.

Drought rigor, 539.

Dry weight, decrease of, in darkness, 191 ;
increase of, in water cultures, 80, 82.

Duramen, conduction of water in, 62.

Dynamical layers, 412.

Ecballium.) ejaculatory movements of, 42 4.

Egg, development of, inhibition of, 368 ;
development of, stimulation of, 370.

Ejaculatory movements, of Catasetum, 426 ;
of ferns, 415 ; of fruits, 424 ; of Fungi,
423 ; of stamens, 425 ; causes of, 410,
415,422.

Elastic stretching of the cell-wall, 263.

Electricity, production of, 402.

Electrotropism, 480.

Elements of the ash, 80.

Elongation, 285.

Embryonic substance, 273 ; continuity of,
284 ; growth of, 284.

Endosmosis, 13.

Endosperm, 148, 155 ; depletion of, in ab-
sence of embryo, 155; depletion of, effects
of oxygen and chloroform on, 1 56.

Energy, conservation of, 397 ; forms of, in
the plant, 397 ; sources of, 397 ; me-
chanical, 403 ; due to respiration, 403 ;
due to other processes, 404 ; transforma-
tion of, 2, 397.

Enzymes, 149, 152; hydrolytic, 152; in-
organic, 153; catalytic action of, 152;
synthetic action of, 154; incomplete re-
actions by, 153, 154; specific action of,
154 ; action of, on peroxide of hydrogen,
153 ; oxidizing, 204 ; zymotic, 211.

Epinasty, autonomous, 530 ; induced, 449.

Epithem, 58.

Equimolecular, 16.

Ether, chemotactic influence of, 542 ; in-
fluence of, on assimilation, 195 ; influence
of, on respiration, 195, 202 ; lowering of
sensitivity by, 516; influence of, on the
formation of shoots, 345.

Ethereal oils, 4, 176.

Ethyl-alcohol, see Alcohol.

Etiolation, by withdrawal of light, 304 ;
significance of, 306 ; by withdrawal of
nitrogen, 315.

Excitation, geotropic, 438 ; heliotropic,

469 ; conduction of, 444, 469.
Excreta, 176.
Exosmosis, 14.

Experiment, 2.

Extensibility of the cell-wall, 420 520.

Factors concerned, in p'ant form 339 ; in
plant habit, 255 : in a mechanism. 255.

Fats, 4, 158; as products of assimilation,
158; as respiratory material oo for-
mation of, from carbohydra es, 75 ; hy-
drolysis of, 159; as reserves, 59 1 62 ;
transformation of, into caiboayd.-ates,
158.

Felspar, weathering of, 91.

Fermentation, 214; alcoholic, of sugar,
208; significance of, 2 1 3 ; influence of
oxygen on, 213; material o<, 2 8 : by-
products in, 2 1 o ; products of, 210; re-
lation to respiration of, ?I2 ; zy n;»se in,
211 ; see also Butyric acid, Butyl-* cohol,
Cellulose, Acetic acid, Pec.in Organic
acids.

Ferments, see Enzymes.

Fertilization, 354, 358, 361 ; significance of,
367, 376; of the embryo-sac nucl us. 361,
370.

Flanks, 276.

Flower, 349, 360 ; formation of the. due to
specific materials, 349; on cut i igs 364;
causes of, 363, 364; and vegetative
growth, 364.

Flowering plants, functions of rvg;ms in,
250; differentiation and division ot labour
in, 250; development of, 250 ; s gnenta-
tion of, 250.

Fluctuating variations 387.

Foliage- ieat and scale- leaf, 349.

Form and material, 256.

Formal conditions, 253 427, 522 526-8.

Formaldehyde, as a product of assimila-
tion, 112.

Formation, 258.

Formative stimuli, 298.

Freezing, 300.

Fruits, ejaculatory movements of, 410, 424.

Function, inhibition of, results or, 331 ;
transference of, 331.

Functional adaptation, 251, 331, 392; sti-
muli, 335.

Fungi, influence of substratum on, 320;
nutrition of, see Heterotrophic phnts.

Fusion in fertilization, 354, 355, 358 361.

Galls, 320; of Dryophanta, 324 ue to
insects, 321 ; due to Fungi 321 ; of
Sp .ithegaster> 323 ; causes of. 3:4; pur-
pose of, 324.

Gaitonian curve, 387.

Galvanotaxis, 551.

Ga vanot opisin, 481.

Gametes, 355.

Gases see Carbon -dioxide, Oxyg n, &c.

Geotaxis, 551.

Geotropism, 429; in dorsiventral t c'ures,
452 ; excitation in, 439 ; in ui\ inate

INDEX

559

leaves, 454 ; intermittent stimulation in,
438 ; use of klinostat, • 30 : Knight's
research, 430: correlative influences in,
450 : curvature in, see Curvature ; after-
effects in, 437 ; n- gative, 431 ; in ortho-
tropic organs, 429 : perception in, 440 ;
in p agiotropic organs, 445 ; positive,
431 ; latent period in, 439 ; protoplasmic
movement in, 444 ; chain of stimuli in,
444 ; in rhizomes, 446 ; rest position in,
440. 447 ; in lateral roots, 447; in lateral
tranches, 448 ; stato ith theory in, 442 ;
disposition of organs to, 450-2 ; torsion
in see Tor- ions ; in twining plants, see
Twining plants ; in conjunction with
heliotropism. 476

Cermi nation, capacity for, 341 ; of seeds,
transU cati< ,n of materials in, 147; con-
ditions of, 252.

Glucosides, 4, 176.

Glutamin, see Amido-compounds.

Glycerine, diosmosis of, 19; as a product of
fermentation, 210; as a nutrient, 179;
esters, 4.

Glycogen, 189.

Graft hybri is, 381.

Grafts, 333.

Gravity, its activity cancelled by the klino-
stat. 429 ; as a teleasing stimulus, 436;
influence of, on secondary growth in
thickness, 3 1 4 ; on growth in length, 314;
on direction, see Geotaxis, Geotropism ; on
symmetry 313: replaced by centrifugal
force, 430 ; intensity of, 430 ; direction
°f 3 3» 44° ? Hminal intensity of, 439.

Growth, 25 ; influence of poisons on, 316 ;
of oxygen on, 316; duration of, 294;
embryonic 284 ; and reproduction, 358 ;
without reproduction 362 ; rate of, 293 ;
of branches, see Branches; of proto-
plasm, 259 ; spasmodic variations in,
292 ; distribution of, in branches, see
Branches ; of the cell, artificial, 260 ; of
the cell- wall, see Cell- wall; movements,
421,529; capacity for, 300 ; periodicity
in, 284, 95 ; zones of, 288.

Habitat, 389 ; adaptation to, 253.

Halophytes, 97, 319.

Haptotropism, 4*7; in Algae, 499; in Dro-
sera, 496 ; in Fungi, 499 ; in tendrils, 487.

Harmonic dwarfing, 3 1 5.

Harmonious development, 252.

Haustoria, induced by contact, 496.

Heat, production of, 399.

Heating, due to respiration, 400; due to
radiation, 4*.

Heat-rigor, 30x5.

Heliotropism 460 ; in Avena, 469 ; rela-
tion of, to geotropism, 460; in diffuse
and direct light, 466 ; in dorsiventral
organs, 464, 466; sensitivity in, 463,
469 ; excitation in, 469 ; surface position

in, 466 ; curvature in, see Curvature ;
light intensity in, 463 ; in nature, 464 ;
negative, 460 ; in orthotropic organs,
461 ; in Paniceae, 468 ; in conjunction
with geotropism, 476 ; perception of in-
tensity of light in, 472, 475 ; of direction
of light in, 471 ; localization of percep-
tion in, 468, 470, 471 ; in plagiotropic
organs, 464, 4674 positive, 460 ; latent
period in, 473 ; prifttery influence of light
in, 474 ; profile position in, 466 ; move-
ment in, influenced by stimuli, 462 ; se-
quence of stimuli in, 475 ; liminal inten-
sity of stimulus in, 462, 473 ; torsions in,
see Torsions ; Weber's Law applied to,
473 ; wave lengths of light in, 474.

Heredity, 376, 383 ; material basis of, 376,
377.

Heterotrophic plants, nutrition of, in rela-
tion to carbon, 178 ; to nitrogen, 181 ; to
humus, 183 ; enzyme formation in, 183 ;
metabolism in, 188.

Hibernation, 341, 348 ; respiration during,

341.

Holdfasts, 315.

Hook climbers, 455.

Humidity of air, influence of, on transpira-
tion, 39; on growth, 318, 342.

Humus plants, 183, 241.

Humus soils, 100.

Hybrids, hetero- and homo-dynamic, 373 ;
intermediate forms in, 373 ; new charac-
ters in, 374 ; reversion in, 374 ; segrega-
tion in, 375 ; sterility of, 376 ; vegetative
power of, 374.

Hydathodes, 57.

Hydrocarbon, as a product of assimilation,
no.

Hydrogen, as a product of fermentation,
218.

Hydrolysis, 150, 152.

Hydrophytes, 253.

Hydrotaxis, 548.

Hydrotropism, 484.

Hygrophilous plants, 390.

Hygroscopic movements, 406.

Hypertrophy, 321.

Hyponasty, 530.

Ice, formation of, 300.

Idioplasm, 377; distribution of, in the

plant, 379 ; in the cell, 377.
Imbibition water, 415.
Impacts, external, in slinging movements,

426, see Stimuli.

Impatient, ejaculatory movements in, 424.
Increment, in growth, 288 ; maximal, 293.
Indefinite shoot, 277.
Individual variation, 387.
Induced movement, 428, 521.
Influences, external, on growth, 253, 298 ;

correlative, 251, 326 ; of other organisms,

320.

INDEX

Inhibiting agents, 156.

Initials, 371 ; latent, 374, 377, 395.

Inorganic constituents, see Ash.

Insectivorous plants, 184 ; movements in
response to stimuli in, see Drosera ; diges-
tion of proteid by, 185.

Intercalary growing points, 273, 291 ;
growth, 259-63.

Intercellular protoplasmic threads, signi-
ficance of, for conduction of stimuli, 475 ;
significance of, for translocation of mate-
rials, 170.

Intercellular spaces, 37, 104, 120, 199, 296.

Intermittent stimulation, 439.

Intra-molecular respiration, 203 ; relation to
fermentation, 207 ; products of, 203.

Intussusception, see Cell-wall.

Inulin, 162.

Invertase, 152.

Ions, 1 6.

Iron, 85 ; -bacteria, 224.

Kinoplasm, 369.

Klinostat, 430 ; theory of, 438, 449.

Lactic acid fermentation, 217.

Lactose, 210.

Latent characters, 376.

Latent period, 437.

Lateral roots, geotropism in, 446.

Laticiferous tubes, 172.

Leaf-fall, 351.

Leaves, 277 ; acropetal expansion of, 292 ;
assimilation of carbon-dioxide by the,
107, 251 ; absorption of ammonia by the,
137 ; materials of the ash by the, 78 ; of
carbon-dioxide by the, 1 20 ; of organic
substances by the, 112, 186 ; of water by
the, 33 ; translocation of materials from
the, 162 ; movements of, due to stimuli, see
Stimuli ; autonomous, see Autonomous ;
etiolation of, 305 ; light position of, 464,
466; regeneration of, 329; growth of,
292-4 ; formation of, 281 ; climbers, 495 ;
arrangement, 275 ; mechanical theory of,

Leaves, unfolding of, 292.

Lecithin, 4.

Leguminosae, nitrogen fixation in, 237.

Leucin, 140, 144, 174.

Levulose, as a chemical stimulus, 317; see
Carbohydrates.

Lichen-symbiosis, chemistry of, 243 ; mor-
phogenic results of, 325.

Life, interpretation of, 254 ; causes of, 254;
conditions of, 298 ; duration of, 351.

Light, absorption of, in carbon assimila-
tion, 130; influence of, in flower forma-
tion, 364 ; in protoplasmic movement,
540 ; intensity of, significance of, in car-
bon-dioxide assimilation, 125 ; on forma-
tion, 306, 310; on growth, 301, 302; pro-
duction of, 401 ; quality of, influence of,

in carbon-dioxide assimilation, 126 ; in
formation of chlorophyll, 311 ; in forma-
tion, 311; in growth, 310; direction of,
influence of on movements, see Helio-
tropism, Phototaxis ; influence of, on
polarity and symmetry, 310 ; as inhibit-
ing growth, 303 ; energy of, in relation to
carbon-dioxide assimilation, 130 ; amount
of, made use of, 307 ; position of leaves,
466; alteration in, influence of, on growth,
343 ; as a stimulus, see Nyctitropism,
Phototaxis, Apobatic.

Lime, 98.

Limiting angle, 449 ; in geotropism, 446.

Lipase, 152, 159.

Lithium, 83.

Locomotion, 405 ; autonomous, 532 ; in-
duced, 532, 541.

Longitudinal growth, 286 ; measurement
of, 286 ; secondary, 295 ; distribution of,
in the stem, leaf and root, 290, 291.

Magnesium, 84.

Malformations, due to mutation, 393.

Malic acid, chemotaxis by, 543.

Maltose, derived from starch, 150.

Manganese, 87.

Mannite, 112, 175, 179.

Manuring, 101.

Mass, movement of water in, 47 ; of nu-
trients, 170.

Material and form, 256.

Materials, absorption of, by land plants,
24 ; by the cell, n, 24 ; translocation of,
1 66 ; organs concerned in, 169, 170 ;
causes of, 167-9.

Maximum, see Cardinal points.

Mechanical stimuli, see Pressure, Contact,
Shock, Tension.

Mechanism, structure of, 522.

Mechanism and organism, 255.

Merogeny, 371.

Metabolism, 2.

Metamorphosis, I, 247, 249, 256.

Metamorphosis, due to gall insects, 320;
natural, 349.

Metamorphosis of organs, due to correla-
tive influences, 330 ; due to intensity of
light, 310 ; due to Fungi, &c., 321.

Metaphase, in nuclear division, 268.

Methane, in fermentation of cellulose, 218 ;
derivatives of, as nutrients, 180.

Methods of Physiology, 2.

Micella, 407.

Mimosa, chemical stimulation of, 516 ;
nyctitropic movements in, 504 ; periodic
movements in, 510; movements in re-
sponse to shock in, 516 ; significance of,
513; mechanics of, 514; resistance to
flexion in, 514 ; transmission of stimulus
in, 518 ; in response to wounds in, 516.

Minimum, see Cardinal points ; law of the,

INDEX

Mixing of initials, 371.

Molecular weight, determined by plasmo-
lysis, 16.

Mosaic hybrids, 373.

Mosses, desiccation of, 317.

Mould-fungi, nutrition of, 178.

Movement, I, 403 ; active, 405 ; autono-
mous, 428 ; by torsion, 406 ; by curvature,
406 ; by twining, 406 ; rectilinear, 405 ;
hygroscopic, 410; induced, 428; rotatory,
455 ; locomotory, 428, 532 ; paratonic, 428 ;
passive, 405 ; causes of, see Cilia, Cohesion,
Protoplasm, Swelling, Turgor, Growth,
Shrivelling.

Mucilage, as a reserve, 162.

Multicellular formation, 271.

Mutation, 393.

Mycoderma, 180, 217.

Mycorhiza, 240.

Myxomycetes, alteration of form in, 247.

Narcotics, chemotactical influence of, 542 ;
influence of, on assimilation, 195 ; on
respiration, 195, 202; on Mimosa, 516;
on perception, 527 ; on tendrils, 492 ; on
reaction, 527 ; on development of shoots,
345 ; want of, as a formal condition, 539 ;
osmosis of, 19.

Nasties, 428, 449, 500, 530 ; transitions to
tropism, 499.

Natatory movements, 532.

Nectaries, 59.

Night position, 500.

Nitrate formation, 228.

Nitric acid, assimilation of, 133 ; formation
of, 226; occurrence of, 135, 136.

Nitrification, see Nitrogen-Bacteria.

Nitrogen, assimilation of atmospheric, 232 ;
by autotrophic plants, 133, see Ammonia
and Nitric acid; in proteid formation,
138; by heterotrophic plants, 181 ; in-
fluence of light on, 142 ; organic com-
pounds of, 143; atmospheric, 134, 136,
232, 236 ; combination of, in soil, 233 ;
by Clostridium, 233 ; by Leguminosae,
237 ; by other Phanerogams, 239 ; by
Fungi, 235 ; manuring with, 138 ; gain
in, 135 ; etiolation in absence of, 315 ; as
a nutrient, 86; loss of, 136.

Nitrogen-Bacteria, 228 ; respiration of. 229;
nutrition of, by carbon-dioxide, 228;
morphology of, 227 ; as nitrate formers,
226 ; as nitrite formers, 226 ; oxygen
requirements of, 226; injury to, by organic
substances, 224 ; occurrence of, 230.

Non-cellular, 258.

Nuclein, in sexual cells, 367.

Nucleus, 6; division of, 268, 271 ; as the

bearer of hereditary characters, 377.
Nutation, 293, 421 ; simple, 531 ; transi-
tory, 530 ; epinastic, 531 ; hyponastic
531 ; periodic, 530 ; revolving, 530 ; un-
dulatory, 531.

Nutrients, incombustible, 86 ; combustible,
86 ; absence of, influence on growth,

3I5-
Nutritive solutions for autotrophic plants,

8 1 ; for heterotrophic plants, 178.
Nyctitropism, 500; cooling as a stimulus,

502 ; significance of, 511 ; in flowers, 501 ;
heating as a stimulus in, 501, 505; of
foliage-leaves, 503 ; alteration in light as
a stimulus in, 502, 504 ; periodic move-
ments in, 509 ; backward curvature in,

503 ; influence of gravity in, 508 ; varia-
tion movements in, 504; resistance to
flexion in the pulvinus in, 506 ; mechanics
of, 505.

Oenothera, mutation in, 393.

Oil emulsion, 538.

Oils, ethereal, 4, 176 ; fatty, see Fats.

Omnivora, 180, 182.

Opening of flowers, 500.

Optimum, see Cardinal points ; in inorganic
processes, 526.

Orchidaceae, autonomous movements in
flowers of, 528.

Organic acids, 4 ; formation of, by Fungi,
197 ; by succulents, 198 ; as nutrients,
178 ; osmotic activity of, 404 ; materials,
as nutrients in autotrophic plants, 112,
143 ; in heterotrophic plants, 177.

Organic characters, 386.

Organism and mechanism, 255.

Organization, 9.

Organs, formation of, in regeneration, 331 ;
at growing points, 274, 280, 281.

Orientation, movements, 429.

Orthotropic, 445.

Osmosis, II.

Osmotaxis, 548.

Oxalic acid, 85, 197.

Oxydases, 204.

Oxygen, loosely combined, 215, 216 ; need
for by anaerobes, 215; for respiration,
192 ; for movement, 539 ; for fermenta-
tion, 212; for growth, 316; occurrence
of in the cell, 196 ; rigor, 526.

Pangenesis, 391.

Parasites, 178, 186.

Parastichies, 275.

Paratonic movements, 521.

Parthenogenesis, 369.

Partition walls, arrangement of, 270.

Passive layers, 409.

Pectin, 157; fermentation of, 218.

Pepsin, 1 60.

Peptone, 139, 178 ; organisms, 182, 241.

Perception, 440, 469 ; see individual stimu-
lating agents ; distinct from reaction, 441 ,
469, 527 ; apparatus, 441, 443> 523-

Pericline, 280.

Period, grand, 288, 529.

0 O

562

INDEX

Periodic movements, 508 ; origin of, 509 ;
mechanics of, 510.

Periodicity, 340 ; in leaf-formation, 343,
348 ; in secondary growth, 350 ; in
development, as a whole, 351; in longi-
tudinal growth, 344; yearly, 343~8 ;
daily, 342 ; in tropical plants, 347.

Performance of work, 247, 397 ; in geotropic
curvature, 437 ; in imbibition, 408 ; in
turgor and growth, 422.

Permanent tissue, 296.

Petiole, 292.

Phosphorus, 83, 145.

Photonastic, 500, 502, 531.

Phototaxis, 549, 552.

Phototropism, 460.

Physiology, problems of, I.

Pilobohts, ejaculatory movements of, 423.

Pith, geotropic behaviour of, 435.

Pits, 46, 170.

Plagiotropic, 445.

Plant materials, 4.

Plasmatic layer, osmotic characters, 22.

Plasmodia, movements of, 534 ; chemical
composition of, 8.

Plasmolysis, 14.

Plastic stretching, 265.

Pneumathodes, 38.

Poisons, effect of, on growth, 316; as
stimuli, 87.

Polarity, 330, 333, 336.

Poles, 273.

Pollen-tube, conditions of germination of,
317 ; directive movements in, 483.

Polygonum amphibium, adaptation in, 253.

Posterior side, 276.

Potassium, 83.

Potometer, 30.

Precipitation membranes, 13.

Predominant characters, 373.

Pressure, influence of, on organogenesis,
338; on lateral roots, 315; on division
planes in the cell, 314; on growth and
formation, 314; negative, of the air in
vessels, 70 ; osmotic, in external medium,
179, 418 ; variations in, 318 ; significance
of, 20, 419; determination of, 18, 420;
amount of, 18, 419; regulation of, 419;
theory of, 17 ; effect of, 420 ; as a stimulus,
314, see Contact ; exerted by the growing
plant, 422.

Primitive organisms, 258.

Production of energy in the plant, 397-404.

Progressive development, 274.

Prophase, in nuclear division, 268.

Protease, 152, 160.

Proteids, 4, 140; formation of, 115, 138;
from amido-compounds, 173; chemistry
of, 139; subdivision of, 140; as reserves,
159, 163, 165 ; decomposition of, 139, 161 ;
respiration of, 200; fermentation of, 219;
organisms, 182.

Protein, see Proteid.

Protoplasm, 6; amoeboid movements of,
534 ; explanation of, 537 ; formal condi-
tions of, 539 ; rotation of, 536 ; circula-
tion of, 536 ; permeability of, 19, 20 ;
causes of, 21 ; structure of, 9, 10 ; growth
of, 259 ; composition of, 7.

Putrefaction, 219.

Qualitative division, 380.

Qualities, mechanical explanation of, 254.

Racemic acid, 181.

Races, 384, 388.

Radial growing point, 276.

Ranunculus aquatilis, adaptation in, 253.

Reaction, see Perception ; purposeful, 254 ;
forms of, 428.

Recessive characters, 373.

Reduction division, 367, 378.

Reflex actions, 525.

Regeneration, 328-9 ; from the single cell,
380 ; from somatic cells, 380 ; causes
of, 336.

Rejuvenescence, 366, 371.

Releasing agents, 255, 339, 428, 522, 527.

Reproduction, 353 ; in Algae, 353 ; in
Basidiobolus, 249; digenous, 371; in
ferns, 358 ; sexual, 249, 355 ; by embryos
and buds, 355; monogenous, 371; in
Phanerogams, 360 ; asexual, 249, 355 ;
causes of, 355 ; relationship of, to growth,
357 ; organs of, 353 ; accessory, 359, 362.

Reproductive-leaves, 349.

Reserve organs, 329.

Reserves, 147, 148, 161, 162.

Resins, 4.

Resistance to flexion in articulations, 506,
514.

Respiration, 190; dependence of, on de-
velopmental condition, 194 ; on light, 201;
on oxygen, 202 ; on material influences,
20 1 ; on temperature, 201 ; on water, 341 ;
significance of, 205 ; history of our know-
ledge of, 205 ; amount of, 1 93 ; intra-
molecular, 203, 316; coefficient of, 197,
199; dependent on materials, 199; and
production of light, 401 ; materials used
in, 196, 208 ; demonstration of, 192 ; in
the green cell, 194 ; primary decomposi-
tion in, 204; products of: alcohol, 204;
carbon-dioxide, 192 ; organic acids, 197 ;
water, 196 ; causes of, 203, 204 ; occur-
rence of, 191 ; production of heat in, 201,
400.

Rest period, 345.

Retting, 218.

Reversible enzyme actions, 154.

Rheotaxis, 548.

Rheotropism, 486.

Rhinanthaceae, parasitism in, 188.

Rhizomes, geotropism in, 446, 448.

Rigidity, of the stem, 251 ; of the cell, 251,
419.

INDEX

563

Rigor, 526.

Ringing, 48, 171.

Root, conditions of origin of, 338 ; functions
of, 26 ; size of, 28 ; periodicity of growth
in, 346 ; branching of, 26, 97, 283 ; ab-
sorption of water by, 28 ; growth of, 289 ;
corrosion figures of, on limestone, 94 ;
pressure, see Bleeding ; hairs, 28 ; func-
tions of, 93, cap, 283 ; tendrils, 495.

Rotation, 536.

Rubidium, 83.

SaccharomyceS) as causing alcoholic fer-
mentation, 208.

Saprophytes, 178.

Scale and foliage-leaves, 349.

Scion, 334.

Secondary growth, 294; eccentric, 314;
contraction during, 295.

Secretion of peptic enzymes, 184 ; of acids,
184; of water, 50, 57.

Seeds, see Germination.

Segment cells, 280.

Segregation in hybrids, 375 ; in Cytisus
adami, 381.

Selection, 384-6; of nutrients, 180.

Self-regulation, phenomena of, 184, 255, 525.

Semipermeability, 12, 13.

Sexual-cells, injurious influence of, 376.

Shock-stimuli, 516; in Cynareae, 519; in
Mimosa, 511 ; in styles, 520; in other
structures, 517, 520.

Shoots, sprouting of, 290, 344, 345 ; elonga-
tion of, 290 ; development of, 345, 346.

Shrivelling, movements due to, 409.

Sieve-tubes, 172.

Silica, 87.

Siliciferous plants, 99.

Silicon, 87.

Size, specific, 340.

Soap films, 270.

Sodium, 83, 86.

Soil, absorption by, 92 ; employment of, by
the root, 26, 96 ; occupation of, by plants,
97 : origin of, 90 ; nutrients in the, 91 ;
oxygen in the, 25 ; primitive, 90 ; alluvial,
90 ; water contents of, 25 ; and agricul-
ture, 101 ; and plant distribution, 97 ;
plants local and indifferent to, 97.

Solution, theory of, 16 ; colloidal, 153.

Soma, somatic, 273.

Somatic cells, initials of, 380.

Specialists, 180, 182.

Species, definition of, 384 ; primary or sub-,
385 ; origin of, by mutation, 393 ; origin
of, according to Darwin, 384 ; characters
of, 385.

Spermatozoa, 359, 543.

Spindle threads, 268.

Spiral arrangement of members, 275.

Spiral, fundamental, 275.

Spontaneous movements, 521.

Sporangium, dehiscence of, 415.

Spores, ejection of, 422.

Spring plants, periodicity in, 351 ; shoots
of, 346.

Stamens, movements of, 425, 520.

Starch, as a product of assimilation, 1 10 ;
translocation of, 147 ; formation of, from
sugar, &c., 112; transitory, 171; disso-
lution of, 149.

Statocysts, statoliths, 442.

Stem climbers, 495.

Stigma, stimulation of, by shock, 520.

Stimuli, 255, 298, 496 ; in general, 427,
522 ; chemical, &c., see the individual
stimuli ; special, 427 ; specific, 522 ; ap-
plication of, 524 ; movements in response
to, general characters of, 521 ; chain of,
525 ; intensity of, in relation to reactions,
526; transmission of, in Drosera, 498;
in heliotropism, 468, 469, 470, 475 ; in
Mimosa, 518 ; in tendrils, 493 ; media of,
428, 524 ; materials acting as, 88 ; re-
action to, 428.

Stock, 334.

Stomata, 37 ; absorption of carbon-dioxide
by, 120 ; structure of, 38 ; mechanics of,
39 ; opening of, effect of external factors
on, 40-42.

Stretching of the cell-wall, due to osmotic
pressure, 419.

Strontium, 85.

Structure, specific, 339.

Style, sensitive, 520.

Stylidium, autonomous movements in, 529.

Suberin, 4.

Substance, embryonic, 273.

Substratum, orientation in reference to,
486.

Succinic acid, in fermentation, 210.

Sugar, formation of, from fat, 199 ; pro-
moted by low temperatures, 175.

Sulphur, 83, 145 ; in Beggiatoa, 220; Bac-
teria, colourless, 223 ; new group of, 229;
red, 223.

Sulphuretted hydrogen, 221.

Sulphuric acid, assimilation of, 145 ; for-
mation of, by Beggiatoa, 221.

Superelongation in darkness, 305 ; in ab-
sence of nitrogen, 315.

Surface tension, 537.

Surfaces of minimum area, 270.

Swarmspores, 354.

Swelling, 406.

Symbiosis, 237, 325.

Symmetry, 274.

Taxis, 541.

Temperature, influence of, on growth, 299 ;
as a formal condition, 252, 526, 540;
formative effect of, 301 ; cardinal points
of, 300, 526; of the plant, 399; alteration
in, effects of, on growth, 342 ; as a sti-
mulus to movement, 500, 505 ; reduction

564

INDEX

of, by conduction, 398 ; by radiation, 398 ;
by transpiration, 44, 398.

Tendrils, reacting all round, 490; senile
curvature in, 495 ; autotropism in, 493 ;
reacting unilaterally, 489 ; distribution
of sensitivity in, 490 ; curvature in, on
stimulation, 499 ; circumnutation in, 489,
530 ; perception in, 490, 491 ; stimula-
tion of both sides of, 490 ; due to chejnical
influences, 492 ; due to electricity, 492 ;
due to contact, 491 ; due to temperature,
492 ; tropistic curvature in, 490 ; clasp-
ing of supports by, 495 ; distribution of
growth in, 488 ; nastic curvature in, 499.

Tension, influence of on growth, 261.

Tetrathionic acid, 229.

Thermonastic, 499, 500, 502.

Thermotaxis, 548.

Thigmotaxis, 548.

Thiosulphate, 229.

Thiothrix, 223.

Thyloses, 296.

Tissue tensions, 297, 421.

Tonotaxis, see Osmotaxis.

Torsion, 406; autonomous, 531 ; geotropic,
452, 453 ; heliotropic, 465 ; due to shrivel-
ling, 414.

Tracheae and tracheides, see Vessels.

Transitory nutation, 530.

Translocation products, 148, 172; of
materials : from the leaf, 166 ; from store-
houses of reserves, 167 ; from seeds, 155,
168.

Transpiration, 25, 35 ; dependence of, on
external conditions, 38 ; on stomata, 39 ;
on the structure of the plant, 37 ; in-
fluence of, on absorption of nutrients, 43 ;
on the temperature of the plant, 43 ; on
ascent of water, 74 ; acceleration of, 43 ;
formative results of, 317 ; amount of, 43 ;
significance of, 43 ; inhibition of, 42 ; in-
ternal, 37 ; cuticular, 38 ; stomatal, 38.

Transplantation, 332.

Traumatotaxis, 553.

Traumatotropism, 486.

Trees, annual periodicity in, 344; winter
transformation of starch into fat in,

175-

Tropism, 428, 499.

Turgor, 13, 263, 418.

Twining plants, 455 ; geotropism in, 457 ;
spiral movements of, 456; supports to,
458; torsions in, 458 ; twining of, 455.

Twinings, 406 ; autonomous, 530 ; in ten-
drils, see Tendrils ; due to shrivelling,
412 ; in twining plants, 457.

Tyrosin, 140, 174, 444.

Undulatory nutation, 531.

Unicellular, 272.

Urea, as a nutrient, 143, 144 ; fermentation

of, 224.

Uro-Bacteria, 224.
Urticaceae, stimulus movements in, 425.

Vacuoles, 6.

Variation, see Adaptation, Species, Muta-
tion ; fluctuating, 387 ; movements, 421 ;
autonomous, 528 ; geotropic, 453 ; helio-
tropic, 465 ; paratonic, 428.

Vegetative growing point, 272 ; basal, 273 ;
structure of, 279-81 ; form of, 276 ; in-
tercalary, 273, 292 ; of the stem, 277 ;
symmetry of, 274, 276 ; capacity of, 273 ;
terminal, 273 ; growth in, 280 ; distri-
bution of, 284 ; of the root, 283 ; arrange-
ment of cells in, 279.

Ventral, side, 276.

Vessels, structure of, 66-70 ; functions of,
47, 172 ; contents of, 70; absorption of
water by, 49.

Volume, alteration in, in growth, 258.

Water, excretion of, see Bleeding, Tran-
spiration ; absorption of, by aerial organs,
32 ; by roots, 29 ; by cells, 24 ; influence
of different factors on, 31 ; excretion of, by
hydathodes, 58 ; significance of, in move-
ment, 539; movement of, in the plant,
45-7 ; influence of, in germination, 252 ;
in growth, 317; withdrawal of, 318;
conduction of, 25 ; influence of living
cells on, 74-6 ; through vessels, 47 ; by
capillarity, 72; rate of, 62 ; height of, 62 ;
amount of, 61 ; by parenchyma, 46 ; di-
rection of, 6 1 ; by suction, 64 ; in dead
trees, 75 ; by root pressure, 63 ; glands,
58 ; amount of in soil, 26, 318 ; in air,
318; capacity, 26; culture, 80; plants,
absorption by, 24 ; cause of form, 319.

Wax, 4.

Whorl, 274.

Working energy, 205, 403, 404.

Working materials, 148.

Wounds, healing of, 328; regeneration
from, 329 ; as stimuli to protoplasmic
movement, 540; as directive stimuli,
486, 553-

Xerophytes, adaptation in, 253.

Zinc, 87.

Zone of maximal growth, 289.

Zymase, 211.

Oxford : Printed at the Clarendon Press by HORACE HART, M.A.

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